Calculation of leaf photosynthetic parameters from light-response curves for ecophysiological applications

Measurement of the light response of photosynthetic CO₂ uptake is often used as an implement in ecophysiological studies. A method is described to calculate photosynthetic parameters, such as the maximum rate of whole electron transport and dissimilative respiration in the light, from the light response of CO₂ uptake. Examples of the light-response curves of flag leaves and ears of wheat (Triticum aestivum cv. ARKAS) are shown.Abbreviations and symbols A net photosynthesis rate - D ¹ rate of dissimilative respiration occurring in the light - f loss factor - I incident PPFD - I effective absorbed PPFD - J rate of whole electron transport - J _m maximum rate of whole electron transport - p ^c intercellular CO₂ partial pressure - PPFD photosynthetic photon flux density - q effectivity factor for the use of light (electrons/quanta) - absorption coefficient - I ^* CO₂ compensation point in the absence of dissimilative respiration (bar) - II conversion factor for calculation of CO₂ uptake from the rate of whole electron transport - convexity factor Gas-exchange rates relate to the projective area and are given in mol·m^-2·s^-1. Electron-transport rates are given in mol electrons·m^-2·s^-1; PPFD is given in mol quanta·m^-2·s^-1.     

施氮和供水对混播和单播白羊草叶片叶绿素荧光特性的影响

为明确干旱条件下混播和施氮对白羊草〔Bothriochloa ischaemum(Linn.)Keng〕叶片叶绿素荧光参数的影响,采用盆栽法并设置不同混播比例〔白羊草与柳枝稷(Panicum virgatum Linn.)混播比例分别为8:0、6:2、4:4和2:6〕、施氮水平(即不施氮和1kg干土施01g纯氮)和供水条件(即正常供水和干旱胁迫6d后复水),对白羊草叶片叶绿素荧光参数的变化进行比较分析;在此基础上,采用一般线性模型分析这3个因素及其交互作用对白羊草叶绿素荧光参数的影响效应.结果显示:正常供水条件下,各处理组白羊草的最大光化学效率(Fv/Fm)、实际光化学效率(ΦPSⅡ)、表观光合电子传递速率(ETR)、光化学淬灭系数(qP)和非光化学淬灭系数(NPQ)均无明显变化.干旱及复水条件下,不施氮处理组白羊草的Fv/Fm值在干旱胁迫6 d(即土壤相对含水量最低)时降至最低值,并在复水2 d后恢复至与正常供水条件下相近的水平,而施氮处理组的Fv/Fm值则一直保持与正常供水条件下相近的水平;不论施氮与否,各处理组白羊草的ΦPSⅡ、ETR、qP和NPQ值基本上均表现为在干旱胁迫6 d时达到最高值,并在复水2 d后恢复至正常供水条件下各参数值的90%以上.总体上看,混播白羊草的qP和ETR值均高于单播白羊草,而其NPQ值则低于后者.统计分析结果表明:混播比例、施氮水平和供水条件3个因素间的交互作用对白羊草的ΦPSⅡ、qP和ETR值无显著影响,施氮水平对NPQ值的单独作用、施氮水平和混播比例的交互作用对Fv/Fm值以及施氮水平和供水条件的交互作用对ETR值也无显著影响,但这3个因素的单独作用及两两因素间的交互作用对白羊草其余叶绿素荧光参数均有显著或极显著影响.研究结果表明:一定程度的干旱胁迫有利于提高白羊草叶片PSⅡ反应中心的开放程度、光合电子传递速率和热耗散过剩光能的能力;在干旱胁迫条件下,施氮有助于白羊草叶片维持PSⅡ反应中心的活性和光化学效率;并且,与柳枝稷适度混播可改善白羊草叶片的光合性能,提高其种间竞争适应性.     

Nitrogen nutrition and water stress effects on leaf photosynthetic gas exchange and water use efficiency in winter wheat

The responses of gas exchange and water use efficiency to nitrogen nutrition for winter wheat were investigated under well-watered and drought conditions. The photosynthetic gas exchange parameters of winter wheat are remarkably improved by water and nitrogen nutrition and the regulative capability of nitrogen nutrition is influenced by water status. The effects of nitrogen nutrition on photosynthetic characteristics and on the limited factors to photosynthesis are not identical under different water status. Intrinsic water use efficiency (WUE(i)) of the plants at the high-N nutrition was decreased by a larger value than that of the plants in the low-N treatment due to a larger decrease in photosynthetic rate than in transpiration rate. Carbon isotope composition of plant material (delta(p)) is increased by the increase of drought intensity. The delta(p) at a given level of C(i)/C(a) is reduced by nitrogen deficiency. Leaf carbon isotope discrimination (Delta) is increased by the increase of nitrogen nutrition and decreased by the increase of drought intensity. Transpirational water use efficiency (WUE(t)) is negatively correlated with Delta in both nitrogen supply treatments and increased with the nitrogen supply.     

