不同生育期玉米叶片光合特性及水分利用效率对水分胁迫的响应

利用大型移动防雨棚开展了玉米水分胁迫及复水试验,通过分析玉米叶片光合数据,揭示了不同生育期水分胁迫及复水对玉米光合特性及水分利用效率的影响。结果表明:水分胁迫导致玉米叶片整体光合速率、蒸腾速率和气孔导度下降以及光合速率日变化的峰值提前;水分胁迫后的玉米叶片蒸腾速率、光合速率和气孔导度为适应干旱缺水均较对照显著下降,从而提高了水分利用效率,缩小了与水分充足条件下玉米叶片的水分利用效率差值;在中度和重度水分胁迫条件下,玉米叶片的水分利用效率降幅低于光合速率、蒸腾速率和气孔导度的降幅, 有时甚至高于正常供水条件下的水分利用效率;适度的水分胁迫能提高玉米叶片的水分利用效率,从而增强叶片对水分的利用能力,抵御干旱的逆境;水分亏缺对玉米光合速率、蒸腾速率及水分利用效率的影响具有较明显滞后效应,干旱后复水,光合作用受抑制仍然持续;水分胁迫时间越长、胁迫程度越重,叶片的光合作用越呈不可逆性;拔节-吐丝期水分胁迫对玉米叶片光合作用的逆制比三叶-拔节期更难恢复。     

黄土丘陵区柳枝稷光合生理生态特性的初步研究

研究了黄土丘陵区引种草种柳枝稷（Panicum virgatum)的光合生理生态特性。比较了不同叶位叶片光合速率（Pn)、蒸腾速率（Tr)、水分利用效率（WUE）的日变化以及环境因子的作用,结果表明,柳枝稷叶片Pn日变化曲线为双峰型,中午“光合降低”主要是由于叶温过高导致呼吸高引起的净化合速率降低,叶龄增大,叶片Pn日变化相对较平缓,其中壮龄叶Pn日变化最为平缓,幼龄叶Tr的日变化为双峰型,随叶龄增大（叶位下降）而成为单峰型。Pn日变化最为平缓,幼龄叶Tr的日变化为双峰型,随叶龄增大（叶位下降）而成为单峰型,WUE的日变化可划分为上午的降低和下午的波动2个阶段,最上充分展开叶（旗叶）的WUE始终最高。     

An attempt to establish a synthetic model of photosynthesis-transpiration based on stomatal behavior for maize and soybean plants grown in field

A synthetic model of photosynthesis-transpiration was established based on a comprehensive consideration of models of CO₂ and H₂O fluxes controlled by stomata of plant leaves.The synthetic model was developed by introducing the internal conductance to CO₂ assimilation, g_ic, and the general equation of stomatal conductance model to H₂O diffusion, g_sw = g₀+a₁A_mf(D_s)/(C_s-Γ), into models of CO₂ and H₂O diffusion through the plant leaves stomata. In the above expression, g₀ and a₁ are coefficients, C_s ambient CO₂ concentration at leaf surface, Γ CO₂ compensation point, and f(D_s) the general function describing the response of stomatal conductance to humidity. Using the data observed in maize (Zea mays L.) and soybean (Glycine max Merr.) plants grown in the field, the parameters in the model were identified, and the applicability of the model was examined. The verification indicated that the developed model could be used to estimate net assimilation rate, transpiration rate, and water use efficiency with a high enough level of precision. The examination also showed that when f(D_s) = h_s or f(D_s) = (1+D_s/D₀)⁻¹ was employed, the estimation precision of the synthetic model was highest. In the study, the parameter g_ic was estimated by means of a linear function of Q_P because it was shown to be mostly correlated with photosynthetic photon flux, Q_P, among various environmental factors.     

