柑橘属光合作用的环境调节

光合机构的运转受环境影响很大,与柑橘的生长发育、产量和品质密切相关.结合我们的工作,综合论述了柑橘光合作用环境调节的研究进展.强光和紫外光导致光合作用下降与PSⅡ反应中心失活有关,光呼吸和叶黄素循环对光合机构有保护作用.温度胁迫下,光合作用下降主要是RuBPCase活性下降和PSⅡ反应中心失活引起,品种间存在差异.轻度水分胁迫引起的光合作用下降是气孔限制的结果,而严重水分胁迫导致光合作用的非气孔限制.提高CO₂浓度,能够促进柑橘的光合作用,进而促进柑橘的生长和提高其品质.阐述了N、P、S、Fe等矿质元素调节光合作用的机理及盐胁迫对光合作用的影响,指出了今后柑橘光合作用的研究方向.     

喜光榕树和耐荫榕树光适应机制的差异

100%和36%光强下生长的喜光的斜叶榕的光合能力高于耐荫的假斜叶榕,而热耗散能力与之相似,说明强光下斜叶榕主要通过光合作用利用光能和热耗散、假斜叶榕主要通过热耗散防御光破坏.100%光强下生长的两种榕树的日间光抑制程度相似,但叶表光强相同情况下各光强下生长的假斜叶榕的光抑制均比斜叶榕严重.100%光强下假斜叶榕叶片悬挂角大于斜叶榕,导致日间叶表光强低于斜叶榕,这可能是两种榕树日间光抑制程度相似的原因,表明叶片悬挂角的适应变化对假斜叶榕有重要的意义.     

Drought-inhibition of photosynthesis in C3 plants: stomatal and non-stomatal limitations revisited

There is a long-standing controversy as to whether drought limits photosynthetic CO2 assimilation through stomatal closure or by metabolic impairment in C3 plants. Comparing results from different studies is difficult due to interspecific differences in the response of photosynthesis to leaf water potential and/or relative water content (RWC), the most commonly used parameters to assess the severity of drought. Therefore, we have used stomatal conductance (g) as a basis for comparison of metabolic processes in different studies. The logic is that, as there is a strong link between g and photosynthesis (perhaps co-regulation between them), so different relationships between RWC or water potential and photosynthetic rate and changes in metabolism in different species and studies may be 'normalized' by relating them to g. Re-analysing data from the literature using light-saturated g as a parameter indicative of water deficits in plants shows that there is good correspondence between the onset of drought-induced inhibition of different photosynthetic sub-processes and g. Contents of ribulose bisphosphate (RuBP) and adenosine triphosphate (ATP) decrease early in drought development, at still relatively high g (higher than 150 mmol H20 m(-2) s(-1)). This suggests that RuBP regeneration and ATP synthesis are impaired. Decreased photochemistry and Rubisco activity typically occur at lower g (<100 mmol H20 m(-2) s(-1)), whereas permanent photoinhibition is only occasional, occurring at very low g (<50 mmol H20 m(-2) s(-1)). Sub-stomatal CO2 concentration decreases as g becomes smaller, but increases again at small g. The analysis suggests that stomatal closure is the earliest response to drought and the dominant limitation to photosynthesis at mild to moderate drought. However, in parallel, progressive down-regulation or inhibition of metabolic processes leads to decreased RuBP content, which becomes the dominant limitation at severe drought, and thereby inhibits photosynthetic CO2 assimilation.     

