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Changes in calcification of coccolithophores may affect their photosynthetic responses to both, ultraviolet radiation (UVR, 280–400 nm) and temperature. We operated semi-continuous cultures of Emiliania huxleyi (strain CS-369) at reduced (0.1 mM, LCa) and ambient (10 mM, HCa) Ca2+ concentrations and, after 148 generations, we exposed cells to six radiation treatments (>280, >295, >305, >320, >350 and >395 nm by using Schott filters) and two temperatures (20 and 25 °C) to examine photosynthesis and calcification responses. Overall, our study demonstrated that: (1) decreased calcification resulted in a down regulation of photoprotective mechanisms (i.e., as estimated via non-photochemical quenching, NPQ), pigments contents and photosynthetic carbon fixation; (2) calcification (C) and photosynthesis (P) (as well as their ratio) have different responses related to UVR with cells grown under the high Ca2+ concentration being more resistant to UVR than those grown under the low Ca2+ level; (3) elevated temperature increased photosynthesis and calcification of E. huxleyi grown at high Ca2+ concentrations whereas decreased both processes in low Ca2+ grown cells. Therefore, a decrease in calcification rates in E. huxleyi is expected to decrease photosynthesis rates, resulting in a negative feedback that further reduces calcification.

The influence of vertical mixing on phytoplankton sensitivity to UV light has been assessed over an annual cycle. Photosynthesis rates of natural assemblages were compared in samples that were incubated at fixed position in a light gradient and with duplicate samples that simulated vertical mixing by movement in the same gradient with a periodicity of 4 h. This is the typical time-scale of vertical mixing in coastal waters in the English Channel. There were clear seasonal differences in the short-term response of phytoplankton to enhanced UVA+UVB. For most of the year, there was no detectable effect of UV on photosynthetic carbon fixation. But natural assemblages in late winter/early spring, when high UV light may sporadically occur at this latitude, were sensitive to UVA+UVB. In some samples, primary production was 40% of that measured in the absence of UV light. At the time of maximum sensitivity to UV, the phytoplankton assemblage was dominated by diatoms. Simulated vertical mixing resulted in more inhibition of photosynthesis by UVA+UVB light than when samples were at constant light with the same time-integrated irradiance. Transient increases in UVA+UVB due to ozone depletion, such as have been observed over Northern Europe, could have a serious impact on coastal phytoplankton production in late winter / early spring.

The Western Photosynthesis Conference is a regional conference that is held on an annual basis to bring together researchers primarily from the Western United States to share their newest research advances on photosynthetic processes. The 23rd conference was focused on both fundamental and more applied research on the biological conversion of solar energy to various energy storage forms. Several particular areas of solar energy conversion were emphasized in this conference (see below). Some of these topics, such as carbon limitations on photosynthesis, biomimicry and phenotyping, have traditionally not been incorporated extensively in the Western Photosynthesis Conference. We found that these topics have substantially broadened of the scope of this meeting.

Rubisco catalyses the first step in photosynthetic carbon fixation, but it can be easily poisoned by side-products of its activity. Structural and functional analyses of a protein conserved across plants, algae and bacteria shows how one such blockage is both removed and recycled.

Crassulacean acid metabolism (CAM), a key adaptation of photosynthetic carbon fixation to limited water availability, is characterized by nocturnal CO2 fixation and daytime CO2 re-assimilation, which generally results in improved water-use efficiency. However, CAM plants display a remarkable degree of photosynthetic plasticity within a continuum of diel gas exchange patterns. Genotypic, ontogenetic and environmental factors combine to govern the extent to which CAM is expressed. The ecological diversity of CAM is mirrored by plasticity in a range of biochemical and physiological attributes. In C3/CAM-intermediate plants, limited water availability can induce or enhance the expression of CAM. CAM induction is controlled by a combination of transcriptional, post-transcriptional and post-translational regulatory events. Early events in CAM induction point to a requirement for calcium and calcium-dependent protein kinase activities. Gene discovery efforts, improved transformation technologies and genetic models for CAM plants, coupled with detailed physiological investigations, will lead to new insights into the molecular genetic basis of induction processes and the circadian oscillator that governs carbon flux during CAM. Future integration of genomic, biochemical and physiological approaches in selected CAM models promise to provide a detailed view of the complex regulatory dynamics involved in CAM induction and modulation by water deficit. Such information is expected to have broad significance as the ecological and agricultural importance of CAM species increases in the face of global warming trends and the associated expansion of desertification in semi-arid regions around the world.