Cytological and embryological studies on apospory in Bothriochloa ischaemum L

Cytological and embryological studies on apomictic species Bothriochloa ischaemum L. were carried out. Our studies revealed that the chromosome number of its root apical cells was 40, indicating that it was a tetraploid cytotype. During the stage of microsporogenesis, meiosis seemed irregular, as the pairing chromosome number of microspore mother cell was more than 20. It was often found that some chromosomes did not assemble in the equatorial plane or moved to the two poles of the cell, a few laggards were seen. Multiporate pollens (22.3%) were often observed. The studies showed that a high frequency (87.8%) of 1-3 or more aposporous embryo sacs developed in one ovule of the species. The mature aposporous sac was usually characterized by an egg cell and one polar nucleus. The egg cell could develop spontaneously into a large proembryo (100-200 microm) mass prior to anthesis. When several aposporous sacs occurred in the same ovule, usually 2 aposporous sacs were involved in pseudogamy and developed into separate endosperm masses in the same ovary. In the low frequency of mature seed, 13.5% twin-embryo seedlings could be obtained after mature seeds germinated.     

Effects of mannose on photosynthetic gas exchange in spinach leaf discs

When mannose is provided in the transpiration stream to spinach (Spinacia oleracea) leaf discs, a series of specific and nonspecific changes occur in CO₂ and H₂O vapor exchange as a function of feeding time. The initial increases in apparent photosynthesis and transpiration are nonspecific effects due to osmotic changes leading to passive stomatal opening. The mannose-specific effects are: (a) time-dependent changes in the CO₂ concentration required for saturation; (b) complex kinetics of the inhibition of CO₂ assimilation dependent on CO₂ and O₂ concentrations and the duration of feeding (high CO₂ and low O₂ lead to rapid inhibitions of photosynthesis); (c) elimination of the capacity of 2% O₂ to stimulate photosynthesis; and (d) oscillations in the CO₂ exchange rate following transitions from 20% to 2% O₂. The mannose-specific effects are reversible by orthophosphate. The mannose-dependent changes in gas exchange are attributed to altered [ATP]/[ADP] ratios.     

Effects of water stress and high temperature on gas exchange and chlorophyll fluorescence in Triticum aestivum L.

Wheat plants grown in controlled growth chambers were exposed to drought stress (DS) and high temperature (HT) singly and in combination (DS+HT). The effects of these two stresses on net photosynthetic rate (P _N), stomatal conductance (g _s), intercellular CO₂ concentration (C _i), quantum efficiency of photosystem 2 (Φ_PS2), variable to maximum chlorophyll (Chl) fluorescence (F_v/F_m), photochemical (q_p) and non-photochemical (NPQ) Chl fluorescence, and yield were investigated. Grain yield was decreased by 21 % due to DS, while it was increased by 26 % due to HT. P _N, g _s, C _i, and Chl fluorescence were dramatically reduced to DS, HT, and their interaction, except NPQ which showed an increase due to HT.     

干旱胁迫和CO2浓度升高条件下白羊草的光合特征

采用盆栽控制试验,研究了黄土丘陵区乡土种白羊草在不同水分水平(80％ FC和40％ FC)和CO2浓度(375和750 μmol·m-2·s-1)处理下的光合生理变化特征.结果表明:干旱胁迫使白羊草的最大净光合速率(Pnmax)、表观量子效率(AQE)、气孔导度(gs)、蒸腾速率(Tt)、最大光化学效率(Fv/Fm)、潜在光化学效率(Fv/Fo)和光合色素含量降低,丙二醛(MDA)和脯氨酸(Pro)含量升高.水分充足条件下,与正常大气CO2浓度相比,大气CO2浓度倍增下白羊草的PnmaxMDA和Pro含量无显著差异.干旱胁迫下,CO2浓度升高提高了白羊草的最大荧光(Fm)、Fv/Fm、Fv/Fo、叶绿素含量和AQE,Pnmax比正常CO2浓度下高23.3％,差异达到显著水平,而MDA和Pro含量均显著降低.CO2浓度升高对干旱胁迫引起的白羊草光合能力下降有一定的补偿作用,减轻了干旱胁迫对白羊草的伤害.     