开顶式气室内外冬小麦光合特性差异比较

为探明开顶式气室(OTC)内外冬小麦光合特性的差异,测定了OTC内(T1处理组,冬小麦整个生育期生长在OTC内)和OTC外(T2处理组,大田自然环境组)冬小麦(扬麦16)不同生育期的气体交换参数、光合色素含量和叶绿素荧光参数。结果表明,T1处理的净光合速率Pn、气孔导度Gs、胞间CO2浓度Ci、最大光合速率Pm和半饱和光强Ik大部分生育期均大于T2;灌浆之前T1的表观量子效率AQY较高,蒸腾速率Tr和暗呼吸速率Rd较低,灌浆之后则出现逆转。大部分生育期T1处理叶绿素和类胡萝卜素含量均显著大于T2。T1处理的初始荧光Fo和最大荧光Fm均大于T2,而两者最大光量子产量Fv/Fm大部分生育期无显著差异,孕穗期和扬花期T1处理光化学淬灭系数q P显著低于T2。T1和T2处理PSⅡ的实际光化学量子效率Y(II)大部分生育期无显著差异,但灌浆期以后T1的非光化学淬灭系数NPQ和PSⅡ处调节性能量耗散的量子产量Y(NPQ)显著大于T2,而PSⅡ处非调节性能量耗散的量子产量Y(NO)显著低于T2。由此可见,OTC内冬小麦的气体交换能力和光响应能力均优于OTC外,光合色素含量也更高;OTC内外冬小麦内禀光能转换效率和实际光化学量子效率相当,但OTC内冬小麦过剩光能向调节性热耗散分配的比例较高而向非调节性热耗散分配的比例较低,其光保护能力更强,光能分配也更加合理。     

Effects of simulated acid rain on the photosynthetic characteristics of Phaseolus vulgaris L.

A single treatment with a low pH solution of bean plants led to serious changes in the net photosynthetic rate (P _N) as well as in various parameters of photosystem 2 (PS2) activity. A considerable suppression of P _N was established already in the first hours (3 and 5) following the acid treatment (pH 2.4-1.8). The period of strong inhibition of CO₂ uptake and photochemical activity was followed by the period of recovery (24-72 h). At a single spraying, pH values exceeding 2.0 did not lead to irreversible damages of the photosynthetic apparatus. The damages resulting from treatments with pH 2.0 and 1.8 were on the threshold of irreversible ones and were the cause of faster ageing. This revised version was published online in June 2006 with corrections to the Cover Date.     

氮、磷、钾营养对冬小麦光合作用及水分利用的影响

在中国科学院封丘农业生态实验站长期肥料实验的基础上,选择氮磷(T1)、氮钾(T2)、磷钾(T3)和氮磷钾(T4)4个肥料处理,以不施肥为对照(CK),研究了长期施用氮、磷、钾肥对小麦抽穗期和灌浆期净光合速率(Pn)、蒸腾速率(Tr)和水分利用效率(WUE)日变化的影响.结果表明:小麦抽穗期和灌浆期T4处理的Pn日变化值显著高于CK,其他处理与CK未达到显著性差异,各处理Pn日积累量大小为T4＞T1＞T2＞T3＞CK;缺氮、缺磷和缺钾均降低了叶片Pn,影响大小依次为缺氮＞缺磷＞缺钾;小麦抽穗期和灌浆期Tr日变化曲线呈单峰型,处理间日变化值差异不显著;缺氮、缺磷和缺钾均降低了叶片WUE,抽穗期,T1、T4处理的WUE日变化值显著高于CK,而T3、T2与CK差异不显著,说明缺氮、缺磷对WUE的影响最大,灌浆期,T4处理的WUE日变化值显著高于CK;不同肥料处理仅改变了小麦光合日变化的幅度,而未改变其变化规律,氮、磷、钾复合施肥有效地提高了小麦的光合生产和水分利用效率.     