The control of gaseous exchange between the leaf and bulk atmosphere by stomata governs CO2 uptake for photosynthesis and transpiration, determining plant productivity and water use efficiency. The balance between these two processes depends on stomatal responses to environmental and internal cues and the synchrony of stomatal behavior relative to mesophyll demands for CO2. Here we examine the rapidity of stomatal responses with attention to their relationship to photosynthetic CO2 uptake and the consequences for water use. We discuss the influence of anatomical characteristics on the velocity of changes in stomatal conductance and explore the potential for manipulating the physical as well as physiological characteristics of stomatal guard cells in order to accelerate stomatal movements in synchrony with mesophyll CO2 demand and to improve water use efficiency without substantial cost to photosynthetic carbon fixation. We conclude that manipulating guard cell transport and metabolism is just as, if not more likely to yield useful benefits as manipulations of their physical and anatomical characteristics. Achieving these benefits should be greatly facilitated by quantitative systems analysis that connects directly the molecular properties of the guard cells to their function in the field.

▪ Abstract Guard cells surround stomatal pores in the epidermis of plant leaves and stems. Stomatal pore opening is essential for CO2influx into leaves for photosynthetic carbon fixation. In exchange, plants lose over 95% of their water via transpiration to the atmosphere. Signal transduction mechanisms in guard cells integrate hormonal stimuli, light signals, water status, CO2, temperature, and other environmental conditions to modulate stomatal apertures for regulation of gas exchange and plant survival under diverse conditions. Stomatal guard cells have become a highly developed model system for characterizing early signal transduction mechanisms in plants and for elucidating how individual signaling mechanisms can interact within a network in a single cell. In this review we focus on recent advances in understanding signal transduction mechanisms in guard cells.

Microscopic stomatal pores in the epidermes of aerial plant organs allow the loss of water vapor to the atmosphere in a process known as transpiration and the entry of CO2 into the plant for photosynthetic carbon fixation. Stomatal apertures are rapidly and reversibly regulated by pairs of guard

To test the possibility of inorganic carbon limitation of the marine unicellular alga Emiliania huxleyi (Lohmann) Hay and Mohler, its carbon acquisition was measured as a function of the different chemical species of inorganic carbon present in the medium. Because these different species are interdependent and covary in any experiment in which the speciation is changed, a set of experiments was performed to produce a multidimensional carbon uptake scheme for photosynthesis and calcification. This scheme shows that CO2 that is used for photosynthesis comes from two sources. The CO2 in seawater supports a modest rate of photosynthesis. The HCO3- is the major substrate for photosynthesis by intracellular production of CO2 (HCO3- + H+ → CO2 + H2O → CH2O + O2). This use of HCO3- is possible because of the simultaneous calcification using a second HCO3- , which provides the required proton (HCO3- + Ca2+ → CaCO3 + H+). The HCO3- is the only substrate for calcification. By distinguishing the two sources of CO2 used in photosynthesis, it was shown that E. huxleyi has a K½ for external CO2 of ‘‘only’’ 1.9 ± 0.5 µM (and a Vmax of 2.4 ± 0.1 pmol·cell-1·d-1). Thus, in seawater that is in equilibrium with the atmosphere ([CO2] = 14 µM, [HCO3-] = 1920 µM, at fCO2 = 360 µatm, pH = 8, T = 15° C), photosynthesis is 90% saturated with external CO2. Under the same conditions, the rate of photosynthesis is doubled by the calcification route of CO2 supply (from 2.1 to 4.5 pmol·cell-1·d-1). However, photosynthesis is not fully saturated, as calcification has a K½ for HCO3- of 3256 ± 1402 µM and a Vmax of 6.4 ± 1.8 pmol·cell-1·d-1. The H+ that is produced during calcification is used with an efficiency of 0.97 ± 0.08, leading to the conclusion that it is used intracellularly. A maximum efficiency of 0.88 can be expected, as NO3- uptake generates a H+ sink (OH- source) for the cell. The success of E. huxleyi as a coccolithophorid may be related to the efficient coupling between H+ generation in calcification and CO2 fixation in photosynthesis.

Among the numerous species of the family Euphorbiaceae the photosynthetic carbon fixation follows three pathways: C3, C4, CAM. The object of the present research was Euphorbia peplis L., grown and collected on a Mediterranean coast. Leaf anatomy and ultrastructure together with carbon isotope discrimination measurements indicated that E. peplis is ascribable to the NADP-malic enzyme C4 photosynthetic type.

Plants, restricted by their environment, need to integrate a wide variety of stimuli with their metabolic activity, growth and development. Sugars, generated by photosynthetic carbon fixation, are central in coordinating metabolic fluxes in response to the changing environment and in providing cells and tissues with the necessary energy for continued growth and survival. A complex network of metabolic and hormone signaling pathways are intimately linked to diverse sugar responses. A combination of genetic, cellular and systems analyses have uncovered nuclear HXK1 (hexokinase1) as a pivotal and conserved glucose sensor, directly mediating transcription regulation, while the KIN10/11 energy sensor protein kinases function as master regulators of transcription networks under sugar and energy deprivation conditions. The involvement of disaccharide signals in the regulation of specific cellular processes and the potential role of cell surface receptors in mediating sugar signals add to the complexity. This chapter gives an overview of our current insight in the sugar sensing and signaling network and describes some of the molecular mechanisms involved.

