Diffusive and metabolic limitations to photosynthesis under drought and salinity in C(3) plants

Drought and salinity are two widespread environmental conditions leading to low water availability for plants. Low water availability is considered the main environmental factor limiting photosynthesis and, consequently, plant growth and yield worldwide. There has been a long-standing controversy as to whether drought and salt stresses mainly limit photosynthesis through diffusive resistances or by metabolic impairment. Reviewing in vitro and in vivo measurements, it is concluded that salt and drought stress predominantly affect diffusion of CO(2) in the leaves through a decrease of stomatal and mesophyll conductances, but not the biochemical capacity to assimilate CO(2), at mild to rather severe stress levels. The general failure of metabolism observed at more severe stress suggests the occurrence of secondary oxidative stresses, particularly under high-light conditions. Estimates of photosynthetic limitations based on the photosynthetic response to intercellular CO(2) may lead to artefactual conclusions, even if patchy stomatal closure and the relative increase of cuticular conductance are taken into account, as decreasing mesophyll conductance can cause the CO(2) concentration in chloroplasts of stressed leaves to be considerably lower than the intercellular CO(2) concentration. Measurements based on the photosynthetic response to chloroplast CO(2) often confirm that the photosynthetic capacity is preserved but photosynthesis is limited by diffusive resistances in drought and salt-stressed leaves.     

Photosynthetic performance of two coffee species under drought

Coffea arabica cv. Red Catuaí and C. canephora cv. Kouillou were grown in pots beneath a plastic shelter. When they were 14 months old, irrigation was withheld until the leaf pre-dawn water potential was about -1.5 and -2.7 MPa (designated mild and severe water stress, respectively). Under mild stress, net photosynthetic rate (PN) decreased mainly as a consequence of stomatal limitations in Kouillou, whereas such decreases were dominated by non-stomatal limitations in Catuaí. Under severe drought, further decreases in PN and apparent quantum yield were not associated to any changes in stomatal conductance in either cultivar. Decreases were much more pronounced in Catuaí than in Kouillou, the latter maintained carbon gain at the expense of water conservation. In both cultivars the initial chlorophyll (Chl) fluorescence slightly increased with no changes in the quantum efficiency of photosystem 2. In response to rapidly imposed drought, the Chl content did not change while saccharide content increased and starch content decreased. Photoinhibition and recovery of photosynthesis, as evaluated by the ratio of variable to maximum fluorescence and by the photosynthetic O2 evolution, were unaffected by mild drought stress. Photoinhibition was enhanced under severe water deficit, especially in Catuaí. In this cultivar the O2 evolution did not resume upon reversal from photoinhibition, in contrast to the complete recovery in Kouillou.     

Photosynthetic performance of two coffee species under drought

Coffea arabica cv. Red Catuaí and C. canephora cv. Kouillou were grown in pots beneath a plastic shelter. When they were 14 months old, irrigation was withheld until the leaf pre-dawn water potential was about -1.5 and -2.7 MPa (designated mild and severe water stress, respectively). Under mild stress, net photosynthetic rate (PN) decreased mainly as a consequence of stomatal limitations in Kouillou, whereas such decreases were dominated by non-stomatal limitations in Catuaí. Under severe drought, further decreases in PN and apparent quantum yield were not associated to any changes in stomatal conductance in either cultivar. Decreases were much more pronounced in Catuaí than in Kouillou, the latter maintained carbon gain at the expense of water conservation. In both cultivars the initial chlorophyll (Chl) fluorescence slightly increased with no changes in the quantum efficiency of photosystem 2. In response to rapidly imposed drought, the Chl content did not change while saccharide content increased and starch content decreased. Photoinhibition and recovery of photosynthesis, as evaluated by the ratio of variable to maximum fluorescence and by the photosynthetic O2 evolution, were unaffected by mild drought stress. Photoinhibition was enhanced under severe water deficit, especially in Catuaí. In this cultivar the O2 evolution did not resume upon reversal from photoinhibition, in contrast to the complete recovery in Kouillou. This revised version was published online in June 2006 with corrections to the Cover Date.     

