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In saturating irradiances of red light, photosynthesis of Laminaria saccharina (L.) Lamouroux was stimulated by low irradiances of continuous blue light only when the supply of dissolved inorganic carbon (DIC) was limiting. The degree of this stimulation was inversely proportional to the logarithm of the concentration of free CO2, whether this was adjusted by varying the total DIC or the pH at a given DIC concentration. The final pH reached in a closed system was higher in blue light than in red light. Both acetazolamide and ethoxyzolamide suppressed the responses to blue light almost completely, but reduced photosynthesis in red light by only 30%. Buffering the pH of the seawater also suppressed the stimulation of photosynthesis by blue light without affecting the photosynthetic rate in red light. The transient stimulation of O2 evolution by a blue light pulse was not accompanied by a corresponding increase in CO2 consumption. These observations could be explained if, in analogy to the mechanism proposed for Ectocarpus (Schmid, Mills & Dring 1996, Plant Cell and Environment 19, 373-382, this issue, accompanying paper), photosynthesis was supported by a blue- light-activated release of CO2 from an internal store. We suggest that the store is located in the vacuoles of the cortical tissue of the blades. The main photosynthetic tissue, however, is in the overlying meristoderm, and blue-light- activated mobilization of the store could stimulate O2 evolution only if periplasmic carbonic anhydrase was available to facilitate CO2 uptake from the cortex. [ABSTRACT FROM AUTHOR]

The scaling of CO2 and water vapour transfer from leaf to canopy dimensions was achieved by integrating mechanistic models for physiological (photosynthesis, stomatal conductance and soil/root and bole respiration) and micrometeorological (radiative transfer, turbulent transfer and surface energy exchanges) processes. The main objectives of this paper are to describe a canopy photosynthesis and evaporation model for a temperate broadleaf forest and to test it against field measurements. The other goal of this paper is to use the validated model to address some contemporary ecological and physiological questions concerning the transfer of carbon and water between forest canopies and the atmosphere. In particular, we examine the role of simple versus complex radiative transfer models and the effect of environmental (solar radiation and CO2) and ecophysiological (photosynthetic capacity) variables on canopy-scale carbon and water vapour fluxes. [ABSTRACT FROM AUTHOR]

Many uncertainties remain regarding how climate change will alter the structure and function of forest ecosystems. At the Aspen FACE experiment in northern Wisconsin, we are attempting to understand how an aspen/birch/maple forest ecosystem responds to long-term exposure to elevated carbon dioxide (CO2) and ozone (O3), alone and in combination, from establishment onward. We examine how O3 affects the flow of carbon through the ecosystem from the leaf level through to the roots and into the soil micro-organisms in present and future atmospheric CO2 conditions. We provide evidence of adverse effects of O3, with or without co-occurring elevated CO2, that cascade through the entire ecosystem impacting complex trophic interactions and food webs on all three species in the study: trembling aspen ( Populus tremuloides Michx .), paper birch ( Betula papyrifera Marsh), and sugar maple ( Acer saccharum Marsh). Interestingly, the negative effect of O3 on the growth of sugar maple did not become evident until 3 years into the study. The negative effect of O3 effect was most noticeable on paper birch trees growing under elevated CO2. Our results demonstrate the importance of long-term studies to detect subtle effects of atmospheric change and of the need for studies of interacting stresses whose responses could not be predicted by studies of single factors. In biologically complex forest ecosystems, effects at one scale can be very different from those at another scale. For scaling purposes, then, linking process with canopy level models is essential if O3 impacts are to be accurately predicted. Finally, we describe how outputs from our long-term multispecies Aspen FACE experiment are being used to develop simple, coupled models to estimate productivity gain/loss from changing O3. [ABSTRACT FROM AUTHOR]

CO2‐induced chloroplast movement was reported in the monograph by Gustav Senn in 1908: unilateral CO2 supply to the one cell‐layered moss leaves induced the positively CO2‐tactic periclinal arrangement of chloroplasts. Here, using the model moss plant Physcomitrium patens, we examined basic features of chloroplast CO2‐tactic relocation with a modernized experimental system. The CO2 relocation was light‐dependent and, especially, CO2 relocation in red light was substantially dependent on photosynthetic activity. In blue light, CO2 relocation was mainly dependent on microfilaments while microtubule‐based movement was insensitive to CO2, whereas in red light, both cytoskeletons contributed redundantly to CO2 relocation. The CO2 relocation was observed not only when the two leaf surfaces were exposed to CO2‐free air versus CO2‐containing air, but also by exposing them physiologically relevant differences in CO2 concentrations. In the leaves placed on the surface of a gel sheet, chloroplasts avoided the gel side and positioned in the air‐facing surface, and this phenomenon was also shown to be photosynthesis dependent. Based on these observations, we propose a hypothesis that the threshold light intensity between the light‐accumulation and ‐avoidance responses of the photorelocation would be increased by CO2, resulting in the CO2‐tactic relocation of chloroplasts. Summary Statement: Chloroplasts in one cell‐layered leaf of Physcomitrium patens showed CO2‐tactic relocation in blue and red light. This relocation was based on microfilaments in blue light, while based on microfilaments and microtubules in red light. This relocation was partly photosynthesis dependent. [ABSTRACT FROM AUTHOR]

In order to parametrize a leaf submodel of a canopy level gas-exchange model, a series of photosynthesis and stomatal conductance measurements were made on leaves of white oak (Quercus alba L.) and red maple (Acer rubrum L.) in a mature deciduous forest near Oak Ridge, TN. Gas-exchange characteristics of sun leaves growing at the top of a 30 m canopy and of shade leaves growing at a depth of 3-4 m from the top of the canopy were determined. Measured rates of net photosynthesis at a leaf temperature of 30 °C and saturating photosynthetic photon flux density, expressed on a leaf area basis, were significantly lower (P = 0.01; n = 8) in shade leaves (7.9 μmol m-2 S-1) than in sun leaves (11.5 μmol m-2 S-1). Specific leaf area increased significantly with depth in the canopy, and when photosynthesis rates were expressed on a dry mass basis, they were not significantly different for shade and sun leaves. The percentage leaf nitrogen did not vary significantly with height in the canopy; thus, rates expressed on a per unit nitrogen basis were also not significantly different in shade and sun leaves. A widely used model integrating photosynthesis and stomatal conductance was parametrized independently for sun and shade leaves, enabling us to model successfully diurnal variations in photosynthesis and evapotranspiration of both classes of leaves. Key photosynthesis model parameters were found to scale with leaf nitrogen levels. The leaf model parametrizations were then incorporated into a canopy-scale gas-exchange model that is discussed and tested in a companion paper (Baldocchi & Harley 1995, Plant, Cell and Environment 18, 1157-1173). [ABSTRACT FROM AUTHOR]

This paper describes the construction and performance of branch bags and a CO2 control system used to fumigate branches of mature Sitka spruce trees with air enriched in CO2 (700 μmol mol-1). It contains some examples of results obtained using the system over the course of the first two growing seasons. The branch bags have run continuously for 2 years with very few problems. CO2 concentrations were within 20 μmol mol-1 of the target concentration for more than 90% of the time. Temperatures within the bags were slightly higher than ambient (1-2 °C) and this had some effect on phenology. Attenuation of quantum flux density (photosynthetically active radiation) was 10-45%. The branch bag system has enabled investigation into the effects of elevated CO2 on mature tissue without the problems and expense of fumigating whole trees, Growth in elevated CO2 resulted in an increase in starch and a decrease in soluble protein content of needles, Stomatal conductance was higher in elevated CO2 grown needles, and there was some evidence of an increase in photosynthetic capacity. [ABSTRACT FROM AUTHOR]