CONTENTS INTRODUCTION . . . . . . . . . . . . . . . . . . . . . . ... . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .... . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 578 AN APPROACH TO STUDy . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . 579 PHYSIOLOGICAL EVIDENCE OF INTERACTION . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . SSO CARBON REQUIREMENTS FOR NITROGEN ASSIMILATION .. . . . . . . . . . . . . . . . . . .. . ... . . . . . . . . . . . . . . 581 Photosynthetic Carbon Fixation.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 583 Anapleurotic Carbon Fixation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . 585 The Source ofZ·Oxoglutaratefor Glutamate Synthesis ......... .... . ........ ... . . . . . 585 REGULATION OF CARBON FLOW TO THE TCA CYCLE . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 586 Phosphoenolpyruvate Carboxylase ......... . ... ......... . ......... ... ........ 586 Pyruvate Kinase . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 586 Pyruvate Dehydrogenase Complex . . . . . . . . . . . . . . . . . . . ..... . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ... . . . . . . .. . . . . . . 587 Phosphofructokinase and Pyrophosphate Fructose-6-phosphate I-Phosphotransferase . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ... . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . 587 Fructose-l,6-bisphosphatase . . . . . . . . . . ... . . . . . . . . . . . . . .... . . . . . . . . . . . .. . ... . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . 588 Sucrose Phosphate Synthetase . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ... . . . . . . . . . . . . . ... . . . . . . . . . . . . . . . 589 Starch Metabolism . . . . . . . . . . . . . . . . . . . . . . . . . . ... . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ... . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ... . . . . . . . . . . . . . . . 589 A MODEL FOR ACTIVATION OF RESPIRATORY CARBON FLOW TO NITROGEN ASSIMILATION ....... . . . . . .. . .. . ... . 590 ENERGY CONSIDERATIONS . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 591 The Enzymes of NitraTe Reduction ... . . . . . 591 Cytosolic Reductant Supply....... . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . . . 592 Energy Supply in the Plastid . . . . . . ........ ....... ........ ....... 593 ATP Supply ......... ...... ......... 595 REGULATION IN DIFFERENT SYSTEMS . . . . . . . . . . . . . . . . . . . . . . . . . . ... . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 596

[1] During this decade, the Amazon region has suffered two severe droughts in the short span of five years – 2005 and 2010. Studies on the 2005 drought present a complex, and sometimes contradictory, picture of how these forests have responded to the drought. Now, on the heels of the 2005 drought, comes an even stronger drought in 2010, as indicated by record low river levels in the 109 years of bookkeeping. How has the vegetation in this region responded to this record-breaking drought? Here we report widespread, severe and persistent declines in vegetation greenness, a proxy for photosynthetic carbon fixation, in the Amazon region during the 2010 drought based on analysis of satellite measurements. The 2010 drought, as measured by rainfall deficit, affected an area 1.65 times larger than the 2005 drought – nearly 5 million km2 of vegetated area in Amazonia. The decline in greenness during the 2010 drought spanned an area that was four times greater (2.4 million km2) and more severe than in 2005. Notably, 51% of all drought-stricken forests showed greenness declines in 2010 (1.68 million km2) compared to only 14% in 2005 (0.32 million km2). These declines in 2010 persisted following the end of the dry season drought and return of rainfall to normal levels, unlike in 2005. Overall, the widespread loss of photosynthetic capacity of Amazonian vegetation due to the 2010 drought may represent a significant perturbation to the global carbon cycle.

The study of regulation has previously involved indirect, and largely correlative, approaches. In the following contribution I illustrate the uses and limitations of these traditional approaches, and then discuss how molecular genetics provide a new tool to test directly ideas about regulation in vivo in the intact organism.

We carried out experiments during an expedition (14 August to 14 September, 2007) that covered up to 250,000 km(2) to investigate the effects of solar UV radiation (UVR, 280-400 nm) on the photosynthetic carbon fixation of tropical phytoplankton assemblages in surface seawater of the South China Sea. From coastal to pelagic surface seawaters, UV-B (280-315 nm) caused similar inhibition, while UV-A (315-400 nm) induced photosynthetic inhibition increased from coastal to offshore waters. UV-B resulted in an inhibition by up to 27% and UV-A by up to 29%. Under reduced levels of solar radiation with heavy overcast, UV-A resulted in enhanced photosynthetic carbon fixation by up to 25% in coastal waters where microplankton was abundant. However, such a positive impact was not observed in the offshore waters where piconanoplankton was more abundant. The daily integrated inhibition of UV-A reached 4.3% and 13.2%, and that of UV-B reached 16.5% and 13.5%, in the coastal and offshore waters, respectively.