砂仁光合作用的CO2扩散限制与气孔限制分析

目前常用从气体交换参数计算的胞间CO₂浓度(C_i)来计算气孔限制值(L_s),但由于胁迫情况下计算的C_i偏高常导致结果不准确。该文引入扩散限制分析概念,以砂仁为例介绍了一种不需 C_i的计算扩散限制值(L_d)的新方法。同时通过叶绿素荧光参数间接估算受干旱胁迫植株的C_i(用C_i'表示)计算气孔限制值(L_s')。采用这3种方法分析了生长在100%和40%土壤相对湿度(RSM)下的砂仁(Amomum villosum)净光合速率的限制因素。结果表明两种水分状况下砂仁午后净光合速率的限制因素不同。100%RSM下,午后砂仁L_s没有升高,说明光合作用气孔限制并未增强;午后其L_d升高表明光合作用的CO₂扩散限制增强,这主要是由叶肉阻力相对增大所致。40%RSM下,午后砂仁L_s'升高比L_d升高明显,说明气孔阻力在所有扩散阻力中占主导作用,是限制净光合速率的主要原因;而其 L_s午后并未升高,暗示传统的气孔限制分析会得出非气孔限制的错误结论。C_i'低于C_i,说明干旱胁迫时传统的气体交换方法高估了C_i。上述结果都证明水分胁迫情况下传统方法不可靠,该文介绍的两种新方法比较准确可靠,同时使用两种新方法还可定性推测叶肉阻力的变化方向。     

不同干旱胁迫下欧李光合及叶绿素荧光参数的响应

采用人为控制土壤含水量的方法对欧李进行轻度和重度干旱的处理,测定叶片的气体交换和叶绿素荧光参数的日变化。结果表明,干旱胁迫下欧李叶片净光合速率、蒸腾速率、水分利用效率、气孔导度、PSII最大光化学效率、光化学量子效率显著下降,但胞间CO2浓度、非光化学猝灭系数以及叶黄素循脱环氧化状态（Z＋0.5A）/（V＋A＋Z）和Z含量升高。两干旱处理植株的影响程度存在差异。这表明在长时间干旱条件下,欧李叶片光合作用的降低受到气孔与非气孔因素的双重影响,叶黄素循环的启动增加了胁迫条件下的热耗散能力以保护光合机构免受干旱胁迫的进一步伤害。     

Increased stomatal conductance induces rapid changes to photosynthetic rate in response to naturally fluctuating light conditions in rice

A close correlation between stomatal conductance and the steady-state photosynthetic rate has been observed for diverse plant species under various environmental conditions. However, it remains unclear whether stomatal conductance is a major limiting factor for the photosynthetic rate under naturally fluctuating light conditions. We analysed a SLAC1 knockout rice line to examine the role of stomatal conductance in photosynthetic responses to fluctuating light. SLAC1 encodes a stomatal anion channel that regulates stomatal closure. Long exposures to weak light before treatments with strong light increased the photosynthetic induction time required for plants to reach a steady-state photosynthetic rate and also induced stomatal limitation of photosynthesis by restricting the diffusion of CO₂ into leaves. The slac1 mutant exhibited a significantly higher rate of stomatal opening after an increase in irradiance than wild-type plants, leading to a higher rate of photosynthetic induction. Under natural conditions, in which irradiance levels are highly variable, the stomata of the slac1 mutant remained open to ensure efficient photosynthetic reaction. These observations reveal that stomatal conductance is important for regulating photosynthesis in rice plants in the natural environment with fluctuating light.     

Light Energy Dissipation under Water Stress Conditions: Contribution of Reassimilation and Evidence for Additional Processes

Using ¹⁴CO₂ gas exchange and metabolite analyses, stomatal as well as total internal CO₂ uptake and evolution were estimated. Pulse modulated fluorescence was measured during induction and steady state of photosynthesis. Leaf water potential of Digitalis lanata EHRH. plants decreased to −2.5 megapascals after withholding irrigation. By osmotic adjustment, leaves remained turgid and fully exposed to irradiance even at severe water stress. Due to the stress-induced reduction of stomatal conductance, the stomatal CO₂ exchange was drastically reduced, whereas the total CO₂ uptake and evolution were less affected. Stomatal closure induced an increase in the reassimilation of internally evolved CO₂. This `CO<sub>2</sub> recycling' consumes a significant amount of light energy in the form of ATP and reducing equivalents. As a consequence, the metabolic demand for light energy is only reduced by about 40%, whereas net photosynthesis is diminished by about 70% under severe stress conditions. By CO<sub>2</sub> recycling, carbon flux, enzymatic substrate turnover and consumption of light energy were maintained at high levels, which enabled the plant to recover rapidly after rewatering. In stressed D. lanata plants a variable fluorescence quenching mechanism, termed `coefficient of actinic light quenching,' was observed. Besides water conservation, light energy dissipation is essential and involves regulated metabolic variations.     

