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Background: The environmental stresses caused by climate change have become more severe in recent decades, affecting tree growth and physiology. Tropical forests have great potential for global carbon sequestration. However, they suffer from heavy rainfall and prolonged dry periods due to climate change. Swietenia macrophylla King and Melia azedarach L. are economically valuable trees that are widely planted in southern Taiwan. Plantations are exposed to either prolonged dry periods or heavy rainfall within the seasons of tropical monsoon areas. Photo-physiological comparisons may provide information that can improve management of S. macrophylla and M. azedarach plantations in tropical regions. Results: Both species exhibited a midday depression in leaf photosynthesis regardless of the season. The net photosynthetic rate ( P ), stomatal conductance ( g ), and transpiration rate ( E) in the dry season all significantly decreased in both tree species. In addition, M. azedarach used water more efficiently than did S. macrophylla during the dry season, but S. macrophylla had higher P compared with that in M. azedarach during the wet season. Temperature and vapor pressure deficit influenced P variation in S. macrophylla and M. azedarach, respectively. Our data suggested that the P and g of M. azedarach, but not of S. macrophylla, were linearly correlated during the dry season. The reduction of the leaf area was more sever in M. azedarach than in S. macrophylla, thus preventing water loss more efficiently. Conclusions: M. azedarach adapted to drought by reducing total leaf area and maintaining higher P , g , E, and WUE compared with those measured in S. macrophylla during the dry season. M. azedarach is more drought adaptation and more suitable for both humid and semi-humid areas than S. macrophylla, whereas the latter should be limited to more humid areas. [ABSTRACT FROM AUTHOR]

Plant stomata display a wide range of short-term behavioural and long-term morphological responses to atmospheric carbon dioxide concentration ([CO]). The diversity of responses suggests that plants may have different strategies for controlling gas exchange, yet it is not known whether these strategies are co-ordinated in some way. Here, we test the hypothesis that there is co-ordination of physiological (via aperture change) and morphological (via stomatal density change) control of gas exchange by plants. We examined the response of stomatal conductance ( G) to instantaneous changes in external [CO] ( C) in an evolutionary cross-section of vascular plants grown in atmospheres of elevated [CO] (1,500 ppm) and sub-ambient [O] (13.0 %) compared to control conditions (380 ppm CO, 20.9 % O). We found that active control of stomatal aperture to [CO] above current ambient levels was not restricted to angiosperms, occurring in the gymnosperms Lepidozamia peroffskyana and Nageia nagi. The angiosperm species analysed appeared to possess a greater respiratory demand for stomatal movement than gymnosperm species displaying active stomatal control. Those species with little or no control of stomatal aperture (termed passive) to C were more likely to exhibit a reduction in stomatal density than species with active stomatal control when grown in atmospheres of elevated [CO]. The relationship between the degree of stomatal aperture control to C above ambient and the extent of any reduction in stomatal density may suggest the co-ordination of physiological and morphological responses of stomata to [CO] in the optimisation of water use efficiency. This trade-off between stomatal control strategies may have developed due to selective pressures exerted by the costs associated with passive and active stomatal control. [ABSTRACT FROM AUTHOR]

Crassulacean acid metabolism (CAM) and the capacity to store large quantities of water are thought to confer high water use efficiency (WUE) and survival of succulent plants in warm desert environments. Yet the highly variable precipitation, temperature and humidity conditions in these environments likely have unique impacts on underlying processes regulating photosynthetic gas exchange and WUE, limiting our ability to predict growth and survival responses of desert CAM plants to climate change. We monitored net CO assimilation ( A), stomatal conductance ( g), and transpiration ( E) rates periodically over 2 years in a natural population of the giant columnar cactus Carnegiea gigantea (saguaro) near Tucson, Arizona USA to investigate environmental and physiological controls over carbon gain and water loss in this ecologically important plant. We hypothesized that seasonal changes in daily integrated water use efficiency (WUE) in this constitutive CAM species would be driven largely by stomatal regulation of nighttime transpiration and CO uptake responding to shifts in nighttime air temperature and humidity. The lowest WUE occurred during time periods with extreme high and low air vapor pressure deficit ( D). The diurnal with the highest D had low WUE due to minimal net carbon gain across the 24 h period. Low WUE was also observed under conditions of low D; however, it was due to significant transpiration losses. Gas exchange measurements on potted saguaro plants exposed to experimental changes in D confirmed the relationship between D and g. Our results suggest that climatic changes involving shifts in air temperature and humidity will have large impacts on the water and carbon economy of the giant saguaro and potentially other succulent CAM plants of warm desert environments. [ABSTRACT FROM AUTHOR]