The experiments and simulations reported in this paper show that, for stomata sensitive to both CO2 and water vapour concentrations, responses of stomatal conductance (gsw) to boundary layer thickness have two components, one resulting from changes in intercellular CO2 concentration (χic) and another from changes in leaf surface water vapour saturation deficit (Dsw). The experiments and simulations also show that the boundary layer conductance (gbw) can significantly alter the apparent response of gsw) to ambient air CO2 mole fraction (χac) and water vapour mole fraction (χ aw). Because of the feedback loop involved the responses of gsw) for χac) and χaw) each include responses to both χic) and Dsw), The boundary layer alters the state of the variables sensed by the guard cells-i.e. χic) and Dsw)-and so it is a source of feedback, Thus, when scaling up from responses of stomata to the response of gsw) for a whole leaf, the effect of the boundary layer must be considered. The results indicate that, for given responses of gsw) to χic)and Dsw), the apparent responses of gsw) to Daw and χac) depend on the size of the leaf and wind speed, showing that this effect of the boundary layer should be considered when comparing data measured under different conditions, or with different methods. [ABSTRACT FROM AUTHOR]

While photosynthesis of C3 plants is stimulated by an increase in the spheric CO2 concentration, photosynthetic capacity is often reduced after tong-term exposure to elevated CO2. This reduction appears to be brought about by end product inhibition, resulting from an imbalance in the supply and demand of carbohydrates. A review of the literature revealed that the reduction of photosynthetic capacity in elevated CO2 was most pronounced when the increased supply of carbohydrates was combined with small sink size. The volume of pots in which plants were grown affected the sink size by restricting root growth. While plants grown in small pots had a reduced photosynthetic capacity, plants grown in the field showed no reduction or an increase in this capacity. Pot volume also determined the effect of elevated CO2 on the root/shoot ratio: the root/shoot ratio increased when root growth was not restricted and decreased in plants grown in small pots. The data presented in this paper suggest that plants growing in the field will maintain a high photosynthetic capacity as the atmospheric CO2 level continues to rise. [ABSTRACT FROM AUTHOR]

The CO2 partial pressure in the chloroplasts of intact photosynthetic C3 leaves is thought to be less than the intercellular CO2 partial pressure. The intercellular CO2 partial pressure can be calculated from CO2 and H2O gas exchange measurements, whereas the CO2 partial pressure in the chloroplasts is unknown. The conductance of CO2 from the inter-cellular space to the chloroplast stroma and the CO2 partial pressure in the chloroplast stroma can be calculated if the properties of photosynthetic gas exchange are compared with the kinetics of the enzyme ribulose 1 ,5-bisphosphate carboxylase/ oxygenase (RuBPCO). A discrepancy between gas exchange and RuBPCO kinetics can be attributed to a deviation of CO2 partial pressure in the chloroplast stroma from that calculated in the intercellular space. This paper is concerned with the following: estimation of the kinetic constants of RuBPCO and their comparison with the CO2 compensation concentration; their comparison with differential uptake of 14CO2 and 12CO2 and their comparison with O2 dependence of net CO2 uptake of photosynthetic leaves. Discrepancy between RuBPCO kinetics and gas exchange was found at a temperature of 12.5 °C, a photosynthetic photon flux density (PPFD) of 550 μmol quanta m-2 s-1, and an ambient CO2 partial pressure of 40 Pa. Consistency between RuBPCO kinetics and gas exchange was found if CO2 partial pressure was decreased, temperature increased and PPFD decreased. The results suggest that a discrepancy between RuBPCO kinetics and gas exchange is due to a diffusion resistance for CO2 across the chloroplast envelope which decreases with increasing temperature. At low CO2 partial pressure, the diffusion resistance appears to be counterbalanced by active CO2 (or HCO3-) transport with high affinity and low maximum velocity. At low PPFD, CO2 partial pressure in the chloroplast stroma appears to be in equilibrium with that in the intercellular space due to low CO2 flux. [ABSTRACT FROM AUTHOR]

The paper reports the results of the comprehensive study of crassulacean acid metabolism in two epiphytic tropical ferns, Drymoglossum piloselloides and Pyrrosia longifolia. The plants were investigated under different light, temperature and water status. It was found that both species are obligate CAM plants. The diurnal acidity rhythm is due to the fluctuation in malic acid concentration, which accounts for the change in titratable acidity. Besides malic acid, shikimate and oxalate are found to be present, but not contributing to the CAM acid rhythm. The diurnal rhythm of malic acid content results in a corresponding rhythm in leaf water relations. Both ψπ and ψtotal were lowest at the end of the night, i.e. when the level of malic acid was highest. The effects of temperature on CO2 exchange were inverse to those observed in other CAM plants. In both ferns studied, dark CO2 fixation increased when the night temperature was increased. Increase in day temperature reduced CO2 uptake during phase IV and during the following night. The observed responses of the ferns to temperature changes suggest that the in situ environmental conditions are optimal for their CAM performance. In weak light, the plants showed net CO2 output during the midday deacidification period. Increases in light intensity reduced such CO2 output. Under drought conditions, the CO2 exchange in the ferns was reduced to zero within 5-6d, indication that the ferns studied are more susceptible to water deficiency than other CAM plants. This could be due to a higher cuticular conductance for water. The results are discussed, in particular, in relation to CAM performance of epiphytes growing in the wet tropics. [ABSTRACT FROM AUTHOR]

Each of >20 independent evolutions of C4 photosynthesis in grasses required reorganization of the Calvin–Benson‐cycle (CB‐cycle) within the leaf, along with coordination of C4‐cycle enzymes with the CB‐cycle to maximize CO2 assimilation. Considering the vast amount of time over which C4 evolved, we hypothesized (i) trait divergences exist within and across lineages with both C4 and closely related C3 grasses, (ii) trends in traits after C4 evolution yield the optimization of C4 through time, and (iii) the presence/absence of trends in coordination between the CB‐cycle and C4‐cycle provides information on the strength of selection. To address these hypotheses, we used a combination of optimality modelling, physiological measurements and phylogenetic‐comparative‐analysis. Photosynthesis was optimized after the evolution of C4 causing diversification in maximal assimilation, electron transport, Rubisco carboxylation, phosphoenolpyruvate carboxylase and chlorophyll within C4 lineages. Both theory and measurements indicated a higher light‐reaction to CB‐cycle ratio (Jatpmax/Vcmax) in C4 than C3. There were no evolutionary trends with photosynthetic coordination between the CB‐cycle, light reactions and the C4‐cycle, suggesting strong initial selection for coordination. The coordination of CB‐C4‐cycles (Vpmax/Vcmax) was optimal for CO2 of 200 ppm, not to current conditions. Our model indicated that a higher than optimal Vpmax/Vcmax affects assimilation minimally, thus lessening recent selection to decrease Vpmax/Vcmax. Summary Statement: When C4 evolved in grass lineages affects the current optimization of photosynthetic traits, with two exceptions: 1. the coordination between light reactions, Calvin‐Benson and C4 cycles shows no evolutionary trend, suggesting strong initial selection for tight coordination, and 2. The coordination between Calvin‐Benson and C4 cycle is optimal for CO2 of 200 ppm, as opposed to current CO2. [ABSTRACT FROM AUTHOR]

Cuticular membranes (CMs) were isolated from leaves of amphibious and submerged plants and their CO2 resistances were determined as a contribution to establish quantitatively the series of resistances met by CO2 diffusing from bulk water to the chloroplasts of submerged leaves. The isolation was performed enzymatically; permeabilities were determined and converted to resistances. The range of permeance values was 3 to 43 × 10−6 m s−1 corresponding to resistance values of 23 to 295 × 103 s m−1, i.e. of the same order of magnitude as boundary layer resistances. The sum of boundary layer, CM, leaf cell and carboxylation resistances could be contained within the total diffusion resistance as determined from the photosynthetic CO2 affinity of the leaf. From the same species, the aerial leaf CM resistance was always higher than the aquatic leaf CM resistance. In a terrestrial plant, the CM resistance to CO2 diffusion was found lower in leaves developed submerged. [ABSTRACT FROM AUTHOR]