Leaf respiration of snow gum in the light and dark. Interactions between temperature and irradiance

We investigated the effect of temperature and irradiance on leaf respiration (R, non-photorespiratory mitochondrial CO(2) release) of snow gum (Eucalyptus pauciflora Sieb. ex Spreng). Seedlings were hydroponically grown under constant 20 degrees C, controlled-environment conditions. Measurements of R (using the Laisk method) and photosynthesis (at 37 Pa CO(2)) were made at several irradiances (0-2,000 micromol photons m(-2) s(-1)) and temperatures (6 degrees C-30 degrees C). At 15 degrees C to 30 degrees C, substantial inhibition of R occurred at 12 micromol photons m(-2) s(-1), with maximum inhibition occurring at 100 to 200 micromol photons m(-2) s(-1). Higher irradiance had little additional effect on R at these moderate temperatures. The irradiance necessary to maximally inhibit R at 6 degrees C to 10 degrees C was lower than that at 15 degrees C to 30 degrees C. Moreover, although R was inhibited by low irradiance at 6 degrees C to 10 degrees C, it recovered with progressive increases in irradiance. The temperature sensitivity of R was greater in darkness than under bright light. At 30 degrees C and high irradiance, light-inhibited rates of R represented 2% of gross CO(2) uptake (v(c)), whereas photorespiratory CO(2) release was approximately 20% of v(c). If light had not inhibited leaf respiration at 30 degrees C and high irradiance, R would have represented 11% of v(c). Variations in light inhibition of R can therefore have a substantial impact on the proportion of photosynthesis that is respired. We conclude that the rate of R in the light is highly variable, being dependent on irradiance and temperature.     

Effects of gamma-aminobutyric acid on the photosynthesis and chlorophyll fluorescence parameters of muskmelon seedlings under hypoxia stress

By the method of hydroponic culture, this paper studied the effects of exogenous gamma-aminobutyric acid (GABA) on the photosynthetic pigment contents, photosynthesis, and chlorophyll fluorescence parameters of muskmelon seedlings under hypoxia stress. Hypoxia stress induced a significant decrease of photosynthetic pigment contents, resulting in the decrease of photosynthesis. Applying GABA could significantly increase the photosynthetic pigment contents, net photosynthetic rate (P(n)), stomatal conductance (G(s)), intercellular CO2 concentration (C(i)), carboxylation efficiency (CE), maximal photochemical efficiency of PS II (F(v)/F(m)), photochemical quenching (q(P)), apparent photosynthetic electron transfer rate (ETR), and quantum yield of PS II electron transport (phi(PS II)), and decrease the stomatal limitation value (L(s)), minimal fluorescence (F(o)), and non-photochemical quenching (NPQ) under both hypoxic and normal conditions. The alleviation effect of GABA on photosynthetic characteristics was more obvious under hypoxia stress. However, simultaneously applying GABA and VGB could significantly decrease the alleviation effect of GABA under hypoxia stress.     