施肥水平对巨桉幼树叶片气体交换日变化的影响

以巨桉幼树为对象,研究了不同施肥水平(每株施用含N、P₂O₅和K₂O各15%的复合肥0、90、180和270 g)下巨桉叶片的气孔导度(G_s)、胞间CO₂浓度(C_i)、净光合速率(P_n)、蒸腾速率(T_r)、水分利用效率(WUE)和叶面饱和水汽压亏缺(V_pdl)日变化,及其叶绿素含量变化,探讨巨桉光合特性与施肥和环境因素之间的关系.结果表明:各处理巨桉叶片P_n的日变化呈“单峰”形曲线,峰值出现在14:00,未出现光合“午休”现象;G_s、T_r和V_pdl的日变化与P_n相同,而C_i的最低值出现在14:00;WUE的日变化呈“双峰”形曲线,峰值分别出现在10:00和14:00.与对照处理相比,施肥处理叶片G_s、P_n、T_r、WUE及叶绿素a、叶绿素b和总叶绿素含量平均值分别增加4.6%～15.9%、7.8%～21.8%、4.8%～11.6%、3.2%～8.8%、15.5%～62.0%、14.5%～44.5%和15.3%～57.1%,且随施肥量的增加而增加;而叶片C_i和V_pdl平均值分别降低1.7%～4.6%和6.4%～15.2%,且随施肥量的增加而降低.气温、相对湿度(RH)和光合有效辐射(PAR)与G_s、P_n和T_r,G_s与P_n和Tr均显著相关.施肥可以促进巨桉生长、提高水分利用效率和增加生物固碳能力;气温、RH、PAR和G_s是影响巨桉光合作用和蒸腾作用日变化的主要因素.     

Effect of vacuum infiltration on photosynthetic gas exchange in leaf tissue

Using a manometric method, photosynthetic oxygen evolution and ¹⁴CO₂ fixation have been determined for leaf tissue of Triticum aestivum L., Hordeum vulgare L., Phaseolus vulgaris L., and Lemna minor L. Approximately similar values in the range 0.2 to 0.4 millimoles grams fresh weight⁻¹ hour⁻¹ were obtained for both gases. In tissue subjected to vacuum infiltration, O₂ evolution and ¹⁴CO₂ fixation were barely measurable. It is considered that the elimination of photosynthetic gas exchange results from a decreased supply of CO₂ to the chloroplasts. Chopping wheat laminae also leads to a reduction in photosynthetic gas exchange, slices 1 millimeter or less giving only 10 to 20% of the value for whole tissue. Respiration is unaffected by either treatment. Carbonic anhydrase did not improve photosynthetic gas exchange in infiltrated tissue. The use of sliced or vacuum-infiltrated leaf tissue in photosynthetic studies is discussed.     

干旱胁迫对丹参叶片气体交换和叶绿素荧光参数的影响

研究了干旱处理15d后,大叶型丹参和小叶型丹参2个品种幼苗气体交换和叶绿素荧光参数的变化.结果表明:在干旱胁迫15d后,大叶型丹参叶片净光合速率(Pn)和PSⅡ最大光化学效率(Fv/Fm)分别下降了66.42%和10.98%,而小叶型丹参的Pn和Fv/Fm分别下降了29.32%和5.47%,干旱胁迫对大叶型丹参Pn和Fv/Fm的影响明显大于小叶型丹参.小叶型丹参Pn下降主要由气孔因素造成,而大叶型则主要由非气孔因素所致.干旱胁迫使丹参叶片的气孔导度(Gs)下降,但明显诱导了水分利用效率(WUE)、非光化学猝灭系数(qN)和光呼吸速率与净光合速率比率(Pr/Pn)的增加,以提高干旱胁迫抗性.其中小叶型丹参的增幅明显大于大叶型丹参.表明小叶型丹参的抗干旱胁迫能力更强.     

Effects of water stress on leaf growth and photosynthetic and transpiration rates of Tectona grandis

The response of pear fruit and leaf parameters to shade imposed during development was studied. Whole branches of mature trees of Pyrus communis L. cv. Bartlett growing in the High Valley area of Argentina were covered with a shade cloth (80 % reduction in irradiance) from 6 to 18 weeks after full bloom (WAFB) during the 1995-96 growing season. Fruit diameter was measured at two-weekly intervals; flesh firmness, soluble solids concentration, and leaf area were determined 18 WAFB. Prolonged shading significantly reduced fruiting spur specific leaf mass and consequently resulted in 20.79 % less final fruit fresh mass. However, flesh firmness was 8.07 % lower under full irradiance. This revised version was published online in July 2006 with corrections to the Cover Date.     