This article comments on:Mesophyll conductance decreases in the wild type but not in an ABA‐deficient mutant (aba1) of Nicotiana plumbaginifolia under drought conditions [ABSTRACT FROM AUTHOR]

At leaf level, elevated atmospheric CO2 concentration (eCO2) results in stimulation of carbon net assimilation and reduction of stomatal conductance. However, a comprehensive understanding of the impact of eCO2 at larger temporal (seasonal and annual) and spatial (from leaf to whole‐tree) scales is still lacking. Here, we review overall trends, magnitude and drivers of dynamic tree responses to eCO2, including carbon and water relations at the leaf and the whole‐tree level. Spring and early season leaf responses are most susceptible to eCO2 and are followed by a down‐regulation towards the onset of autumn. At the whole‐tree level, CO2 fertilization causes consistent biomass increments in young seedlings only, whereas mature trees show a variable response. Elevated CO2‐induced reductions in leaf stomatal conductance do not systematically translate into limitation of whole‐tree transpiration due to the unpredictable response of canopy area. Reduction in the end‐of‐season carbon sink demand and water‐limiting strategies are considered the main drivers of seasonal tree responses to eCO2. These large temporal and spatial variabilities in tree responses to eCO2 highlight the risk of predicting tree behavior to eCO2 based on single leaf–level point measurements as they only reveal snapshots of the dynamic responses to eCO2. This review describes overall trends, magnitude and drivers of leaf‐scale and whole‐tree‐scale responses to rising atmospheric CO2 concentration. Results encourage frequent monitoring, as single point measurements provide a limited snapshot of dynamic tree functioning under future CO2 growing conditions. [ABSTRACT FROM AUTHOR]

There have been seven studies of canopy photosynthesis of plants grown iii elevated atmospheric CO2: three of seed crops, two of forage crops and two of native plant ecosystems. Growth in elevated CO2 increased canopy photosynthesis in all cases. The relative effect of CO2 was correlated with increasing temperature: the least stimulation occurred in tundra vegetation grown at an average temperature near 10°C and the greatest in rice grown at 43°C. In soybean, effects of CO2 were greater during leaf expansion and pod fill than at other stages of crop maturation. In the longest running experiment with elevated CO2 treatment to date, monospecific stands of a C3 sedge, Scirpus olneyi (Grey), and a C4 grass, Spartina patens (Alt.) Muhl., have been exposed to twice normal ambient CO2 concentrations for four growing seasons, in open top chambers on a Chesapeake Bay salt marsh. Net ecosystem CO2 exchange per unit green biomass (NCEb) increased by an average of 48% throughout the growing season of 1988, the second year of treatment. Elevated CO2 increased net ecosystem carbon assimilation by 88% in the Scirpus olneyi community and 40% in the Spartina patens community. [ABSTRACT FROM AUTHOR]

At 23°C, both C2H4 and CO2 stimulated the germination of freshly imbibed upper cocklebur (Xanthium pennsylvanicum Wallr.) seeds, but C2H4, unlike CO2, changed to an inhibitor of germination under some soaking conditions. However, when seeds were pre-soaked for more than several hours at 23°C prior to treatment, C2H4 strongly inhibited their germination at 33°C, the degree of inhibition increasing with the duration of pre-soaking. Maximum inhibition occurred at 1-3cm3 m-3 C2H4 when seeds were pre-soaked for 1 week; further increases of C2H4 concentration and pre-soaking period decreased the inhibitory effect. C2H4 was synergistic with CO2 when C2H4 promoted germination, whereas it was antagonistic when inhibitory. Such a transition of the C2H4 action occurred at ca. 27°C. Also l-aminocyclopropane-l-carboxylic acid, a C2H4 precursor, inhibited the germination of presoaked seeds at 33°C, although it promoted the germination at 23°C. When pre-soaked seeds were prepared for germination by chilling at 8°C for 3 d, the inhibitory effect of C2H4 on the subsequent germination was manifested even at 23°C. The reversal of the C2H4 action from promotion to inhibition in cocklebur seed germination is discussed in relation to the engagement of two respiratory pathways in the imbibed seeds. [ABSTRACT FROM AUTHOR]

A method is presented for the control of carbon dioxide concentrations within the roots of Fraxinus pennsylvanica Marsh. The results indicated a linear fit of the root CO2 concentrations to the CO2 levels of the treatment gases: y= 1.1x+105 (r=0.98, 18 d.f.). The method presented for controlling CO2 can be easily modified for other gas mixtures and plant species. [ABSTRACT FROM AUTHOR]

Two experiments are described which test the normal correlations that arise between stomatal conductance, net CO2 assimilation rate, and intercellular CO2 concentration (Ci) using whole shoots of Commelina communis L. In the first, conductance increased with decreasing Ci, at four different quantum flux densities, such that there was no unique relationship between conductance and quantum flux density or Ci. In the second, conductance increased hyperbolically with increasing quantum flux density while Ci was held constant at 466, 302, and 46 μmol mol-1, and the response differed at each Ci. In neither experiment was conductance consistently related to net CO2 assimilation rate in the mesophyll. In both experiments high Ci suppressed the response of conductance to light, while there was a large response of conductance to light at low Ci, indicating an interaction between the effects of light and CO2 on stomata. The results show that the parallel responses of assimilation and conductance to light result in constant intercellular CO2 concentrations, and not that stomata maintain a 'constant Ci.' [ABSTRACT FROM AUTHOR]

An apparatus is described which consists of an assimilation chamber mounted in the centre of a light-integrating Ulbricht sphere, irradiated through two fibre-optic light pipes. This arrangement provides totally diffuse radiation in the sphere. The quantum flux density in the sphere is measured by an integrating quantaspectrometer connected to die sphere by eight fibre-optic light pipes. The quantaspectrometer gives the spectral and the total quantum flux density in the sphere. By measuring the quantum flux density in the sphere before and after the insertion of a plant the number of absorbed quanta per unit time and plant area can be determined. In addition, the spectral distribution of the absorbed quantum flux density is obtained. CO2-assimilation per unit time and plant area is determined in the same apparatus. The totally diffuse radiation within the sphere minimizes mutual shading between branches and leaves. Hence, well-defined light-response-curves of photosynthesis can be obtained by plotting the flux density of CO2-assimilation as a function of the absorbed quantum flux density, and the quantum yields of intact plants can be calculated. Examples of photosynthetic quantum yield determinations and quanta absorption spectra are given for some plant species. [ABSTRACT FROM AUTHOR]

The article announces IEA Carbon Dioxide Disposal Symposium, to be held in Great Britain from March 29-31 1993. The symposium will be divided into a number of major sessions to cover the following subject areas, and their relation to carbon dioxide disposal, environmental impact issues, policy and international initiatives, ocean disposal, aquifers, depleted oil wells/enhanced oil recovery, depleted gas wells and biological processes/chemicals and fuels.