Contrasting effects of N and P deprivation on the regulation of photosynthesis in tomato plants in relation to feedback limitation

The effects were studied of both nitrogen and phosphorus limitation and irradiance on the performance and operation of photosynthesis in tomato leaves (Lycopersicon esculentum Mill.). Plants were grown at low N, high N, low P or high P supply and at two irradiances. Using mature leaves, measurements were made of the irradiance dependencies of the relative quantum efficiencies of photosystems I and II, and of the rate of carbon dioxide fixation. Measurements were also made of foliar starch and chlorophyll concentrations. The results showed that photosynthetic light-harvesting and electron-transport activity acclimate to nutrient stress and growth irradiance such that the internal relationships between electron transport by photosystems I and II do not change; the linear relationship between PhiPSII, and PhiPSI was not affected. It was also evident that under N stress photosynthesis was reduced by a decreased light absorption and by the decreased utilization of assimilates, while P stress mainly affected the carboxylation capacity. Under N stress foliar starch levels increased and the oxygen sensitivity of CO2 fixation decreased, whereas P stress resulted in decreased starch levels and increased oxygen sensitivity of CO2 fixation. The relationship between starch accumulation and oxygen sensitivity (increased starch correlated with decreased oxygen sensitivity) was always the same across the nutrient treatments. These results are consistent with N deprivation producing an increasing limitation of photosynthesis, possibly by feedback from the leaf carbohydrate pool, whereas, although P deprivation produces a decreased rate of CO2 fixation, this is accompanied by a increase in oxygen sensitivity, suggesting that feedback limitation is decreased under P stress.     

The contribution of photosynthesis to the red light response of stomatal conductance

To determine the contribution of photosynthesis on stomatal conductance, we contrasted the stomatal red light response of wild-type tobacco (Nicotiana tabacum 'W38') with that of plants impaired in photosynthesis by antisense reductions in the content of either cytochrome b(6)f complex (anti-b/f plants) or Rubisco (anti-SSU plants). Both transgenic genotypes showed a lowered content of the antisense target proteins in guard cells as well as in the mesophyll. In the anti-b/f plants, CO(2) assimilation rates were proportional to leaf cytochrome b(6)f content, but there was little effect on stomatal conductance and the rate of stomatal opening. To compare the relationship between photosynthesis and stomatal conductance, wild-type plants and anti-SSU plants were grown at 30 and 300 micromol photon m(-2) s(-1) irradiance (low light and medium light [ML], respectively). Growth in ML increased CO(2) assimilation rates and stomatal conductance in both genotypes. Despite the significantly lower CO(2) assimilation rate in the anti-SSU plants, the differences in stomatal conductance between the genotypes were nonsignificant at either growth irradiance. Irrespective of plant genotype, stomatal density in the two leaf surfaces was 2-fold higher in ML-grown plants than in low-light-grown plants and conductance normalized to stomatal density was unaffected by growth irradiance. We conclude that the red light response of stomatal conductance is independent of the concurrent photosynthetic rate of the guard cells or of that of the underlying mesophyll. Furthermore, we suggest that the correlation of photosynthetic capacity and stomatal conductance observed under different light environments is caused by signals largely independent of photosynthesis.     

增强UV-B辐射和NaCl复合胁迫下绿豆光合作用的气孔和非气孔限制

研究了0.35 W/m2的UV-B辐射、0.4%NACl及其复合胁迫下绿豆(Phaseolus radiatus L.)幼苗光合作用的气孔和非气孔限制.发现各胁迫处理下,幼苗净光合速率、气孔导度、光合能力、羧化效率和Rubisco含量均明显降低;细胞间隙CO2浓度在各胁迫处理前期低于对照,后期高于对照;气孔限制值除复合处理第5天外,其余均高于对照;复合处理下上述指标的变化程度均大于两胁迫因子单独处理.表明各胁迫下光合速率的降低既有气孔因素也有非气孔因素,但前期以气孔限制为主,后期以非气孔限制为主;Rubisco含量的降低是各胁迫下光合速率降低的非气孔因素.     

Response of the photosynthetic apparatus of cotton (Gossypium hirsutum) to the onset of drought stress under field conditions studied by gas-exchange analysis and chlorophyll fluorescence imaging.