Root hydraulic conductance,gas exchange and leaf water potential in seedlings of Pistacia lentiscus L. and Quercus suber L. grown under different fertilization and light regimes

Differences in morphology, biomass allocations and physiological responses were investigated in seedlings of Mastic tree (Pistacia lentiscus L.) and Cork oak (Quercus suber L.) submitted to contrasting fertilization and light regimes during early growth. These species are two evergreen sclerophyllous Mediterranean species frequently used in Mediterranean reforestation programmes. Fertilization was the treatment that affected most of the morphological and physiological variables evaluated in P. lentiscus and Q. suber seedlings. Leaf area and specific leaf area (SLA) were affected by shading treatment in both species, showing higher values in seedlings grown under shade. P. lentiscus seedlings showed a high capacity to modify root morphological variables and root hydraulic conductance (K_R) with the fertilization treatment. In contrast, Q. suber showed low to moderate root system changes with the treatments applied, although the fertilization level affected biomass allocation (i.e., root to shoot ratio) in both species. Under high water demand, P. lentiscus seedlings with high K_R allowed transpiration (E) to increase without increasing the water potential gradient between soil and leaves. In Q. suber, high fertilization induced significant increases in photosynthesis (A), as well as a tendency to increase E with significantly lower leaf water potential (ψ_L).     

Impacts of leaf age and heat stress duration on photosynthetic gas exchange and foliar nonstructural carbohydrates in Coffea arabica

Given future climate predictions of increased temperature, and frequency and intensity of heat waves in the tropics, suitable habitat to grow ecologically, economically, and socially valuable Coffea arabica is severely threatened. We investigated how leaf age and heat stress duration impact recovery from heat stress in C. arabica. Treated plants were heated in a growth chamber at 49°C for 45 or 90 min. Physiological recovery was monitored in situ using gas exchange, chlorophyll fluorescence (the ratio of variable to maximum fluorescence, F_V/F_M), and leaf nonstructural carbohydrate (NSC) on mature and expanding leaves before and 2, 15, 25, and 50 days after treatment. Regardless of leaf age, the 90‐min treatment resulted in greater F_V/F_M reduction 2 days after treatment and slower recovery than the 45‐min treatment. In both treatments, photosynthesis of expanding leaves recovered more slowly than in mature leaves. Stomatal conductance (g_s) decreased in expanding leaves but did not change in mature leaves. These responses led to reduced intrinsic water‐use efficiency with increasing heat stress duration in both age classes. Based on a leaf energy balance model, aftereffects of heat stress would be exacerbated by increases in leaf temperature at low g_s under full sunlight where C. arabica is often grown, but also under partial sunlight. Starch and total NSC content of the 45‐min group significantly decreased 2 days after treatment and then accumulated 15 and 25 days after treatment coinciding with recovery of photosynthesis and F_V/F_M. In contrast, sucrose of the 90‐min group accumulated at day 2 suggesting that phloem transport was inhibited. Both treatment group responses contrasted with control plant total NSC and starch, which declined with time associated with subsequent flower and fruit production. No treated plants produced flowers or fruits, suggesting that short duration heat stress can lead to crop failure.     

短期CO2浓度升高和干旱胁迫对白羊草土壤碳氮和微生物根际效应的影响

采用人工气候室和盆栽控水试验研究黄土丘陵区典型草本植物白羊草在倍增CO₂浓度(800 μmol·mol^-1)下和充分供水(75%～80%的田间持水量)、轻度干旱胁迫(55%～60%的田间持水量)和重度干旱胁迫(35%～40%的田间持水量)下根际和非根际土壤碳氮含量和微生物群落结构及其根际效应.结果表明: CO₂浓度升高和干旱胁迫对白羊草根际和非根际土壤有机碳、全氮和水溶性有机碳(DOC)含量及其根际效应均无显著影响.轻度干旱胁迫下CO₂浓度升高显著促进了根际土壤水溶性有机氮(DON)的消耗,导致DOC/DON升高,提高了DON的负根际效应和DOC/DON的正根际效应.干旱胁迫和CO₂浓度升高对土壤总磷脂脂肪酸(总PLFA)和细菌PLFA的根际效应无显著影响.CO₂浓度升高条件下干旱胁迫显著提高了根际土壤G⁺/G^- PLFA,降低了非根际土壤G⁺/G^- PLFA,导致其根际效应显著提高,表明根际微生物群落由自养微生物群落向异养微生物群落的转变.     