Phytoplankton cells in estuary waters usually experience drastic changes in chemical and physical environments due to mixing of fresh and seawaters. In order to see their photosynthetic performance in such dynamic waters, we measured the photosynthetic carbon fixation by natural phytoplankton assemblages in the Jiulong River estuary of the South China Sea during April 24-26 and July 24-26 of 2008, and investigated its relationship with environmental changes in the presence or the absence of UV radiation. Phytoplankton biomass (Chl a) decreased sharply from the river-mouth to seawards (17.3-2.1 μg L(-1)), with the dominant species changed from chlorophytes to diatoms. The photosynthetic rate based on Chl a at noon time under PAR-alone increased from 1.9 μg C (μg Chl a)(-1) L(-1) in low salinity zone (SSS10) to 12.4 μg C (μg Chl a)(-1) L(-1) in turbidity front (SSS within 10-20), and then decreased to 2.1 μg C (μg Chl a)(-1) L(-1) in mixohaline zone (SSS20); accordingly, the carbon fixation per volume of seawater increased from 12.8 to 149 μg C L(-1) h(-1), and decreased to 14.3 μg C L(-1) h(-1). Solar UVR caused the inhibition of carbon fixation in surface water of all the investigated zones, by 39% in turbidity area and 7-10% in freshwater or mixohaline zones. In the turbidity zone, higher availability of CO2 could have enhanced the photosynthetic performance; while osmotic stress might be responsible for the higher sensitivity of phytoplankton assemblages to solar UV radiation.

Cyanobacteria are remarkable for their ability to flourish in environments with widely fluctuating chemical and physical parameters, such as nutrient availability, light quality and intensity, temperature and osmotic conditions. This chapter focuses on the responses of cyanobacteria to changes in light intensity, light quality and macronutrient availability. Suboptimal light and nutrient conditions result in a number of specific and general responses that strongly influence the physiology of the cell. Some of the changes can be striking, while others are subtle; they can occur in minutes or take place over several days. In recent years knowledge of the molecular responses of cyanobacteria to stress conditions has increased considerably. Capacity to acclimate to suboptimal light levels for photosynthetic carbon fixation is unique among the bacteria. In contrast, an understanding of acclimation to nutrient deprivation has gained much from the information available from analyses of enteric bacteria, although cyanobacteria have a number of interesting deviations in their responses. Perhaps most provocative in studies of acclimation processes in cyanobacteria is the overlap among the responses elicited by different environmental stimuli, which suggests the existence of a hierarchy of responses and the involvement of global regulatory circuits.

Limitations of carbon fixation within spinach leaves due to light and CO2 were investigated. Under equivalent photon fluxes, carbon fixation was higher when leaves were irradiated adaxially compared to abaxially. Maximal carbon fixation occurred in the middle of the palisade mesophyll under adaxial illumination, whereas, maximal carbon fixation occurred in the spongy mesophyll under abaxial illumination. Total carbon fixation and the pattern of carbon fixation across leaves were similar, when leaves were irradiated with 800 micromol quanta m(-2) s(-1) either adaxially alone or adaxially plus abaxially (1,600 micromol quanta m(-2) s(-1)). In contrast, when both leaf surfaces were irradiated simultaneously with 200 micromol quanta m(-2) s(-1), total carbon fixation increased and carbon fixation in the middle of the leaf was higher compared to leaves irradiated unilaterally with the low light. Feeding 14CO2 through either the adaxial or abaxial leaf surface did not change the pattern of carbon fixation across the leaf. Increasing 14CO2 pulse-feeding times from 5 s to 60 s allowed more 14CO2 to be fixed but did not change the pattern of 14CO2 fixation across the leaf. We concluded that in spinach, the distribution of both light and Rubisco activity within leaves has significant effects on the patterns of carbon fixation across leaves; whereas CO2 diffusion does not appear to affect the carbon fixation pattern within spinach leaves.

Stomatal opening in wilted leaves at low temperatures has previously been reported for bean and soybean (1, 2). Photosynthetic net CO2 uptake by leaves of plants subjected to a mild drought decreases mainly because of the stomatal closure which occurs under these conditions (3). The resulting decrease in intercellular CO2 partial pressure favors photorespiration relative to photosynthetic carbon fixation and is expected to shift the optimal temperature for net photosynthesis downwards. This effect is further enhanced by the changes in the O2/CO2 solubility ratio with temperature. Here, we show results on pea leaves. We show some evidence that the temperature response of stomata can occur independently of photosynthetic fixation and reduction of carbon dioxide and root ABA signals.

The goals of our DOE OMP project are to (1) understand regulation of ribulose bisphosphate carboxylase (RubisCO) in phytoplankton cultures in response to light regime; (2) determine regulation of RubisCO in response to light during nutrient limitation in these cultures; (3) to determine mechanisms of RubisCO regulation in natural populations of phytoplankton on the ocean margins in the Gulf of Mexico and (4) to measure regulation of RubisCO in phytoplankton of the Hatteras System. Two goals are laboratory-based, and two are ship-based.