Drought stress is known to limit plant performance in Mediterranean-type ecosystems. We have investigated the dynamics of the hydraulics, gas exchange and morphology of six co-existing Mediterranean woody species growing under natural field conditions during a drought that continued during the entire summer. Based on the observed minimum leaf water potentials, our results suggest that the six co-existing species cover a range of plant hydraulic strategies, from isohydric to anisohydric. These differences are remarkable since the selected individuals grow within several meters of each other, sharing the same environment. Surprisingly, whatever the leaf water potentials were at the end of the dry period, stomatal conductance, photosynthesis and transpiration rates were relatively similar and low across species. This result contradicts the classic view that anisohydric species are able to maintain gas exchange for longer periods of time during drought stress. None of the plants showed the expected structural acclimation response to the increasing drought (reduction of leaf-to-sapwood area ratio), thereby rejecting the functional equilibrium hypothesis for our study system. Instead, three of the six species increased photosynthetic area at the branch level. The observed dissimilar patterns of gas exchange, hydraulics and morphology across species seem to be equally successful given that photosynthesis at the leaf level was maintained at similar rates over the whole dry period. [ABSTRACT FROM AUTHOR]

mesoscale meteorological model (FOOT3DK) is coupled with a gas exchange model to simulate surface fluxes of CO and HO under field conditions. The gas exchange model consists of a C3 single leaf photosynthesis sub-model and an extended big leaf (sun/shade) sub-model that divides the canopy into sunlit and shaded fractions. Simulated CO fluxes of the stand-alone version of the gas exchange model correspond well to eddy-covariance measurements at a test site in a rural area in the west of Germany. The coupled FOOT3DK/gas exchange model is validated for the diurnal cycle at singular grid points, and delivers realistic fluxes with respect to their order of magnitude and to the general daily course. Compared to the Jarvis-based big leaf scheme, simulations of latent heat fluxes with a photosynthesis-based scheme for stomatal conductance are more realistic. As expected, flux averages are strongly influenced by the underlying land cover. While the simulated net ecosystem exchange is highly correlated with leaf area index, this correlation is much weaker for the latent heat flux. Photosynthetic CO uptake is associated with transpirational water loss via the stomata, and the resulting opposing surface fluxes of CO and HO are reproduced with the model approach. Over vegetated surfaces it is shown that the coupling of a photosynthesis-based gas exchange model with the land-surface scheme of a mesoscale model results in more realistic simulated latent heat fluxes. [ABSTRACT FROM AUTHOR]

The carbon isotope signature (δ13C) of foliar cellulose from sunlit tops of trees typically becomes enriched as trees of the same species in similar environments grow taller, indicative of size-related changes in leaf gas exchange. However, direct measurements of gas exchange in common environmental conditions do not always reveal size-related differences, even when there is a distinct size-related trend in δ13C of the very foliage used for the gas exchange measurements. Since δ13C of foliage predominately reflects gas exchange during spring when carbon is incorporated into leaf cellulose, this implies that gas exchange differences in different-sized trees are most likely to occur in favorable environmental conditions during spring. If gas exchange differs with tree size during wet but not dry conditions, then this further implies that environmental sensitivity of leaf gas exchange varies as a function of tree size. These implications are consistent with theoretical relationships among height, hydraulic conductance and gas exchange. We investigated the environmental sensitivity of gas exchange in different-sized Douglas-fir (Pseudotsuga menziesii) via a detailed process model that specifically incorporates size-related hydraulic conductance [soil–plant–atmosphere (SPA)], and empirical measurements from both wet and dry periods. SPA predicted, and the empirical measurements verified, that differences in gas exchange associated with tree size are greatest in wet and mild environmental conditions and minimal during drought. The results support the hypothesis that annual net carbon assimilation and transpiration of trees are limited by hydraulic capacity as tree size increases, even though at particular points in time there may be no difference in gas exchange between different-sized trees. Maximum net ecosystem exchange occurs in spring in Pacific Northwest forests; therefore, the presence of hydraulic limitations during this period may play a large role in carbon uptake differences with stand-age. The results also imply that the impacts of climate change on the growth and physiology of forest trees will vary depending on the age and size of the forest. [ABSTRACT FROM AUTHOR]