A widespread perception is that, with increasing wind speed, transpiration from plant leaves increases. However, evidence suggests that increasing wind speed enhances carbon dioxide (CO2) uptake while reducing transpiration because of more efficient convective cooling (under high solar radiation loads). We provide theoretical and experimental evidence that leaf water use efficiency (WUE, carbon uptake per water transpired) commonly increases with increasing wind speed, thus improving plants' ability to conserve water during photosynthesis. Our leaf-scale analysis suggests that the observed global decrease in near-surface wind speeds could have reduced WUE at a magnitude similar to the increase in WUE attributed to global rise in atmospheric CO2 concentrations. However, there is indication that the effect of long-term trends in wind speed on leaf gas exchange may be compensated for by the concurrent reduction in mean leaf sizes. These unintuitive feedbacks between wind, leaf size and water use efficiency call for re-evaluation of the role of wind in plant water relations and potential re-interpretation of temporal and geographic trends in leaf sizes. [ABSTRACT FROM AUTHOR]

Changes in net CO2 assimilation rate ( A) are often partitioned into contributions from changes in different variables using an approach that is based on an expression from calculus: namely the definition of the exact differential of A, which states that an infinitesimal change in A ( dA) is equal to the sum of infinitesimal changes in each of the underlying variables, each multiplied by the partial derivative of A with respect to the variable. Finite changes in A can thus be partitioned by integrating this sum across a finite interval. The most widely used method of estimating that integral is a coarse discrete approximation that uses partial derivatives of the natural logarithm of A rather than A itself. This yields biased and ambiguous estimates of partitioned changes in A. We present an alternative partitioning approach based on direct numerical integration of dA. The new approach does not require any partial derivatives to be computed, and it can be applied under any conditions to estimate the contributions from changes in any photosynthetic variable. We demonstrate this approach using field measurements of both seasonal and diurnal changes in assimilation rate, and we provide a spreadsheet implementing the new approach. [ABSTRACT FROM AUTHOR]

Stem respiration plays a role in species coexistence and forest dynamics. Here we examined the intra- and inter-specific variability of stem CO2 efflux ( E) in dominant and suppressed trees of six deciduous species in a mixed forest stand: F agus sylvatica L., Q uercus petraea [ Matt.] Liebl, Q uercus pyrenaica Willd., P runus avium L., S orbus aucuparia L. and C rataegus monogyna Jacq. We conducted measurements in late autumn. Within species, dominants had higher E per unit stem surface area ( Es) mainly because sapwood depth was higher than in suppressed trees. Across species, however, differences in Es corresponded with differences in the proportion of living parenchyma in sapwood and concentration of non-structural carbohydrates ( NSC). Across species, Es was strongly and NSC marginally positively related with an index of drought tolerance, suggesting that slow growth of drought-tolerant trees is related to higher NSC concentration and Es. We conclude that, during the leafless period, E is indicative of maintenance respiration and is related with some ecological characteristics of the species, such as drought resistance; that sapwood depth is the main factor explaining variability in Es within species; and that the proportion of NSC in the sapwood is the main factor behind variability in Es among species. [ABSTRACT FROM AUTHOR]

Alpine treelines are temperature-limited vegetation boundaries. Understanding the effects of elevated [ CO2] and warming on CO2 and H2O gas exchange may help predict responses of treelines to global change. We measured needle gas exchange of Larix decidua Mill. and Pinus mugo ssp. uncinata DC trees after 9 years of free air CO2 enrichment (575 µmol mol−1) and 4 years of soil warming (+4 °C) and analysed δ13 C and δ18 O values of needles and tree rings. Tree needles under elevated [ CO2] showed neither nitrogen limitation nor end-product inhibition, and no down-regulation of maximal photosynthetic rate ( Amax) was found. Both tree species showed increased net photosynthetic rates ( An) under elevated [ CO2] ( L. decidua: +39%; P. mugo: +35%). Stomatal conductance ( gH2O) was insensitive to changes in [ CO2], thus transpiration rates remained unchanged and intrinsic water-use efficiency ( iWUE) increased due to higher An. Soil warming affected neither An nor gH2O. Unresponsiveness of gH2O to [ CO2] and warming was confirmed by δ18 O needle and tree ring values. Consequently, under sufficient water supply, elevated [ CO2] induced sustained enhancement in An and lead to increased C inputs into this ecosystem, while soil warming hardly affected gas exchange of L. decidua and P. mugo at the alpine treeline. [ABSTRACT FROM AUTHOR]

This article comments on: Temperature response of in vivo Rubisco kinetics and mesophyll conductance in Arabidopsis thaliana: comparisons to Nicotiana tabacum [ABSTRACT FROM AUTHOR]

Crop growth models dynamically simulate processes of C, N and water balance on daily or hourly time-steps to predict crop growth and development and at season-end, final yield. Their ability to integrate effects of genetics, environment and crop management have led to applications ranging from understanding gene function to predicting potential impacts of climate change. The history of crop models is reviewed briefly, and their level of mechanistic detail for assimilation and respiration, ranging from hourly leaf-to-canopy assimilation to daily radiation-use efficiency is discussed. Crop models have improved steadily over the past 30-40 years, but much work remains. Improvements are needed for the prediction of transpiration response to elevated CO2 and high temperature effects on phenology and reproductive fertility, and simulation of root growth and nutrient uptake under stressful edaphic conditions. Mechanistic improvements are needed to better connect crop growth to genetics and to soil fertility, soil waterlogging and pest damage. Because crop models integrate multiple processes and consider impacts of environment and management, they have excellent potential for linking research from genomics and allied disciplines to crop responses at the field scale, thus providing a valuable tool for deciphering genotype by environment by management effects. [ABSTRACT FROM AUTHOR]

ABSTRACT ROP GTPases function as molecular switches in diverse cellular processes. Previously, we showed that ROP2 GTPase is activated upon light irradiation, and thereby negatively regulates light-induced stomatal opening. Here we studied the role of ROP2 during stomatal closure. The expression of a constitutively active form of ROP2 (CA-rop2) in Arabidopsis thaliana and Vicia faba resulted in slower and reduced stomatal closure in response to abscisic acid (ABA) and CO2. In contrast, the expression of a dominant-negative form of ROP2 (DN-rop2) and the knockout mutation of ROP2 ( rop2 KO) promoted ABA-induced stomatal closure in Arabidopsis. As early as 10 min after ABA treatment, ROP2 was inactivated and translocated to the cytoplasm of the stomatal guard cells. To elucidate the mechanism by which active ROP2 suppresses stomatal closure, we monitored endocytotic membrane trafficking, which is regulated by Rho GTPases in animal cells. We found that the endocytosis of plasma membrane (PM), as tracked by FM4-64, was lower in CA-rop2-expressing guard cells than in those of wild-type plants, which suggests that active ROP2 suppresses the endocytotic internalization of PM, a process required for stomatal closure. Together, our results suggest that ROP2 is inactivated by ABA, and that this inactivation is required for the timely stomatal closure. [ABSTRACT FROM AUTHOR]

The Farquhar-von Caemmerer-Berry (FvCB) model of photosynthesis is a change-point model and structurally overparameterized for interpreting the response of leaf net assimilation (A) to intercellular CO concentration (Ci). The use of conventional fitting methods may lead not only to incorrect parameters but also several previously unrecognized consequences. For example, the relationships between key parameters may be fixed computationally and certain fits may be produced in which the estimated parameters result in contradictory identification of the limitation states of the data. Here we describe a new approach that is better suited to the FvCB model characteristics. It consists of four main steps: (1) enumeration of all possible distributions of limitation states; (2) fitting the FvCB model to each limitation state distribution by minimizing a distribution-wise cost function that has desirable properties for parameter estimation; (3) identification and correction of inadmissible fits; and (4) selection of the best fit from all possible limitation state distributions. The new approach implemented theoretical parameter resolvability with numerical procedures that maximally use the information content of the data. It was tested with model simulations, sampled A/Ci curves, and chlorophyll fluorescence measurements of different tree species. The new approach is accessible through the automated website leafweb.ornl.gov. [ABSTRACT FROM AUTHOR]