Marine diatoms are major contributors to coastal and oceanic primary productivity. Assimilation of nitrate, the predominant limiting nutrient, is usually tightly coupled to photosynthetic carbon fixation; however, during darkness or periods of rapid nitrate assimilation, stored carbon reserves are respired for the energetic requirements of the cell and for protein synthesis (Turpin and Weger 1990).

This article analyses the evolution of a major photorespiratory enzyme, glycolate oxidase (GOX), finding that plant GOX was phylogenetically derived from cyanobacterial lactate oxidase. Cyanobacterial GOX-like proteins are today found only in N2-fixing strains, in which they play an important role in the protection of nitrogenase.

The author gives an account of his life and work in scientific research. At the prompting of Govindjee, this is a quite personal account and, I hope, not too serious. The circumstances surrounding the discovery of C4 photosynthesis are mentioned together with some aspects of the subsequent development of this field. Since it seems to be expected on such occasions, there is also some reminiscences and some unsolicited advice. I hope there are no gross or libelous inaccuracies.

Guard cells are located in the leaf epidermis and pairs of guard cells surround and form stomatal pores, which regulate CO(2) influx from the atmosphere into leaves for photosynthetic carbon fixation. Stomatal guard cells also regulate water loss of plants via transpiration to the atmosphere. Signal transduction mechanisms in guard cells integrate a multitude of different stimuli to modulate stomatal apertures. Stomata open in response to light. Stomata close in response to drought stress, elevated CO(2), ozone and low humidity. In response to drought, plants synthesize the hormone abscisic acid (ABA) that triggers closing of stomatal pores. Guard cells have become a highly developed model system for dissecting signal transduction mechanisms in plants and for elucidating how individual signaling mechanisms can interact within a network in a single cell. Many new findings have been made in the last few years. This chapter is an update of an electronic interactive chapter in the previous edition of The Arabidopsis Book (Mäser et al. 2003). Here we focus on mechanisms for which genes and mutations have been characterized, including signaling components for which there is substantial signaling, biochemical and genetic evidence. Ion channels have been shown to represent targets of early signal transduction mechanisms and provide functional signaling and quantitative analysis points to determine where and how mutations affect branches within the guard cell signaling network. Although a substantial number of genes and proteins that function in guard cell signaling have been identified in recent years, there are many more left to be identified and the protein-protein interactions within this network will be an important subject of future research. A fully interactive clickable electronic version of this publication can be accessed at the following web site: http://www-biology.ucsd.edu/labs/schroeder/clickablegc2/. The interactive clickable version includes the following features: Figure 1. Model for the roles of ion channels in ABA signaling.Figure 2. Blue light signaling pathways in guard cells.Figure 3. ABA signaling pathways in guard cells.Figure 1 is linked to explanations that appear upon mouse-over. Figure 2 and Figure 3 are clickable and linked to info boxes, which in turn are linked to TAIR, to relevant abstracts in PubMed, and to updated background explanations from Schroeder et al (2001), used with permission of Annual Reviews of Plant Biology.

Modelling the diurnal course of photosynthesis in oak leaves (Quercus robur L.) requires appropriate description of the dynamics of leaf photosynthesis of which diurnal variations in leaf conductance and in CO2 assimilation are essential components. We propose and analyse a simple photosynthesis model with three variables: leaf conductance (gs), the CO2 partial pressure inside the leaf (pi), and a pool of Calvin cycle intermediates (aps). The environmental factors light (I) and vapour pressure deficit (VPD) are used to formulate a target function G(I, VPD) from which the actual leaf conductance is calculated. Using this gs value and a CO2 consumption term representing CO2 fixation, a differential equation for pi is derived. Carboxylation corresponds to the sink term of the pi pool and is assumed to be feedback-inhibited by aps. This simple model is shown to produce reasonable to excellent fits to data on the diurnal time courses of photosythesis, pi and gs sampled for oak leaves.

Summary Calcium is a strong inhibitor of phosphoenolpyruvate carboxylase at concentrations low enough (0.5-1.0 mM) to be of physiological significance. The inhibition is competitive in respect to magnesium and affects both C 3 - and C 4 -phosphoenolpyruvate carboxylases. The results are consistent with the hypothesis that low soluble calcium is a prerequisite for C 4 -photosynthetic carbon fixation.