Effects of drought stress on photosynthetic gas exchange, chlorophyll fluorescence and stem diameter of soybean plants

Changes in plant growth, photosynthetic gas exchange, chlorophyll fluorescence and stem diameter of soybean [Glycine max (L.) Merr.] plants under drought stress were studied. Total plant dry mass was reduced by 30 % compared to well-watered control plants. Leaf water potential was slightly decreased by water stress. Water stress induced daytime shrinkage and reduced night-time expansion of stem. Photosynthetic rate, stomatal conductance and transpiration rate were significantly declined by water stress, while the intercellular CO₂ concentration was changed only slightly at the initiation of stress treatment. The maximum photochemical efficiency of photosystem 2 and apparent photosynthetic electron transport rate were not changed by water stress.     

鸢尾(Iris L.)叶片取向与其光合特性及光抑制的关系

通过气体交换、叶绿素荧光、反射光谱等方法,研究了鸢尾叶片取向对植株光合特性及光抑制的影响.自然状态下,鸢尾的叶片不同取向影响植株对光能的截获;叶片净光合速率Pn与光合有效辐射PAR呈极显著相关;东西取向叶片的Pn要大于南北取向.南北取向的植株中叶片叶绿素(Chl a和Chl b),类胡萝卜素(Car)含量略高于东西取向.日进程中,各取向的叶片在一天中均没有发生明显的光抑制.相对于东西取向的植株,南北取向植株发生了明显的倾斜;在两种取向的植株中,叶片东侧和南侧的光化学反射指数(PRI)下调幅度较大;PRI的变化量(△PRI)大小依次为:东侧>南侧>西侧>北侧.鸢尾植株取向改变了叶片倾斜角度,两者共同导致光能截获减小;同时,叶片光能利用效率下调和叶黄素循环增强,这可能是不同取向植株均未发生严重光抑制的原因.     

Comparative study of water uptake and photosynthetic gas exchange between scrub and fringe red mangroves,Rhizophora mangle L.

Summary The red mangrove (Rhizophora mangle L.) occurs frequently in both scrub and fringe mangrove forests. Our previous study demonstrated that individuals of this mangrove species growing in scrub and fringe forests differ significantly in both morphological and physiological characteristics. To further characterize physiological differences between scrub and fringe mangroves, we compared their differences in water uptake and photosynthetic gas exchange during different seasons. In the wet season (June–October, 1990), scrub mangroves showed lower D and ¹⁸O values of stem water than fringe mangroves, indicating more usage of rain-derived freshwater. In the dry season (Jan–April, 1991), however, scrub mangroves utilized the same water source as fringe mangroves, reflected by their similar D and ¹⁸O values of stem water. Consistently, there were significant differences in predawn water potentials between scrub and fringe mangroves in the wet season (October 1990) with higher values for scrub mangroves, but no significant differences in the dry season (January 1991). Higher elevation in the scrub forest seems to be the major factor responsible for the shift of water sources in scrub mangroves. On Apr. 27 and Aug. 8, 1990, scrub mangroves showed lower CO₂ assimilation rate, stomatal conductance, and intercellular CO₂ concentration than fringe mangroves. There were no differences in these gas exchange characteristics on the other two measuring dates: Oct. 17, 1990 and Jan. 11, 1991. Instantaneous water use efficiency was significantly higher for scrub mangroves than for fringe mangroves on three of the four sampling dates. Similarly, leaf carbon isotope discrimination of scrub mangroves was always significantly lower than that of fringe mangroves, indicating higher long-term water use efficiency. Higher water use efficiency in scrub mangroves is a result of stomatal limitation on photosynthesis, which may entail considerable carbon cost to the plants.     

长期施肥对水稻光合特性及水分利用效率的影响

在实施了27年的长期田间定位试验区,研究了长期不同施肥对红壤区水稻光合特性及水分利用效率的影响.结果表明:在不施肥(CK)、无机肥(N、NP、NPK)、有机肥(猪粪+紫云英绿肥,M)和无机肥与有机肥配施(NPKM)处理中,长期施用肥料,特别是有机肥与无机肥配施提高水稻各生育期剑叶叶绿素含量、净光合作用速率、气孔导度、蒸腾速率、水分利用效率和水稻产量,降低水稻剑叶胞间CO_2浓度;水稻剑叶叶绿素含量、净光合作用速率、气孔导度、蒸腾速率随发育阶段演进而减小,孕穗期>齐穗期>乳熟期,而胞间CO_2浓度相反;水分利用效率以齐穗期为最大;水稻发育阶段叶绿素含量、净光合速率和水稻产量之间均呈显著正相关;长期施用肥料,特别是有机肥与无机肥配施更有利于红壤区水稻的生长发育、产量和水分利用效率的提高.