准噶尔荒漠3种短命植物气体交换特征的日变化

采用LI-6400便携式光合测定仪,在晴天条件下对准噶尔荒漠3种典型短命植物东方旱麦草(Eremopyrum orientale)、卷果涩芥(Malcolmia scorpioides)和四齿芥(Tetracme quadricormis)的气体交换特征的日变化规律进行了研究.结果表明:①东方旱麦草和卷果涩芥的净光合速率(简称Pn,后同)的日变化呈"双峰"型,14:00(采用时间均为北京时间,后同)左右存在明显的光合"午休"现象,四齿芥Pn的日变化呈"单峰"型,峰值出现在12:00与前两者的第一峰值出现时刻相同.3种植物蒸腾速率(简称E,后同)的日变化均呈"单峰"型,但不同植物的峰谷值出现时刻不同.水分利用效率(WUE)日变化,四齿芥呈"单峰"型,东方旱麦草和卷果涩芥呈"双峰"型,峰值分别出现在8:00～10:00之间,后两者第二峰值分别出现在16:00和18:00.②根据Pn、胞间CO2浓度(Ci)和气孔限制值(Ls)的变化方向,推测3种短命植物的光合"午休",东方旱麦草和卷果涩芥主要受非气孔因素限制,而四齿芥主要受气孔因素限制.③卷果涩芥和四齿芥两种十字花科草本日平均Pn、E 和WUE 均高于禾本科东方旱麦草,尤其卷果涩芥是一种高光合、高蒸腾、高水分利用率的物种.④相关分析结果表明,对Pn影响最显著的环境因子是光合有效辐射(PAR),对E影响最显著的因子3种植物各不相同.     

黄土高原4种豆科牧草的净光合速率和蒸腾速率日动态及水分利用效率

在晴天条件下 ,研究了 4年生甘肃红豆草 (Onobrychis viciaefolia scop.cv.‘Gansu’)、沙打旺 (Astragalus adsurgens)、东方山羊豆 (Galega orientalis)和多年生香豌豆 (L athyruslatifolius)人工种群花期 (5月 31日 )和再生期 (7月 10日 )的净光合速率、蒸腾速率、气孔导度、水分利用效率以及土壤贮水量和水分利用特征。结果表明 ,自 5月 31日 (花期 )至 7月 10日 (再生期 ) ,4种牧草对土壤水分消耗由大到小依次为 :沙打旺 119.2 mm、多年生香豌豆 91.6 mm、山羊豆 81.9m m和红豆草 73.8m m。红豆草在花期和再生期的净光合速率分别为 12 .4 1和 9.0 6μ mol CO2 / (m2 · s) ,沙打旺为 10 .10和 7.0 1μ m ol CO2 / (m2 · s) ;红豆草在花期和再生期的日均蒸腾速率 8.13和 9.0 5 m m ol H2 O/ (m2· s) ,沙打旺刈割前和刈割后蒸腾速率分别为 7.4 0和 6 .5 4mmol H2 O/ (m2· s) ,属于高光合、高蒸腾型。而山羊豆和多年生香豌豆则属于低蒸腾、低光合类型 ,花期和再生期 ,山羊豆的日均光合速率分别为 4 .74和 4 .88μm ol CO2 / (m2· s) ,多年生香豌豆为 4 .4 1和 4 .6 4 μ mol CO2 / (m2· s) ,相应的蒸腾速率分别达到 3.75和 5 .4 2 m mol H2 O/ (m2 · s) ,4 .74和 4 .34m mol H2 O/ (m2 · s)。     

Requirements for obtaining maximum indices of photosynthesis and transpiration in attached leaves of willow plants grown in short-rotation forest

The results of multiyear studies of gas exchange in intact attached leaves of several willow species (Salix sp.) were analyzed. Measurements were performed with a portable Li-6400 infrared gas analyzer both on plants in their natural environment and on rooted cuttings grown in a greenhouse. Individual attached leaves were placed into the leaf chamber where climatic conditions were either similar to or different from those outside the chamber. The maximal rates of net photosynthesis (P _n) and transpiration (E) were only observed with the provision that the environmental variables inside and outside the chamber were identical. On rainy or cloudy days, the P _n and E values observed under optimum conditions inside the leaf chamber were lower than their potential maxima by 12–18% and 35–45%, respectively. Deviation of temperature in the chamber by 5–7°C from the external level and fluctuations of ambient temperature affected P _n but not E rates of tested leaves. Variations in relative air humidity in the chamber directly influenced E but had no effect on P _n of attached leaves. It was shown that the maximum rates of gas exchange in the attached willow leaf could be only attained by providing optimum conditions for the whole plant.     