Eucalyptus pauciflora Sieber ex Sprengel. (snow gum) was grown under ambient (370 µL L−1) and elevated (700 µL L−1) atmospheric [CO2] in open-top chambers (OTCs) in the field and temperature-controlled glasshouses. Nitrogen applications to the soil ranged from 0.1 to 2.75 g N per plant. Trees in the field at high N levels grew rapidly during summer, particularly in CO2-enriched atmosphere, but suffered high mortality during summer heatwaves. Generally, wider and more numerous secondary xylem vessels at the root–shoot junction in CO2-enriched trees conferred fourfold higher below-ground hydraulic conductance. Enhanced hydraulic capacity was typical of plants at elevated [CO2] (in which root and shoot growth was accelerated), but did not result from high N supply. However, because high rates of N application consistently made trees prone to dehydration during heatwaves, glasshouse studies were required to identify the effect of N nutrition on root development and hydraulics. While the effects of elevated [CO2] were again predominantly on hydraulic conductivity, N nutrition acted specifically by constraining deep root penetration into soil. Specifically, 15–40% shallower root systems supported marginally larger shoot canopies. Independent changes to hydraulics and root penetration have implications for survival of fertilized trees under elevated atmospheric [CO2], particularly during water stress. [ABSTRACT FROM AUTHOR]

We showed that temperature responses of dark respiration for foliage of Pinus radiata could be approximated by Arrhenius kinetics, whereby E0 determines shape of the exponential response and denotes overall activation energy of respiratory metabolism. Reproducible and predictable deviation from strict Arrhenius kinetics depended on foliage age, and differed between RCO2 and RO2. Inhibition of oxygen reduction ( RO2) by cyanide (inhibiting COX) or SHAM (inhibiting AOX) resulted in reproducible changes of the temperature sensitivity for RO2, but did not affect RCO2. Enthalpic growth – preservation of electrons in anabolic products – could be approximated with knowledge of four variables: activation energies ( E0) for both RCO2 and RO2, and basal rates of respiration at a low reference temperature ( RREF). Rates of enthalpic growth by P. radiata needles were large in spring due to differences between RREF of oxidative decarboxylation and that of oxygen reduction, while overall activation energies for the two processes were similar. Later during needle development, enthalpic growth was dependent on differences between E0 for RCO2 as compared with RO2, and increased E0( RO2) indicated greater contributions of cytochrome oxidase to accompany the switch from carbohydrate sink to source. Temperature-dependent increments in stored energy can be calculated as the difference between RCO2▵ HCO2 and RO2▵ HO2. [ABSTRACT FROM AUTHOR]

We used an advanced radiogasometric method to study the effects of short-term changes in CO2 concentration ([CO2]) on the rates and substrates of photorespiratory and respiratory decarboxylations under steady-state photosynthesis and in the dark. Experiments were carried out on Plantago lanceolata, Poa trivialis, Secale cereale, Triticum aestivum, Helianthus annuus and Arabidopsis thaliana plants. Rates of photorespiration and respiration measured at a low [CO2] (40 µmol mol−1) were equal to those at normal [CO2] (360 µmol mol−1). Under low [CO2], the substrates of decarboxylation reactions were derived mainly from stored photosynthates, while under normal [CO2] primary photosynthates were preferentially consumed. An increase in [CO2] from 320 to 2300 µmol mol−1 brought about a fourfold decrease in the rate of photorespiration with a concomitant 50% increase in the rate of respiration in the light. Respiration in the dark did not depend on [CO2] up to 30 mmol mol−1. A positive correlation was found between the rate of respiration in the dark and the rate of photosynthesis during the preceding light period. The respiratory decarboxylation of stored photosynthates was suppressed by light. The extent of light inhibition decreased with increasing [CO2]; no inhibition was detected at 30 mmol mol−1 CO2. [ABSTRACT FROM AUTHOR]

The effects of short-term (minutes) variations of CO2 concentration on mesophyll conductance to CO2 ( gm) were evaluated in six different C3 species by simultaneous measurements of gas exchange, chlorophyll fluorescence, online carbon isotope discrimination and a novel curve-fitting method. Depending on the species, gm varied from five- to ninefold, along the range of sub-stomatal CO2 concentrations typically used in photosynthesis CO2-response curves ( AN– Ci curves; where AN is the net photosynthetic flux and Ci is the CO2 concentrations in the sub-stomatal cavity), that is, 50 to 1500 µmol CO2 mol−1 air. Although the pattern was species-dependent, gm strongly declined at high Ci, where photosynthesis was not limited by CO2, but by regeneration of ribulose-1,5-bisphosphate or triose phosphate utilization. Moreover, these changes on gm were found to be totally independent of the velocity and direction of the Ci changes. The response of gm to Ci resembled that of stomatal conductance ( gs), but kinetic experiments suggested that the response of gm was actually faster than that of gs. Transgenic tobacco plants differing in the amounts of aquaporin NtAQP1 showed different slopes of the gm– Ci response, suggesting a possible role for aquaporins in mediating CO2 responsiveness of gm. The importance of these findings is discussed in terms of their effects on parameterization of AN– Ci curves. [ABSTRACT FROM AUTHOR]

Photosynthetic responses to carbon dioxide concentration can provide data on a number of important parameters related to leaf physiology. Methods for fitting a model to such data are briefly described. The method will fit the following parameters: Vcmax, J, TPU, Rd and gm[maximum carboxylation rate allowed by ribulose 1·5-bisphosphate carboxylase/oxygenase (Rubisco), rate of photosynthetic electron transport (based on NADPH requirement), triose phosphate use, day respiration and mesophyll conductance, respectively]. The method requires at least five data pairs of net CO2 assimilation ( A) and [CO2] in the intercellular airspaces of the leaf ( Ci) and requires users to indicate the presumed limiting factor. The output is (1) calculated CO2 partial pressure at the sites of carboxylation, Cc, (2) values for the five parameters at the measurement temperature and (3) values adjusted to 25 °C to facilitate comparisons. Fitting this model is a way of exploring leaf level photosynthesis. However, interpreting leaf level photosynthesis in terms of underlying biochemistry and biophysics is subject to assumptions that hold to a greater or lesser degree, a major assumption being that all parts of the leaf are behaving in the same way at each instant. [ABSTRACT FROM AUTHOR]

Combined δ13C and δ18O analyses of leaf material were used to infer changes in photosynthetic capacity ( Amax) and stomatal conductance ( gl) in Fagus sylvatica and Picea abies trees growing under natural and controlled conditions. Correlation between gl and δ18O in leaf cellulose ( δ18Ocel) allowed us to apply a semi-quantitative model to infer gl from δ18Ocel and also interpret variation in δ13C as reflecting variation in Amax. Extraction of leaf cellulose was necessary, because δ18O from leaf organic matter ( δ18OLOM) and δ18Ocel was not reliably correlated. In juvenile trees, the model predicted elevated carbon dioxide (CO2) to reduce Amax in both species, whereas ozone (O3) only affected beech by reducing CO2 uptake via lowered gl. In adult trees, Amax declined with decreasing light level as gl was unchanged. O3 did not significantly affect isotopic signatures in leaves of adult trees, reflecting the higher O3 susceptibility of juvenile trees under controlled conditions. The isotopic analysis compared favourably to the performance of leaf gas exchange, underlining that the semi-quantitative model approach provides a robust way to gather time-integrated information on photosynthetic performance of trees under multi-faced ecological scenarios, in particular when information needed for quantitative modelling is only scarcely available. [ABSTRACT FROM AUTHOR]