Germanic acid (/sup 68/Ge) was used as a tracer to study the uptake and incorporation of silicic acid by natural phytoplankton populations in the Gulf of California. The technique was useful in measuring silicic acid incorporation rates as low as 0.5 nM h/sup -1/. The incorporation followed saturation kinetics. K/sub s/ values at two stations were 1.59 and 2.53 ..mu..M. The incorporation was light dependent, maximum rates being achieved at 0.28 x 10/sup 21/ photons cm/sup -2/ d/sup -1/ (0.044 ly min/sup -1/; 9 percent of surface irradiance) and half-maximum rate at 0.02 x 10/sup 21/ photons cm/sup -2/ d/sup -1/ (0.003 ly min/sup -1/; 0.6 percent of surface irradiance); rate of incorporation in the dark was 44 percent of that at light saturation. Enrichment with silicic acid caused stimulation of photosynthetic carbon fixation at two stations.

Photosynthetic fixation of 14CO2 in the bean Phaseolus vulgaris, cv. Pencil Pod Black Wax, resulted in the appearance of labeled compounds that were characterized as phosphoserine and phosphohydroxypyruvate by chromatographic separation and by the synthesis of chemical derivatives. In 14CO2/12CO2 pulse-chase experiments these metabolites demonstrated the rapid pool saturation and depletion of 14C characteristic of early intermediates in photosynthetic carbon fixation. They were present in sufficient amounts to account for about 35% of total carbon fixed in 1 minute.

Most studies of limiting factors to algal growth seek to quantify the effect of individual factors. However, rates and amounts of photosynthetic carbon fixation in laboratory cultures of Anacystis nidulans show simultaneous dependence on the availability of phosphorus, carbon dioxide, and light. Thus, not all limiting factors may act independently to control algal growth in aquatic systems, and further research into the extent of such interactions is needed.

The photosynthetic carbon fixation pathways of four seagrass species, Zostera marina L. from Alaska and Thalassia testudinum Banks ex Konig, Syringodium filiforme Kutz. and Halodule wrightii Aschers. from the Gulf of Mexico, were investigated with a 14 C pulse-chase technique. All species were found to be principally of the C 3 type. However, Thalassia and Halodule had higher initial incorporation rates into organic acids than is typical for terrestrial C 3 plants. Of 11 seagrass species investigated thus far for C 3 or C 4 metabolism using this technique, 10 were found to be principally of the C 3 type while only one exhibited C 4 metabolism.

Dunaliella tertiolecta (Butcher) was grown in chemostat culture over a wide range of ammonium limited growth rates. The addition of ammonium caused a rapid temporary suppression of photosynthetic carbon fixation. The magnitude of ammonium induced photosynthetic suppression increased with the severity of ammonium limitation. Cells growing at rates greater than ca. 80%μmax exhibited no photosynthetic suppression in response to additions of the limiting nutrient. The duration of photosynthetic suppression was related to the concentration of added ammonium. Immediately following the suppression, photosynthesis was enhanced with respect to the controls. The eventual degree of enhancement increased with the concentration of added ammonium. Steady-state cellular chlorophyll concentrations, photosynthetic rates, and assimilation numbers are reported.

Precambrian reduced carbon is more depleted in13C than what would be expected from the carbon isotopic composition of modern marine algae and algal mats. Since the photosynthetic carbon fixation by algae is the most likely source of the reduced carbon, the depletion has been considered an anomaly.

The hypothesis that active photosynthesis results in a more severe cellular response to ozone (O3) was tested. Using suspensions of isolated soybean (Glycine max (L.) Merr. cv. Chippewa 64) mesophyll cells, we measured photosynthetic carbon fixation rates and cell viability, as determined by fluorescein diacetate staining, before and after treatment with O3 in the light and dark in the presence or absence of NaHCO3. Ozone reduced the photosynthesis rates of isolated mesophyll cells to a greater degree than cell viability, suggesting greater sensitivity of photosynthesis. Posttreatment photosynthesis rates were higher in cells that were fumigated in media containing NaHCO3 than in cells fumigated in its absence. The only interaction detected was between the gaseous treatments and light, in which relative cell viability was reduced more by O3 in the light than in the dark, in the second experiment. The interaction was, at least in part, a reflection of experimental error. Of greater significance was the observation that the photosynthetic function could be affected equally by O3 in the dark and in the light.

Summary Audouinella violacea , a fresh water red alga usually growing epiphytically on Lemanea fluviatilis , accumulates Mannitol as reserve carbohydrate. During photosynthetic carbon fixation serine is rapidly synthetized, comparable to results obtained with other red algae. Monogalactosyl diglyceride, digalactosyl diglyceride, sulfoquinovosyl diglyceride and steryl glycoside were demonstrated to be present in the lipid fraction.