The functioning of the photosynthetic apparatus of cotton (Gossypium hirsutum) grown during the onset of water limitation was studied by gas-exchange and chlorophyll fluorescence to better understand the adaptation mechanisms of the photosynthetic apparatus to drought conditions. For this, cotton was grown in the field in Central Asia under well-irrigated and moderately drought-stressed conditions. The light and CO(2) responses of photosynthesis (A(G)), stomatal conductance (g(s)) and various chlorophyll fluorescence parameters were determined simultaneously. Furthermore, chlorophyll fluorescence images were taken from leaves to study the spatial pattern of photosystem II (PSII) efficiency and non-photochemical quenching parameters. Under low and moderate light intensity, the onset of drought stress caused an increase in the operating quantum efficiency of PSII photochemistry (varphi(PSII)) which indicated increased photorespiration since photosynthesis was hardly affected by water limitation. The increase in varphi(PSII) was caused by an increase of the efficiency of open PSII reaction centers (F(v)'/F(m)') and by a decrease of the basal non-photochemical quenching (varphi(NO)). Using a chlorophyll fluorescence imaging system a low spatial heterogeneity of varphi(PSII) was revealed under both irrigation treatments. The increased rate of photorespiration in plants during the onset of drought stress can be seen as an acclimation process to avoid an over-excitation of PSII under more severe drought conditions.     

Nonstomatal inhibition of photosynthesis by water stress. Reduction in photosynthesis at high transpiration rate without stomatal closure in field-grown tomato

Large underestimates of the limitation to photosynthesis imposed by stomata can occur because of an error in the standard method of calculating average substomatal pressures of carbon dioxide when heterogeneity of those pressures occurs across a leaf surface. Most gas exchange data supposedly indicating nonstomatal inhibition of photosynthesis by water stress could have this error. However, if no stomatal closure occurs, any reduction in photosynthesis must be due to nonstomatal inhibition of photosynthesis. Net carbon dioxide exchange rates and conductances to water vapor were measured under field conditions in upper canopy leaves of tomato plants during two summers in Beltsville, Maryland, USA. Comparisons were made near midday at high irradiance between leaflets in air with the ambient water vapor content and in air with a higher water content. The higher water content, which lowered the leaf to air water vapor pressure difference (VPD), was imposed either one half hour or several hours before measurements of gas exchange. In both seasons, and irrespective of the timing of the imposition of different VPDs, net photosynthesis increased 60% after decreasing the VPD from 3 to 1 kPa. There were no differences in leaf conductance between leaves at different VPDs, thus transpiration rates were threefold higher at 3 than at 1 kPa VPD. It is concluded that nonstomatal inhibition of photosynthesis did occur in these leaves at high transpiration rate.     

Stomatal limitation of photosynthesis as affected by water stress and CO2 concentration

A water stress effect on photosynthesis and transpiration of wheat seedlings at 50-500 µmol(CO2) mol-1 was measured in an open gas exchange system. The limitation of photosynthesis by stomatal conductance was quantified by a stomatal control coefficient of the net photosynthetic rate. The stomatal control coefficient increased linearly as the water potential of root media decreased to -1 MPa, and it decreased with increasing CO2 concentration.     

Effects of Nitrogen Nutrition Level on Photosynthesis of Wheat Under Rapid Water Stress

Effects of nitrogen (N) nutrition level on photosynthesis of wheat were studied using method of quick drying of detached leaves, under rapid water stress. The results showed that in the case, leaf water potential (Ψw), net photosynthetic rate (Pn) and stomatal conductance (Gs) of high N (HN) leaves decreased more quickly than that of low N (LN) leaves. Therefore, the difference of Pn between HN and LN leaves became less and less with increasing water stress. Under severe water stress, the Pn of HN leaves were lower than that of LN leaves. The intercellular concentration of CO2 (Ci) of HN leaves were lower than that of LN leaves, and the value of stomatal limitation of photosynthesis (Ls) of HN leaves were higher during rapid water stress. However, the mesophllous conductance of CO2 (Gm) and photosynthetic activity of mesophyll of HN leaves were still higher than that of LN leaves.     