Associations between δ13C values and leaf gas exchanges and tree-ring or needle growth, used in ecophysiological compositions, can be complex depending on the relative timing of CO2 uptake and subsequent redistribution and allocation of carbon to needle and stem components. For palaeoenvironmental and dendroecological studies it is often interpreted in terms of a simple model of δ13C fractionation in C3 plants. However, in spite of potential complicating factors, few studies have actually examined these relationships in mature trees over inter- and intra-annual time-scales. Here, we present results from a 4 years study that investigated the links between variations in leaf gas-exchange properties, growth, and dated δ13C values along the needles and across tree rings of Aleppo pine trees growing in a semi-arid region under natural conditions or with supplemental summer irrigation. Sub-sections of tissue across annual rings and along needles, for which time of formation was resolved from growth rate analyses, showed rapid growth and δ13C responses to changing environmental conditions. Seasonal cycles of growth and δ13C (up to ~4‰) significantly correlated ( P<0.01) with photosynthetically active radiation, vapour pressure deficit, air temperature, and soil water content. The irrigation significantly increased leaf net assimilation, stomatal conductance and needle and tree-ring growth rate, and markedly decreased needle and tree-ring δ13C values and its sensitivity to environmental parameters. The δ13C estimates derived from gas-exchange parameters, and weighted by assimilation, compared closely with seasonal and inter-annual δ13C values of needle- and tree-ring tissue. Higher stomatal conductances of the irrigated trees (0.22 vs. 0.08 mol m−2 s−1 on average) corresponded with ~2.0‰ lower average δ13C values, both measured and derived. Derived and measured δ13C values also indicated that needle growth, which occurs throughout the stressful summer was supported by carbon from concurrent, low rate assimilation. For Aleppo pine under semi-arid and irrigated conditions, the δ13C of tree-ring and needle material proved, in general, to be a reasonable indicator of integrated leaf gas-exchange properties. [ABSTRACT FROM AUTHOR]

Growth, gas exchange and water relations have been studied on hydroponically grown peach (cv. Armking) plants, grafted on GF677 (Arm/GF) and Mr.S.2/5 (Arm/MrS), exposed to 0, 40, 80 and 120 mM NaCl concentration, over a four-week period. Plant performance was also evaluated during a subsequent four-week period of relief from stress, by supplying the plants with a complete nutrient solution. Salinity stress reduced growth and net assimilation rate to a greater extent in Arm/GF than in Arm/MrS plants. Salt-induced water stress was more severe in Arm/GF than Arm/MrS leaves, and particularly during the first two weeks of treatment. On the other hand, leaf osmotic potential at full turgor (ΨπFT) of salt-treated Arm/MrS received a markedly greater contribution from Na+ and Cl- than salt-treated Arm/GF plants. By contrast, divalent cations and K+ made a substantially greater contribution to leaf ΨπFT of salt-treated Arm/GF than to the corresponding ΨπFT of Arm/MrS plants. Salinity stress markedly altered leaf carbohydrate composition and led to a preferential accumulation of sorbitol, independent on the scion/rootstock combination. Salt-treated Arm/GF but not Arm/MrS plants stored most of the Na+ and Cl- loaded in the shoot, into basal (old) leaves. The rootstock ability to control the accumulation of salt in the scion leaves during the salinity stress, determined striking differences in the recovery of photosynthetic performances, during the relief period. A full recovery of A and gs was detected in both 40 and 80 mM salt-treated Arm/GF leaves, but only in 40 mM salt-treated Arm/MrS, at the end of the relief treatment. These data indicate a greater efficiency of GF677 than Mr.S.2/5 in protecting peach plants from the deleterious effects of NaCl stress. [ABSTRACT FROM AUTHOR]