Kinetic studies of photosynthetic O2 evolution as a function of pH were conducted to investigate the nature of the inorganic carbon used during photosynthesis by Anacystis nidulans. At pH 5, the apparent affinity for carbon during photosynthesis was similar in air-grown and high CO2 grown cells, but at alkaline pH, the apparent affinity was much greater in air-grown cells. The substrate concentration for half-maximum rates of photosynthesis in air-grown cells remained constant as a function of pH when the substrate was expressed as total carbon, suggesting that these cells were capable of using varying proportions of CO2 and [Formula: see text]. Photosynthesis in high CO2 grown algae appeared to be more dependent on CO2 over the pH range, indicating that CO2 was the predominant carbon species used, but [Formula: see text] uptake was also indicated. Internal inorganic carbon and photosynthetic carbon fixation in air-grown cells were determined at pH 8.5, using silicone oil centrifugation. Anacystis accumulated inorganic carbon in large excess of that in the external medium by a mechanism which is sensitive to inhibitors of energy metabolism and independent of concurrent carbon fixation; light was required to accumulate and maintain the internal carbon pool. The degree of accumulation was a function of the carbon concentration in the external medium; at 12 μM external carbon, the accumulation ratio was in excess of 100-fold, whereas at 4.76 mM, the ratio was only 5-fold. The rates of carbon transport were always sufficient to maintain photosynthesis. Carbon efflux rates approaching 40% of the influx rate were found at equilibrium internal carbon concentrations. Kinetic parameters of photosynthesis are discussed with reference to the known properties of algal ribulose bisphosphate (RuBP) carboxylase–oxygenase. It is concluded that the internal inorganic carbon pool serves as an intermediate for photosynthetic carbon fixation and that, if CO2 and [Formula: see text] are in equilibrium, the carbon accumulation at ambient CO2 and O2 is sufficient to suppress RuBP oxygenase activity.

Organelles morphologically similar to microbodies have been found in several tissues of atmospheric species of Tillandsia from different habitats. The presence of catalase was demonstrated by the DAB reaction thus confirming the microbody nature of these organelles. They are a feature of the Tillandsia species with normal photosynthetic carbon fixation and with CAM. Their size is consistently small. The nucleoid observed in the microbodies shows a characteristic morphology which has not been reported before within other plant microbodies. This nucleoid is composed of minute tubular structures, for which the authors here propose a three-dimensional arrangement.

Excised apical portions of green wheat leaf sections were treated with aminotriazole to prevent formation of new chloroplasts. Illumination retarded the decline in chlorophyll content per leaf section, the disintegration of chloroplast ultrastructure, and the loss of capacity for photosynthetic carbon fixation. We interpret these 3 effects of illumination as facets of a single light effect in retarding chloroplast senescence. This light effect in retarding chloroplast senescence has features differing from characteristics of photosynthetic carbon fixation. For example, A) application of the photosynthetic inhibitor 3-(3,4-dichlorophenyl)-1, 1-dimethylurea did not decrease, and may have even slightly increased, the effectiveness of light; B) although the action spectrum contains peaks in the blue and red regions, it differs from the action spectrum for photosynthetic CO(2) assimilation in wheat; C) in nonphotosynthesizing tissue, application of sugars did not retard chloroplast senescence; D) light saturation was achieved by only a few hundred microwatts/cm(2). Considered together with the well-known light requirement for chloroplast formation, our results indicate that light has a dual, photomorphogenetic control in maintaining the green status of the plant by also exerting a second effect: retarding of senescence of chloroplasts already present.

1. The effect of varying light intensities on the rates of calcification and CO2 fixation in the coralMontastrea annularis is described. 2. High light intensities, above the saturation point of symbiont photosynthesis, are shown to inhibit the rate of calcification in both shallow and deep specimens. 3. Light enhanced calcification in reef-building corals appears to be an energy-requiring process involving the active transport of calcium and carbonate ions, and the translocation of symbiont photosynthate.

We propose that plants possessing the C4-dicarboxylic acid pathway of photosynthetic carbon fixation are generally inferior food sources for herbivores, and are often avoided by them, relative to plants possessing only the C3-Calvin cycle pathway. As initial support of this hypothesis, we present data from the literature, dealing primarily with insects, that suggest that C4 species are ingested in proportions lower than their availability in a number of natural situations, that they tend to be avoided in laboratory preference tests, and that they result in lower survival and fecundity in herbivores feeding on them. We suggest a number of physiological, anatomical, and nutritional differences between C3 and C4 species that may be involved in generating the observed pattern of herbivore preference. The ecological relevance of the differences between C3 and C4 species has been discussed to date only in relation to hypothetical effects on interspecific competition. If our hypothesis is true, selective herbivo...