It is anticipated that enrichment of the atmosphere with CO2 will increase photosynthetic carbon assimilation in C3 plants. Analysis of controlled environment studies conducted to date indicates that plant growth at concentrations of carbon dioxide ([CO2]) anticipated for 2050 (∼ 550 µmol mol−1) will stimulate leaf photosynthetic carbon assimilation ( A) by 20 to 40%. Simultaneously, concentrations of tropospheric ozone ([O3]) are expected to increase by 2050, and growth in controlled environments at elevated [O3] significantly reduces A. However, the simultaneous effects of both increases on a major crop under open-air conditions have never been tested. Over three consecutive growing seasons > 4700 individual measurements of A, photosynthetic electron transport ( JPSII) and stomatal conductance ( gs) were measured on Glycine max (L.) Merr. (soybean). Experimental treatments used free-air gas concentration enrichment (FACE) technology in a fully replicated, factorial complete block design. The mean A in the control plots was 14.5 µmol m−2 s−1. At elevated [CO2], mean A was 24% higher and the treatment effect was statistically significant on 80% of days. There was a strong positive correlation between daytime maximum temperatures and mean daily integrated A at elevated [CO2], which accounted for much of the variation in CO2 effect among days. The effect of elevated [CO2] on photosynthesis also tended to be greater under water stress conditions. The elevated [O3] treatment had no statistically significant effect on mean A, gs or JPSII on newly expanded leaves. Combined elevation of [CO2] and [O3] resulted in a slightly smaller increase in average A than when [CO2] alone was elevated, and was significantly greater than the control on 67% of days. Thus, the change in atmospheric composition predicted for the middle of this century will, based on the results of a 3 year open-air field experiment, have smaller effects on photosynthesis, gs and whole chain electron transport through photosystem II than predicted by the substantial literature on relevant controlled environment studies on soybean and likely most other C3 plants. [ABSTRACT FROM AUTHOR]

Limited information is available on the genetic variation and control for plant growth response to elevated CO2 (e[CO2]). Such information is necessary to understand plant adaptation and evolution in future rising CO2. Here, quantitative trait loci (QTL) for leaf growth, development, quality and leaf senescence were determined in a tree pedigree – an F2 hybrid of Populus trichocarpa T. & G and Populus deltoides Marsh, following season-long exposure to either current day ambient carbon dioxide (a[CO2]) or e[CO2] at 600 µL L−1. Leaf growth and development differed between the grandparents such that P. trichocarpa showed greater response to e[CO2]. In the F2 generation, leaf development and quality traits including leaf area, leaf shape, epidermal cell area, and stomatal number, specific leaf area (SLA), and the phenology trait, canopy senescence index, were sensitive to e[CO2]. Sixty-nine QTL were mapped for the 19 traits of plants in a[CO2] while 60 QTL were mapped for plants in e[CO2]. The results suggest that although many QTL mapped to common positions in a[CO2] and e[CO2], confirming their importance in determining growth, there was also differential genetic control for a number of traits including leaf senescence. Candidate genes were shown to collocate to regions where response QTL mapped. This study is the first to identify candidate genes that may be important in determining plant adaptation to future high-CO2 world. [ABSTRACT FROM AUTHOR]

Estimates of the effect of increased global atmospheric CO2 levels on oceanic primary productivity depend on the physiological responses of contemporary phytoplankton populations. However, microalgal populations will possibly adapt to rising CO2 levels in such a way that they become genetically different from contemporary populations. The unknown properties of these future populations introduce an undefined error into predictions of C pool dynamics, especially the presence and size of the biological C pump. To address the bias in predictions introduced by evolution, we measured the kinetics of CO2 uptake in populations of Chlamydomonas reinhardtii that had been selected for growth at high CO2 for 1000 generations. Following selection at high CO2, the populations were unable to induce high-affinity CO2 uptake, and one line had a lower rate of net CO2 uptake. We attribute this to conditionally neutral mutations in genes affecting the C concentrating mechanism (CCM). Lower affinity CO2 uptake, in addition to smaller population sizes, results in a significant reduction in net CO2 uptake of about 38% relative to contemporary populations under the same conditions. This shows how predictions about the properties of communities in the future can be influenced by the effect of natural selection. [ABSTRACT FROM AUTHOR]

A reduction in leaf stomatal conductance (g) with increasing leaf-to-air difference in water vapour pressure (D) is nearly ubiquitous. Ecological comparisons of sensitivity have led to the hypothesis that the reduction in g with increasing D serves to maintain leaf water potentials above those that would cause loss of hydraulic conductance. A reduction in leaf water potential is commonly hypothesized to cause stomatal closure at high D. The importance of these particular hydraulic factors was tested by exposing Abutilon theophrasti, Glycine max, Gossypium hirsutum and Xanthium strumarium to D high enough to reduce g and then decreasing ambient carbon dioxide concentration ([CO2]), and observing the resulting changes in g, transpiration rate and leaf water potential, and their reversibility. Reducing the [CO2] at high D increased g and transpiration rate and lowered leaf water potential. The abnormally high transpiration rates did not result in reductions in hydraulic conductance. Results indicate that low water potential effects on g at high D could be overcome by low [CO2], and that even lower leaf water potentials did not cause a reduction in hydraulic conductance in these well-watered plants. Reduced g at high D in these species resulted primarily from increased stomatal sensitivity to [CO2] at high D, and this increased sensitivity may mediate stomatal responses to leaf hydraulics at high D. [ABSTRACT FROM AUTHOR]

In illuminated stems and branches, CO2 release is often reduced. Many light-triggered processes are thought to contribute to this reduction, namely photorespiration, corticular photosynthesis or even an inhibition of mitochondrial respiration. In this study, we investigated these processes with the objective to discriminate their influence to the overall reduction of branch CO2 release in the light. CO2 gas-exchange measurements of young birch ( Betula pendula Roth.) branches (< 1.5 cm) performed under photorespiratory (20% O2) and non-photorespiratory (< 2%) conditions revealed that photorespiration does not play a pre-dominant role in carbon exchange. This suppression of photorespiration was attributed to the high CO2 concentrations (Ci) within the bark tissues (1544 ± 227 and 618 ± 43 µmol CO2 mol−1 in the dark and in the light, respectively). Changes in xylem CO2 were not likely to explain the observed decrease in stem CO2 release as gas-exchange measurements before and after cutting of the branches did not effect CO2 efflux to the atmosphere. Combined fluorescence and gas-exchange measurements provided evidence that the light-dependent reduction in CO2 release can pre-dominantly be attributed to corticular refixation, whereas an inhibition of mitochondrial respiration in the light is unlikely to occur. Corticular photosynthesis was able to refix up to 97% of the CO2 produced by branch respiration, although it rarely led to a positive net photosynthetic rate. [ABSTRACT FROM AUTHOR]

Measurements of net fluxes of CO2 and O2 from leaves and chlorophyll a fluorescence were used to determine the role of mitochondrial respiration during nitrate (NO3–) assimilation in both a C3 (wheat) and a C4 (maize) plant. Changes in the assimilatory quotient (net CO2 consumed over net O2 evolved) when the nitrogen source was shifted from NO3– to NH4+ (Δ AQ) provided a measure of shoot NO3– assimilation. According to this measure, elevated CO2 inhibited NO3– assimilation in wheat but not maize. Net O2 exchange under ambient CO2 concentrations increased in wheat plants receiving NO3– instead of NH4+, but gross O2 evolution from the photosynthetic apparatus (JO2) was insensitive to nitrogen source. Therefore, O2 consumption within wheat photosynthetic tissue (ΔΟ2), the difference betweenJO2 and net O2 exchange, decreased during NO3– assimilation. In maize, NO3– assimilation was insensitive to changes in intercellular CO2 concentration (Ci); nonetheless,ΔΟ2 at lowCi values was significantly higher in NO3–-fed than in NH4+-fed plants. Changes in O2 consumption during NO3– assimilation may involve one or more of the following processes: (a) Mehler ascorbate peroxidase (MAP) reactions; (b) photorespiration; or (c) mitochondrial respiration. The data presented here indicates that in wheat, the last process, mitochondrial respiration, is decreased during NO3– assimilation. In maize, NO3– assimilation appears to stimulate mitochondrial respiration when photosynthetic rates are limiting. [ABSTRACT FROM AUTHOR]