Stomatal and nonstomatal limitations to net photosynthesis in seedlings of woody angiosperms

Comparative responses of net photosynthesis (A) to water stress in woody species from a variety of habitats were studied to assess the relationship between photosynthetic attributes and drought tolerance. Stomatal and nonstomatal limitations to A were compared in three-month-old white oak (Quercus alba L.), post oak (Quercus stellata Wangenh.), sugar maple (Acer saccharum Marsh.), and black walnut (Juglans nigra L.) seedlings during a drying cycle. Relative stomatal limitation of photosynthesis (I) was less than 50% in all species except for Q. stellata seedlings subjected to severe water stress. No significant changes in I were observed in Q. alba and J. nigra before, during, and after drought. In A. saccharum, I was generally low and decreased significantly under water stress. Under well-watered conditions, A was highest in Q. stellata, intermediate in Q. alba, and lower in A. saccharum and J. nigra. High A in well-watered Q. stellata was associated with high stomatal conductance and carboxylation efficiency, whereas low A was associated with low stomatal conductance and carboxylation efficiency in A. saccharum and low stomatal conductance, low carboxylation efficiency, and high CO₂ compensation point in J. nigra. Under severe water stress, A, carboxylation efficiency, and stomatal conductance decreased substantially in all species; however, Q. stellata had the highest carboxylation efficiency and lowest CO₂ compensation point under these conditions. After 5 days at high soil moisture after drought, stomatal and mesophyll components of A in A. saccharum and J. nigra had not recovered to predrought levels, whereas they had completely recovered in Q. stellata and Q. alba. The photosynthetic apparatus, especially mesophyll components, of drought-tolerant Quercus species showed either less inhibition under water stress, superior recovery to predrought capacity, or both. Exposure of the leaves to ¹⁴CO₂ indicated apparent asymmetric stomatal closure for mildly water-stressed seedlings, but not for leaves of well-watered, severely stressed, or rehydrated plants. These results suggest that patchy stomatal closure under mild water stress might be important for water stress-induced inhibition of photosynthesis, but not under the more severe water stress imposed in this study.     

Physiological basis of genetic variation in leaf photosynthesis among rice (Oryza sativa L.) introgression lines under drought and well-watered conditions

To understand the physiological basis of genetic variation and resulting quantitative trait loci (QTLs) for photosynthesis in a rice (Oryza sativa L.) introgression line population, 13 lines were studied under drought and well-watered conditions, at flowering and grain filling. Simultaneous gas exchange and chlorophyll fluorescence measurements were conducted at various levels of incident irradiance and ambient CO(2) to estimate parameters of a model that dissects photosynthesis into stomatal conductance (g (s)), mesophyll conductance (g (m)), electron transport capacity (J (max)), and Rubisco carboxylation capacity (V (cmax)). Significant genetic variation in these parameters was found, although drought and leaf age accounted for larger proportions of the total variation. Genetic variation in light-saturated photosynthesis and transpiration efficiency (TE) were mainly associated with variation in g (s) and g (m). One previously mapped major QTL of photosynthesis was associated with variation in g (s) and g (m), but also in J (max) and V (cmax) at flowering. Thus, g (s) and g (m), which were demonstrated in the literature to be responsible for environmental variation in photosynthesis, were found also to be associated with genetic variation in photosynthesis. Furthermore, relationships between these parameters and leaf nitrogen or dry matter per unit area, which were previously found across environmental treatments, were shown to be valid for variation across genotypes. Finally, the extent to which photosynthesis rate and TE can be improved was evaluated. Virtual ideotypes were estimated to have 17.0% higher photosynthesis and 25.1% higher TE compared with the best genotype investigated. This analysis using introgression lines highlights possibilities of improving both photosynthesis and TE within the same genetic background.     