沙柳光合作用和蒸腾作用日动态变化的初步研究

沙柳是毛乌素沙地流动沙丘和固定沙丘上广泛分布的重要固沙植物。叶片的光合速率呈现出不规则的日动态变化 ,在 1 0 :0 0～ 1 1 :0 0及 1 4 :0 0出现光合午休现象。蒸腾速率呈现出单峰型的日动态变化 ,蒸腾在午后 1 5:0 0达到最大。水分利用效率在早晨 9:0 0后保持在 2～ 2 .5μmol CO2 · mmol-1 H2 O左右。叶片水分亏缺也表现出规律性的变化。从早晨开始 ,叶片水分亏缺逐渐增大 ,在 1 8:0 0达到最大值。比较生长季节的不同时期环境因子的变化情况表明 ,春末的光照强度和气温比夏末高 ,但土壤水分含量则相反。沙柳在春末的光合速率、蒸腾速率分别是夏末的 79%和 72 .4% ,表明水分亏缺是影响沙柳生长发育的主要限制因素。沙柳在不同季节对水分的不同利用特点是对环境胁迫的弹性适应     

Vegetative growth and photosynthesis in coffee plants under different watering and fertilization managements in Yunnan,SW China

In a field experiment Coffea arabica L. was subjected to various moisture and fertilizer regimes in Simao, Yunan, SW China. The experimental treatments consisted of eight factorial combinations of two fertilization levels (high and low) and four watering treatments applied in the dry season: application of dry rice straw mulch, drip irrigation, mulching plus drip irrigation on the soil surface, and control (no mulching or irrigation). The growth of the coffee plants was monitored throughout the course of a full year. Two clear growth peaks were detected (one at the beginning and one in the middle of the wet season) in plants subjected to all treatments, and the growth rhythm of coffee plants was not regulated by extrinsic abiotic factors. High fertilization resulted in a significantly higher relative growth rates for both height and length of the branches during the growth peaks than the low fertilization treatment. In the dry season, increasing the soil moisture contents by irrigation and/or mulching enhanced the plants’ gas exchange, but the soil water status had no significant effects on the internal fluorescence parameters of photosystem 2. More fertilized plants had a greater ability to acclimate to high-irradiance environments than the lightly fertilized plants, showing significant lower diurnal photoinhibition, associated with higher energy utilization through photochemistry and energy dissipation through the xanthophyll cycle. Hence the wet season is the optimum period for photosynthetic carbon fixation and vegetative growth of coffee plants. Higher than routinely applied levels of fertilization are required to optimize the coffee plants’ photosynthetic acclimation and growth in the studied environment. Both soil moisture conserving practices tested, mulching and drip irrigation, had significant effects on the growth and photosynthesis of the coffee plants, but the former was more practical than the latter.     

Improved growth and water use efficiency of cherry saplings under reduced light intensity

Cherry (Prunus avium L.) saplings were grown under natural sunlight (controls) or moderate shading (up to 30%, depending on the incident light intensity and the hour of the day). Reduced light intensity increased the dry mass of each of the plant components studied. Consequently, the total dry mass of shaded plants was significantly greater than that of controls at the end of the growing season. However, the diurnal trend in the level of photosynthesis (per unit of leaf area) of shaded plants was similar to the controls in August, but lower in September. As the growing season proceeded, reduced photosynthetic rates, thinner mesophyll and larger specific leaf area in the shaded plants indicated that leaf development had adapted to shaded conditions throughout the growing season. It is suggested that increased growth of shaded plants was caused by a higher initial relative growth rate and a greater whole-plant photosynthesis. Shading consistently reduced transpiration over the season, therefore improving water use efficiency of shaded leaves. Our results suggest that a moderate reduction in light intensity can be a useful method for improving growth and saving water in hot and dry environments.     

Chilling and photosynthetic productivity of field grown maize (Zea mays); changes in the parameters of the light-response curve,canopy leaf CO2 assimilation rate and crop radiation-use efficiency

The effect of growth temperatures on the photosynthetic performance of field grown maize (Zea mays cv. LG11) was examined for crops sown on 1 May and 28 June 1991. During the period of growth, 2 May to 10 August, the early-sown crop experienced temperatures below 10 °C on 33 occasions compared with only one for the crop sown on 28 June. The prolonged period of low temperatures throughout May and beginning of June were associated with a marked depression in CO₂ assimilation rates at all light levels in the early-sown treatment. Chill-induced depression of the photosynthetic light-response curve reflected a sustained reduction in canopy leaf photosynthesis and crop radiation-use efficiency (RUE). During the early stages of growth, RUE was 65% lower in the early- than late-sown treatment, with no marked recovery observed in the former treatment until approximately three weeks after chilling conditions had ceased. Data show a close correlation between chill-induced depression of quantum yield () and RUE, with corresponding reductions in the light-saturated rates of CO₂ assimilation (P_max). The convexity of the light-response curve recovered most rapidly from chilling temperatures, and at least three weeks before any improvement in RUE. It is concluded that photosynthetic productivity of immature maize stands is less sensitive to changes in the convexity of the light response, than to changes in either or P_max.     