Two silver birch ( Betula pendula Roth) clones K1659 and V5952 were grown in open-top chambers over 3 years (age 7–9 years). The treatments were increased CO2 concentration (+CO2, 72 Pa), increased O3 concentration (+O3, 2 × ambient O3 with seasonal AOT40 up to 28 p.p.m. h) and in combination (+CO2 + O3). Thirty-seven photosynthetic parameters were measured in the laboratory immediately after excising leaves using a computer-operated routine of gas exchange and optical measurements. In control leaves the photosynthetic parameters were close to the values widely used in a model (Farquhar, von Caemmerer and Berry, Planta 149, 78–90, 1980). The distribution of chlorophyll between photosystem II and photosystem I, intrinsic quantum yield of electron transport, uncoupled turnover rate of Cyt b6f, Rubisco specificity and Km (CO2) were not influenced by treatments. Net photosynthetic rate responded to +CO2 with a mean increase of 17% in both clones. Dry weight of leaves increased, whereas protein, especially Rubisco content and the related photosynthetic parameters decreased. Averaged over 3 years, eight and 17 mechanistically independent parameters were significantly influenced by the elevated CO2 in clones K1659 and V5952, respectively. The elevated O3 caused a significant decrease in the average photosynthetic rate of clone V5952, but not of clone K1659. The treatment caused changes in one parameter of clone K1659 and in 11 parameters of clone V5952. Results of the combined treatment indicated that +O3 had less effect in the presence of +CO2 than alone. Interestingly, changes in the same photosynthetic parameters were observed in chamberless grown trees of clone V5952 as under +O3 treatment in chambers, but this was not observed for clone K1659. These results suggest that during chronic fumigation, at concentrations below the threshold of visible leaf injuries, ozone influenced the photosynthetic parameters as a general stress factor, in a similar manner to weather conditions that were more stressful outside the chambers. According to this hypothesis, the sensitivity of a species or a clone to ozone is expected to depend on the growth conditions: the plant is less sensitive to ozone if the conditions are close to optimal and it is more sensitive to ozone under conditions of stress. [ABSTRACT FROM AUTHOR]

Unlike behavioural responses to physical gradients, active responses to chemical gradients, and their physiological and ecological implications, have rarely been studied in freshwater phytoplanktonic flagellates. This study used microscale preference chambers to investigate the population and individual cell responses of five species exposed to four chemical gradients which commonly develop with depth in lakes: phosphate, oxygen, carbon dioxide and pH. Upon exposure of nutrient-replete and -depleted cells to a phosphate gradient, only nutrient-depleted cells of the autotrophic Chlamydomonas moewusii responded, accumulating at high concentrations. In contrast, all species responded to an oxygen gradient with Ceratium furcoides, Chlamydomonas moewusii, Dinobryon sertularia and Plagioselmis nannoplanctica preferring high concentrations whereas Euglena gracilis preferred low concentrations. In addition, all species displayed a strong affinity for carbon dioxide which was not mediated by detecting pH. Analysis of the swimming trajectories of individual cells showed that directed chemotaxes, rather than speed-dependent chemokineses, were responsible for the observed preferences. These complex and diverse species-dependent chemosensory responses may optimize photosynthesis, facilitate nutrient retrieval during migration, increase growth rate and may influence spatial and temporal distribution, contributing to the delineation of niche separation in phytoplanktonic flagellates. [ABSTRACT FROM AUTHOR]

ABSTRACT Better understanding of crop responses to projected changes in climate is an important requirement. An experiment was conducted in sunlit, controlled environment chambers known as soil–plant–atmosphere–research units to determine the interactive effects of atmospheric carbon dioxide concentration [CO2 ] and ultraviolet-B (UV-B) radiation on cotton (Gossypium hirsutum L.) growth, development and leaf photosynthetic characteristics. Six treatments were used, comprising two levels of [CO2 ] (360 and 720 µ mol mol-1 ) and three levels of 0 (control), 7.7 and 15.1 kJ m-2 d-1 biologically effective UV-B radiations within each CO2 level. Treatments were imposed for 66 d from emergence until 3 weeks after the first flower stage. Plants grown in elevated [CO2 ] had greater leaf area and higher leaf photosynthesis, non-structural carbohydrates, and total biomass than plants in ambient [CO2 ]. Neither dry matter partitioning among plant organs nor pigment concentrations was affected by elevated [CO2 ]. On the other hand, high UV-B (15.1 kJ m-2 d-1 ) radiation treatment altered growth resulting in shorter stem and branch lengths and smaller leaf area. Shorter plants at high UV-B radiation were related to internode lengths rather than the number of mainstem nodes. Fruit dry matter accumulation was most sensitive to UV-B radiation due to fruit abscission. Even under 7.7 kJ m-2 d-1 of UV-B radiation, fruit dry weight was significantly lower than the control although total biomass and leaf photosynthesis did not differ from the control. The UV-B radiation of 15.1 kJ m-2 d-1 reduced both total (43%) and fruit (88%) dry weights due to smaller leaf area and lower leaf net photosynthesis. Elevated [CO2 ] did not ameliorate the adverse effects of UV-B radiation on cotton growth and physiology, particularly the boll retention under UV-B stress. [ABSTRACT FROM AUTHOR]

ABSTRACT Fluxes and concentrations of carbon dioxide and 13 CO2 provide information about ecosystem physiological processes and their response to environmental variation. The biophysical model, CANOAK, was adapted to compute concentration profiles and fluxes of 13 CO2 within and above a temperate deciduous forest (Walker Branch Watershed, Tennessee, USA). Modifications to the model are described and the ability of the new model (CANISOTOPE ) to simulate concentration profiles of 13 CO2 , its flux density across the canopy–atmosphere interface and leaf-level photosynthetic discrimination against 13 CO2 is demonstrated by comparison with field measurements. The model was used to investigate several aspects of carbon isotope exchange between a forest ecosystem and the atmosphere. During the 1998 growing season, the mean photosynthetic discrimination against 13 CO2 , by the deciduous forest canopy (Δcanopy ), was computed to be 22·4‰, but it varied between 18 and 27‰. On a diurnal basis, the greatest discrimination occurred during the early morning and late afternoon. On a seasonal time scale, the greatest diurnal range in Δcanopy occurred early and late in the growing season. Diurnal and seasonal variations in Δcanopy resulted from a strong dependence of Δcanopy on photosynthetically active radiation and vapour pressure deficit of air. Model calculations also revealed that the relationship between canopy-scale water use efficiency (CO2 assimilation/transpiration) and Δcanopy was positive due to complex feedbacks among fluxes, leaf temperature and vapour pressure deficit, a finding that is counter to what is predicted for leaves exposed to well-mixed environments. [ABSTRACT FROM AUTHOR]

ABSTRACT Dark chilling inhibited photosynthesis in two soybean [Glycine max (L.) Merr.] cultivars (Fiskeby V and Maple Arrow). The inhibition of CO2 assimilation was characterized by a simultaneous decrease in stomatal conductance (G s ) and intercellular CO2 concentration (C i ) in Maple Arrow, whereas a similar decrease in G s in Fiskeby V occurred without any change in C i . Dark chilling had little effect on total ribulose-1, 5-bisphosphate carboxylase/oxygenase (Rubisco) activity, Rubisco protein content or Rubisco activation state in the subsequent light period. Chilling reduced the abundance of the nocturnal Rubisco inhibitor, 2-carboxyarabinitol 1-phosphate, only in Fiskeby V. The abundance of Rubisco small subunit transcripts was enhanced in both cultivars as a result of dark chilling. Dark chilling decreased the maximal extractable activities and activation states of stromal fructose-1,6-bisphosphatase (FBPase) and NADP-malate dehydrogenase but had no effect on sucrose phosphate synthase or leaf sucrose and starch contents. It is concluded that dark chilling-induced limitations on CO2 assimilation are predominantly due to metabolic restrictions rather than to direct effects on electron transport reactions and that stromal FBPase is particularly susceptible to dark chilling. [ABSTRACT FROM AUTHOR]