Controlling Effects of Irradiance and Heterotrophy on Carbon Translocation in the Temperate Coral Cladocora caespitosa

Temperate symbiotic corals, such as the Mediterranean species Cladocora caespitosa, live in seasonally changing environments, where irradiance can be ten times higher in summer than winter. These corals shift from autotrophy in summer to heterotrophy in winter in response to light limitation of the symbiont's photosynthesis. In this study, we determined the autotrophic carbon budget under different conditions of irradiance (20 and 120 μmol photons m(-2) s(-1)) and feeding (fed three times a week with Artemia salina nauplii, and unfed). Corals were incubated in H(13)CO(3) (-)-enriched seawater, and the fate of (13)C was followed in the symbionts and the host tissue. The total amount of carbon fixed by photosynthesis and translocated was significantly higher at high than low irradiance (ca. 13 versus 2.5-4.5 μg cm(-2) h(-1)), because the rates of photosynthesis and carbon fixation were also higher. However, the percent of carbon translocation was similar under the two irradiances, and reached more than 70% of the total fixed carbon. Host feeding induced a decrease in the percentage of carbon translocated under low irradiance (from 70 to 53%), and also a decrease in the rates of carbon translocation per symbiont cell under both irradiances. The fate of autotrophic and heterotrophic carbon differed according to irradiance. At low irradiance, autotrophic carbon was mostly respired by the host and the symbionts, and heterotrophic feeding led to an increase in host biomass. Under high irradiance, autotrophic carbon was both respired and released as particulate and dissolved organic carbon, and heterotrophic feeding led to an increase in host biomass and symbiont concentration. Overall, the maintenance of high symbiont concentration and high percentage of carbon translocation under low irradiance allow this coral species to optimize its autotrophic carbon acquisition, when irradiance conditions are not favourable to photosynthesis.     

Photosynthetic responses of a C3 and three C4 species of the genus Panicum (s.l.) with different metabolic subtypes to drought stress

Young plants of Panicum bisulcatum (C(3)), Zuloagaea bulbosa [NADP-malic enzyme (ME)-C(4)], P. miliaceum (NAD-ME-C(4)) and Urochloa maxima [phosphoenolpyruvate carboxykinase (PCK)-C(4)] were subjected to drought stress (DS) in soil for 6?days. The C(3) species showed severe wilting symptoms at higher soil water potential (-1.1?MPa) and relative leaf water content (77?%) than in the case of the C(4) species (-1.5 to -1.7?MPa; 58-64?%). DS decreased photosynthesis, both under atmospheric and under saturating CO(2). Stomatal limitation of net photosynthesis (P (N)) in the C(3), but not in the C(4) species was indicated by P (N)/C (o) curves. Chlorophyll fluorescence of photosystem II, resulting from different cell types in the four species, indicated NADPH accumulation and non-stomatal limitation of photosynthesis in all four species, even under high CO(2). In the NAD-ME-C(4) and the PCK-C(4) species, DS plants showed increased violaxanthin de-epoxidase rates. Biochemical analyses of carboxylating enzymes and in vitro enzyme activities of the C(4) enzymes identified the most likely non-stomatal limiting steps of photosynthesis. In P. bisulcatum, declining RubisCO content and activity would explain the findings. In Z. bulbosa, all photosynthesis enzymes declined significantly; photosynthesis is probably limited by the turnover rate of the PEPC reaction. In P. miliaceum, all enzyme levels remained fairly constant under DS, but photosynthesis can be limited by feedback inhibition of the Calvin cycle, resulting in asp inhibition of PEPC. In U. maxima, declines of in vivo PEPC activity and feedback inhibition of the Calvin cycle are the main candidates for non-stomatal limitation of photosynthesis under DS.