Compensatory effects of elevated CO<Subscript>2</Subscript> concentration on the inhibitory effects of high temperature and irradiance on photosynthetic gas exchange in carrots

We determined the interactive effects of irradiance, elevated CO₂ concentration (EC), and temperature in carrot (Daucus carota var. sativus). Plants of the cv. Red Core Chantenay (RCC) were grown in a controlled environmental plant growth room and exposed to 3 levels of photosynthetically active radiation (PAR) (400, 800, 1 200 μmol m⁻² s⁻¹), 3 leaf chamber temperatures (15, 20, 30 °C), and 2 external CO₂ concentrations (C _a), AC and EC (350 and 750 μmol mol⁻¹, respectively). Rates of net photosynthesis (P _N) and transpiration (E) and stomatal conductance (g _s ) were measured, along with water use efficiency (WUE) and ratio of internal and external CO₂ concentrations (C _i/C _a). P _N revealed an interactive effect between PAR and C _a. As PAR increased so did P _N under both C _a regimes. The g _s showed no interactive effects between the three parameters but had singular effects of temperature and PAR. E was strongly influenced by the combination of PAR and temperature. WUE was interactively affected by all three parameters. Maximum WUE occurred at 15 °C and 1 200 μmol m⁻² s^{− 1} PAR under EC. The C _i /C _a was influenced independently by temperature and C _a. Hence photosynthetic responses are interactively affected by changes in irradiance, external CO₂ concentration, and temperature. EC significantly compensates the inhibitory effects of high temperature and irradiance on P _N and WUE.     

平茬高度对四合木生长及生理特性的影响

为改善珍稀濒危植物四合木种群衰退现状,促进其更新复壮及可持续发展,对四合木老龄化植株进行了0 cm、5 cm、10 cm、15 cm（文中定义为A、B、C、D）四种平茬高度处理,以未平茬老龄植株为对照（CK）,跟踪测定各处理的生长指标和生理指标变化规律,结果表明：（1）平茬能够促进四合木老龄化植株更新复壮,平茬后第一生长季各处理组株高生长量极显著大于CK（p<0.01）,处理B、C、D冠幅年生长量极显著大于CK（p<0.01）,各处理组新生萌条数量大于CK、萌条基径和萌条枝长极显著大于CK（p<0.01）,其中处理B株高和冠幅生长量分别较对照组增加241.43 %和417.21 %,平均萌条数量为324枝分别大于处理A、C、D和CK的293枝、229枝、221枝159枝;（2）对照植株净光合速率及蒸腾速率日变化均为双峰曲线,四种平茬处理植株平茬后第一个生长季7月份净光合速率和蒸腾速率为单峰曲线,而平茬后第二个生长季7月份变为双峰曲线;（3）平茬后前两个生长季7月份净光合速率日均值排序均为：处理B>D>C>A>CK,其中,处理B日均值分别为CK的2.07倍和1.94倍;（4）平茬处理改善了四合木植株水分状况,提高了四合木抵御干旱胁迫的能力,处理组蒸腾速率和水势均高于对照组;（5）平茬处理提高了四合木水分利用效率,其中,处理B提高程度最为明显,前两个生长季的水分利用效率分别是CK的1.38倍和1.42倍;（6）在平茬后前两个生长季内,5 cm平茬较其他高度平茬在生长和生理特性均有明显优势,但随平茬年限的增加其状况如何尚需进一步研究。     

Gas exchanges of an endangered species Syringa pinnatifolia and a widespread congener S. oblata