ABSTRACT A combined model to simulate CO2 and H2 O gas exchange at the leaf scale was parameterized using data obtained from in situ leaf-scale observations of diurnal and seasonal changes in the CO2 and H2 O gas exchange of four temperate deciduous broad-leaved trees using a porometric method. The model consists of a Ball et al . type stomatal conductance submodel [Ball, Woodrow & Berry, pp. 221–224 in Progress in Photosynthesis Research (ed. I. Biggins), Martinus-Nijhoff Publishers, Dordrecht, The Netherlands, 1987] and a Farquhar et al . type biochemical submodel of photosynthesis (Farquhar, von Caemmerer & Berry, Planta 149, 78–90, 1980). In these submodels, several parameters were optimized for each tree species as representative of the quantitative characteristics related to gas exchange. The results show that the seasonal physiological changes of V cmax25 in the biochemical model of photosynthesis should be used to estimate the long-term CO2 gas exchange. For R d25 in the biochemical model of photosynthesis and m in the Ball et al . type stomatal conductance model, the difference should be counted during the leaf expansion period. [ABSTRACT FROM AUTHOR]

Abstract An investigation to determine whether stomatal acclimation to [CO2 ] occurred in C3 /C4 grassland plants grown across a range of [CO2 ] (200–550 µ mol mol-1 ) in the field was carried out. Acclimation was assessed by measuring the response of stomatal conductance (g s ) to a range of intercellular CO2 (a g s –C i curve) at each growth [CO2 ] in the third and fourth growing seasons of the treatment. The g s –C i response curves for Solanum dimidiatum (C3 perennial forb) differed significantly across [CO2 ] treatments, suggesting that stomatal acclimation had occurred. Evidence of non–linear stomatal acclimation to [CO2 ] in this species was also found as maximum g s (g smax ; g s measured at the lowest C i ) increased with decreasing growth [CO2 ] only below 400 µ mol mol-1 . The substantial increase in g s at subambient [CO2 ] for S. dimidiatum was weakly correlated with the maximum velocity of carboxylation (V cmax ; r 2 = 0·27) and was not associated with CO2 saturated photosynthesis (A max ). The response of g s to C i did not vary with growth [CO2 ] in Bromus japonicus (C3 annual grass) or Bothriochloa ischaemum (C4 perennial grass), suggesting that stomatal acclimation had not occurred in these species. Stomatal density, which increased with rising [CO2 ] in both C3 species, was not correlated with g s . Larger stomatal size at subambient [CO2 ], however, may be associated with stomatal acclimation in S. dimidiatum . Incorporating stomatal acclimation into modelling studies could improve the ability to predict changes... [ABSTRACT FROM AUTHOR]

ABSTRACT A spectrum of models that estimate assimilation rate A from intercellular carbon dioxide concentration (Ci) and measured stomatal conductance to CO2 (gc) were investigated using leaf-level gas exchange measurements. The gas exchange measurements were performed in a uniform loblolly pine stand (Pinus taeda L.) using the Free Air CO2 Enrichment (FACE) facility under ambient and elevated atmospheric CO2 for 3 years. These measurements were also used to test a newly proposed framework that combines basic properties of the A–Ci curve with a Fickian diffusion transport model to predict the relationship between Ci/Ca and gc, where Ca is atmospheric carbon dioxide concentration. The widely used Ball–Berry model and five other models as well as the biochemical model proposed by were also reformulated to express variations in Ci/Ca as a function of their corresponding driving mechanisms. To assess the predictive capabilities of these approaches, their respective parameters were estimated from independent measurements of long-term stable carbon isotope determinations (δ13C), meteorological variables, and ensemble A–Ci curves. All eight approaches reproduced the measured A reasonably well, in an ensemble sense, from measured water vapour conductance and modeled Ci/Ca. However, the scatter in the instantaneous A estimates was sufficiently large for both ambient and elevated Ca to suggest that other transient processes were not explicitly resolved by all eight parameterizations. An important finding from our analysis is that added physiological complexity in modeling Ci/Ca (when gc is known) need not always translate to increased accuracy in predicting A. Finally, the broader utility of these approaches to estimate assimilation and net ecosystem exchange is discussed in relation to elevated atmospheric CO2. [ABSTRACT FROM AUTHOR]

ABSTRACTPrevious modelling exercises and conceptual arguments have predicted that a reduction in biochemical capacity for photosynthesis (Aarea) at elevated CO2 may be compensated by an increase in mesophyll tissue growth if the total amount of photosynthetic machinery per unit leaf area is maintained (i.e. morphological upregulation). The model prediction was based on modelling photosynthesis as a function of leaf N per unit leaf area (Narea), where Narea = Nmass × LMA. Here, Nmass is percentage leaf N and is used to estimate biochemical capacity and LMA is leaf mass per unit leaf area and is an index of leaf morphology. To assess the relative importance of changes in biochemical capacity versus leaf morphology we need to control for multiple correlations that are known, or that are likely to exist between CO2 concentration, Narea, Nmass, LMA and Aarea. Although this is impractical experimentally, we can control for these correlations statistically using systems of linear multiple-regression equations. We developed a linear model to partition the response of Aarea to elevated CO2 into components representing the independent and interactive effects of changes in indexes of biochemical capacity, leaf morphology and CO2 limitation of photosynthesis. The model was fitted to data from three pine and seven deciduous tree species grown in separate chamber-based field experiments. Photosynthetic enhancement at elevated CO2 due to morphological upregulation was negligible for most species. The response of Aarea in these species was dominated by the reduction in CO2 limitation occurring at higher CO2 concentration. However, some species displayed a significant reduction in potential photosynthesis at elevated CO2 due to an increase in LMA that was independent of any... [ABSTRACT FROM AUTHOR]

Elevated CO2 is expected to reduce forest water use as a result of CO2-induced stomatal closure, which has implications for ecosystem-scale phenomena controlled by water availability. Leaf-level CO2 and H2O exchange responses and plant and soil water relations were examined in a maturing loblolly pine (Pinus taeda L.) stand in a free-air CO2 enrichment (FACE) experiment in North Carolina, USA to test if these parameters were affected by elevated CO2. Current-year foliage in the canopy was continuously exposed to elevated CO2 (ambient CO2+200μmol mol–1) in free-air during needle growth and development for up to 400 d. Photosynthesis in upper canopy foliage was stimulated by 50–60% by elevated CO2 compared with ambient controls. This enhancement was similar in current-year, ambient-grown foliage temporarily measured at elevated CO2 compared with long-term elevated CO2 grown foliage. Significant photosynthetic enhancement by CO2 was maintained over a range of conditions except during peak drought. There was no evidence of water savings in elevated CO2 plots in FACE compared to ambient plots under drought and non-drought conditions. This was supported by evidence from three independent measures. First, stomatal conductance was not significantly different in elevated CO2 versus ambient trees of P. taeda. Calculations of time-integrated ci/ca ratios from analysis of foliar δ13C showed that these ratios were maintained in foliage under elevated CO2. Second, soil moisture was not significantly different between ambient and elevated CO2 plots during drought. Third, pre-dawn and mid-day leaf water potentials were also unaffected by the seasonal CO2 exposure, as were tissue osmotic potentials and turgor loss points.... [ABSTRACT FROM AUTHOR]