CONTENTS INTRODUCTION . . . . • . . . . . • • . • . . . . . . • . . . . . . . . . . . . . • . . . . . . . . . . . . . . . . . . . . . . • , 253 TOTIPOTENCY AND PLANT BIOLOGy . . • . . . . . . • . . . . . . . . . . . . . . , . . . . . . . • . . . . . . . . . 255 PHOTORESPIRATION. . . . . • . . . • . . . . • • . • . • . . . . . . . . . . . . . . . . . . . . . . . . . • . . . . . . . . . 255 PRIMARY ENZYMES OF C02 METABOLISM . . . . . . . . . . . . . • . . . . . . . . . 260 Carboxylases. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 260 Ribulose diphosphate carboxylase.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 260 Phosphoenolpyruvate carboxylase ... . . . , . . . . . . . . . . . . . . . . . . . . . . . . . 262 Phosphoenolpyruvate carboxykinase. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 263 Transcarboxylase. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 264 Decarboxylases. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. 264 Carbonic Anhydrase. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 265 THE PENTOSE CYCLE. • . . . . • . . . • . . . . . . . . . . • . . . . . . . . . . . . . . . . . . • . . . . . . . . . . . . . 266 THE C. CYCLE. . • • • . . . . . . . . . . . • . . . . . . . . . . . . . . . • . . . . . . . . . . . . . • . . . . . . . . . . . . . 266 CO2 Assimilation Studies wih Isolated C, Plant Chloroplasts. . . . . . . . . . . 274 Variation in C, Photosynthesis . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . " 275 ATP and NADPH Production in C, Plants..... . . . . . . . . . . . . . . . . . . . . . . 276 CRASSULACEAN ACID METABOLISM . . • . . . . . . . . . " . . . . . . . . . . . . . . . . . . . . . . . . . . . . , 278 CONCLUSIONS ON NET C02 UPTAKE • . . . . . . . . . . . . • . . . . , . , . . . . . . . . . . . . . . . . . . . . . 281 OTHER GROUPS OF HIGHER PLANTS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . , 281

Estimates of the rate of translocation of carbon, T, from sugar beet leaves were made from simultaneous determinations of CO2 exchange and dry weight change. For an illuminated leaf in a constant environment, T increased to a maximum during the first 2 h and was constant for the next 10 h. On darkening, T decreased rapidly in the first 2 to 4 h to a constant minimal value which could continue for as long as 24 h. The average value of T for nine leaves in the light was 4.2 ± 0.4 mg C dm−2 leaf surface h−1, while in the dark the average value for five leaves over 10 h was 1.2 ± 0.2 mg C dm−2 h−1. Generally T was about three-quarters of the net rate of photosynthetic carbon fixation.

An apparatus is described which allows kinetic studies to be made of photosynthetic carbon fixation using pieces of leaf tissue. Eight pieces of tissue 2 cm × 1 cm can be exposed to 14CO2 under identical conditions for periods down to about 1 sec. 90% of the 14CO2 can be flushed from the chamber in 1 sec, and about 99% in 3 sec.

Seven species of marine microalgae including chlorophytes, diatoms, and a chrysomonad, of tropical, Antarctic, and temperate origin, were used to measure photosynthetic carbon fixation as affected by concentration of dissolved O2 at two levels of CO2. An apparent O2 inhibition of carbon fixation was observed in some species, but others were affected little or not at all by concentration of dissolved O2. In several cases, O2 appeared necessary for maximal carbon fixation.

the C4 pathway of photosynthetic carbon fixation is now known to occur in at least 15 families of flowering plants. Accurate techniques (Smith and Epstein, 1971; Tregunna et al., 1970) have been developed that permit the classification of both living and dried plant material into the C3 or C4 pathway category. Surveys of the taxonomic distribution of the C4 pathway have been published (e.g., Downton, 1975; Smith and Brown, 1973), and C4 species are known or suspected to occur in approximately 150 genera, with over half of these genera in the Gramineae. The C4 pathway has fascinated ecologists because of the apparent high degree of sensitivity of C4 species to both spatial and temporal variations in climate. It is now possible to compare the kinds of climates in which large numbers of C4 species are successful with functional interpretations of the C4 pathway based on laboratory investigations at the cellular and molecular levels. The primary functional difference between all C4 plants and all C3 plants is the ability of C4 plants to have more efficient CO2 fixation at low intercellular CO2 concentrations (Bjorkman, 1976). As a result of this ability, C4 plants are generally observed to have high photosynthetic rates at high light intensities, a high rate of CO2 uptake per rate of transpirational water loss, and the ability to maintain high photosynthetic rates at high leaf temperatures (Bjorkman, 1976).

The water extract of a light Nigerian crude oil had pronounced effects on the growth, metabolism and development of radish (Raphanus sativus) plants. In addition to hydrocarbons (ca. 7 mg l−1) the extract contained phenols (ca. 0.6 mg l−1), carbonyl compounds, and carboxylic acids. Although seed germination was virtually unaffected, the growth of both above- and below-ground parts was significantly inhibited when the plants were watered with the extract rather than with well water. In addition, the onset of flowering was significantly delayed in the extract-treated plants, and the number of plants which ever flowered was reduced. Significantly increased respiration was observed in leaf tissue of the extract-treated plants, but photosynthetic carbon fixation was not significantly affected. The extract appeared to elicit effects similar to those caused by other forms of environmental stress, such as infections.