Net photosynthetic rate (P _N), transpiration rate (E), water use efficiency (WUE), stomatal conductance (g _s), and stomatal limitation (L_s) were investigated in two Syringa species. The saturation irradiance (SI) was 400 µmol m^-2s^-1 for S. pinnatifolia and 1 700 µmol m^-2s^-1 for S. oblata. Compared with S. oblata, S. pinnatifolia had extremely low g_s. Unlike S. oblata, the maximal photosynthetic rate (P _max) in S. pinnatifoliaoccurred around 08:00 and then fell down, indicating this species was sensitive to higher temperature and high photosynthetic photon flux density. However, such phenomenon was interrupted by the leaf development rhythms before summer. A relatively lower P _N together with a lower leaf area and shoot growth showed the capacity for carbon assimilation was poorer in S. pinnatifolia.This revised version was published online in March 2005 with corrections to the page numbers.     

中国北方杂草稻光合与水分生理特性

杂草稻是一类重要的稻属种质资源,具有耐寒、耐旱、耐瘠薄等优良特性.本文以88份中国北方杂草稻资源和4份栽培稻为材料,研究了中国北方杂草稻的光合速率、蒸腾速率、气孔导度等光合与水分生理特性及其相互关系.结果表明: 北方杂草稻资源的光合和水分生理特性存在较大差异,具有丰富的多样性.杂草稻的光合速率变化范围在12.47～28.67 μmol CO₂·m^-2·s^-1,瞬时水分利用效率的变化范围在1.39～3.40 mg·g^-1.光合参数中,胞间CO₂浓度的变异系数最小,气孔导度的变异系数最大.光合速率与蒸腾速率、气孔导度呈极显著的二次曲线关系,光合速率与胞间CO₂浓度呈显著的直线关系,瞬时水分利用效率与蒸腾速率、气孔导度呈极显著的二次曲线关系.可用杂草稻材料的优越性能对栽培稻进行品种改良.     

土壤水、氮供应对麻疯树幼苗光合特性的影响

在盆栽半控制试验中, 采用两因素的随机区组设计, 在3个土壤水分梯度(分别为80%、50%和30%的田间持水量(FC))下研究了施氮肥和不施氮肥处理麻疯树(Jatropha curcas)幼苗的光合特性。比较了不同水分和氮素供应条件下麻疯树幼苗的光合-光响应和CO₂响应曲线、PSII的最大光化学效率(F_v/F_m)、氮含量和光合色素含量之间的差异。结果表明: 1)施氮肥处理中, 随着土壤水分含量的增加, 叶片表观量子效率(AQY)、光补偿点、最大净光合速率、羧化效率、光呼吸速率、暗呼吸速率和叶片氮含量均呈现增加的趋势, 而且均在80% FC下最高。2)不施氮肥处理中, 随着土壤水分含量的增加, 麻疯树各光合参数均与施氮肥处理呈现相反的变化趋势。3)在30% FC下, 施氮肥处理和不施氮肥处理相比, 氮含量显著增加, AQY、F_v/F_m、光合色素含量无显著的变化, 其他各项指标均显著降低。这些结果表明, 水氮耦合效应对麻疯树光合特性有显著影响, 尤其是在80% FC下增施氮肥的效果最为明显。因此, 在土壤氮素含量不高的情况下, 麻疯树更适宜在较低的土壤水分下生长, 土壤水分较高反而不利于麻疯树的光合作用; 而如果增施氮肥, 麻疯树在土壤水分含量较高时生长更好。     

土壤水分状况对长白山阔叶红松林主要树种叶片生理生态特性的影响

现在水资源缺乏已成为全球性问题 ,对植物产生极大的影响 ,水分亏缺影响植物的整个生长过程 ,不论是外部形态还是内部结构以及各种代谢过程均受到影响。一般认为 ,植物的不同程度水分亏缺都对其生长不利 ,但也有的研究表明 ,适度的水分亏缺能促进植物的生长[13 ,15] ,这主要是由于不同植物在不同程度水分亏缺条件下碳同化与水分利用机制间存在差异的结果[6,7,14 ] 。本研究以温带典型森林类型阔叶红松林主要树种为研究对象 ,通过观察不同树种光合能力、水分利用率等生理生态特性对不同土壤水分条件响应程度和耐干旱程度 ,为进一步分析未来气…     

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.