Abstract：

A field experiment has being conducted for 27 years in Jinxian County, Institute of Red Soil in Jiangxi Province (116°20'24" E, 28°15'30" N) to study the effects of fertilization on the rice photosynthetic traits and water use efficiency. Four treatments were installed,i. e., no fertilization (CK), chemical fertilization (N, NP, NPK), organic fertilization (M), and chemi-cal and organic fertilization (NPKM). Long-term fertilization, especially treatment NPKM, in- creased the flag leaf chlorophyll content, net photosynthetic rate, stomatal conductance, transpi-ration rate, and water use efficiency of rice at its all growth stages and the rice yield, and de-creased the flag leaf intercellular CO_2 concentration. With the growth of rice, the chlorophyll content, net photosynthetic rate, stomatal conductance, and transpiration rate decreased, but the intercellular CO_2 concentration increased. The water use efficiency was the greatest at full-head-ing stage. There were significant positive correlations between the chlorophyll content and net photosynthetic rate at various growth stages and the rice yield, Long-term fertilization, especially the combined chemical and organic fertilization, was favorable to the rice growth and develop-ment, water use efficiency, and yield production in red soil region.     

Visual estimation of leaf water stress inMercurialis perennis L.

Leaf inclination of three upper leaf pairs of a plant was used as a visual parameter for estimation of leaf (plant) water status inM. perennis. Negative correlation was found between leaf angle and leaf water saturation deficit (WSD). Large angles (between 130 and 80–90°) indicated WSD below approx. 12%, narrow angles expressed negative water balance of the plant and indicated usually strong or moderate water stress. The correlation may be expressed by two regression lines differing in slope. Some differences were observed between three leaf pairs investigated: At the same water deficits, leaves of the first pair (from apex) were inclined to a less degree than leaves of the other two leaf pairs. Leaves of the third pair wilted the most rapidly, therefore their WSD were highest at moderate and strong water stress of the plant. The approximation method tested is suitable for judgment of the plant water status inM. ptrennis and may be used in ecological investigations in forest ecosystems.     

Effect of water deficit on photosynthetic oxygen exchange measured using 18O2 and mass spectrometry in Solanum tuberosum L. leaf discs

The effect of leaf dehydration on photosynthetic O₂ exchange of potato (Solanum tuberosum L., cv. Haig) leaf discs was examined using ¹⁸O₂ as a tracer and mass spectrometry. In normal air (350 μl·l^?1CO₂) and under an irradiance of 390 μmol photons·m^?2·s¹, a relative water deficit (RWD) of about 30% severely decreased net O₂ evolution and increased O₂ uptake by about 50%, thus indicating an enhancement of photorespiration. Increasing CO₂ concentrations diminished O₂ uptake and stimulated net O₂ evolution both in well-hydrated and in dehydrated (RWD of about 30%) leaves. Much higher CO₂ concentrations (up to 4%) were required to observe a complete effect of CO₂ in dehydrated leaves. The chloroplastic CO₂ concentration at the ribulose-1,5-bisphosphate carboxylase-oxygenase (Rubisco) level (C_c) was calculated from O₂-exchange data in both well-hydrated and dehydrated leaves, assuming that the specificity factor of Rubisco was unaffected by desiccation. When plotting net O₂ photosynthesis as a function of C_c, a similar relationship was obtained for well-hydrated and waterstressed leaf discs, thus showing that the main effect of water deficit is a decrease of the chloroplastic CO₂ concentration. At saturating CO₂ levels, the non-cyclic electron-transport rate, measured either as gross O₂ photosynthesis or as the chlorophyll fluorescence ratio (F_m -F_s)/F_m, was insensitive to water deficit, provided RWD was below 40%. In this range of RWD, the decrease in gross O₂ photosynthesis observed in normal air was attributed to the inability of oxidative processes to sustain the maximal electron-flow rate at low chloroplastic CO₂ concentration. The maximal efficiency of photosystem II, estimated as the chlorophyll fluorescence ratio (F_m -F₀)/F_m measured in dark-adapted leaves, was not affected by water deficits up to 60%.