Drought Stress Memory in Sugar Beet: Mismatch Between Biochemical and Physiological Parameters

In a greenhouse study, we aimed to determine whether a temporary water deficit induces ‘drought memory’ in sugar beet (Beta vulgaris L.), and whether this effect can be quantified by alterations in the fluorescence signature of the leaves. Plants were subjected to three consecutive water deficit phases, each followed by a recovery period, and in each cycle new, fully developed leaves were analyzed. Changes in the photosynthetic performance and pigment fluorescence were recorded with a hand-held fluorescence sensor, a laser-induced fluorescence spectrometer, and a leaf gas exchange analyzer. Parameters such as osmotic potential, proline, and chlorophyll content were used as indicators for biochemical modifications and quantification of stress intensity. In general, the evaluated cultivars showed a similar response pattern to water deficit, although the intensity of the stress-induced modification was not always on the same level in the distinct parameters. The long-term and repeated drought caused a decrease of net photosynthesis, increase of far-red fluorescence, and a decrease of both the ‘Simple Fluorescence Ratio’ and the fluorescence lifetime (LT mean) in the blue spectral region. In the second drought cycle, changes in osmotic potential and proline content were lower, but alterations in photosynthesis and fluorescence were as strong as in the first and third drought cycles. This indicates that even if a drought stress memory might occur, it was not possible to precisely identify it using gas exchange and pigment fluorescence determinations. Irrespective of that, the photosynthesis and chlorophyll fluorescence-based parameters (RF, SFR) clearly indicated with high temporal resolution the response of sugar beet plants to the stress, and their partial recovery.     

Influence of drought intensity on the response of six woody karst species subjected to successive cycles of drought and rewatering

Tolerance to the effects of drought and subsequent recovery after a rainfall appear to be critical for plants in the karst regions of southwestern China, which are characterized by frequent but temporary drought events. This study investigated the effects of drought intensity and repetition on photosynthesis and photoprotection mechanisms of karst plants during successive cycles of drought and subsequent recovery. Leaf water potential, gas exchange, chlorophyll fluorescence and several associated metabolic processes were studied in six plant species, including Pyracantha fortuneana (PF), Rosa cymosa (RC), Broussonetia papyrifera (BP), Cinnamomum bodinieri (CB), Platycarya longipes (PL) and Pteroceltis tatarinowii (PT) during three cycles of drought treatments at four different intensities. The four treatments were: well‐watered, mild drought, moderate drought and severe drought, each followed by rewatering events. We found that limitations to CO₂ diffusion accounted for photosynthetic declines under mild and moderate drought treatments, while metabolic limitations dominated the response to severe drought. Repetition of drought did not intensify the impairment of photosynthetic metabolism regardless of drought intensity in the six species studied. Repetition of severe drought delayed the photosynthetic recoveries in PF, RC and CB after rewatering. Repetition of drought increased thermal dissipation in PF, CB and BP, as well as superoxide dismutase (EC 1.15.1.1) activity in RC and CB. Enhanced photosynthetic performance, measured as increased intrinsic water use efficiency, photosynthetic performance per unit of photosynthetic pigment, maintenance of high thermal dissipation and high ratios of carotenoids to chlorophylls, was observed during the rewatering periods. This enhanced photosynthetic performance allowed for the complete recovery of the six karst species from successive intermittent drought events.     

Field Study of the Interaction between Solar Ultraviolet-B Radiation and Drought on Photosynthesis and Growth in Soybean

Soybean, Glycine max (L.) Merr. cv Essex, plants were grown in the field in a 2 × 2 factorial design, under ambient and supplemental levels of ultraviolet-B (UV-B) radiation (supplemental daily dose of 5.1 effective kilojoules per square meter) and were either well-watered or subjected to drought. Soil water potentials were reduced to −2.0 megapascals by the exclusion of natural precipitation in the drought plots and were maintained at approximately −0.5 megapascal by supplemental irrigation in well-watered plots. Plant growth and gas exchange characteristics were affected under both drought and supplemental UV-B radiation. Whole-leaf gas exchange analysis indicated that stomatal limitations on photosynthesis were only significantly affected by the combination of UV-B radiation and drought but substrate (ribulose bisphosphate) regeneration limitations were observed under either stress. The combined effect of both drought and UV-B radiation on photosynthetic gas exchange was a reduction in apparent quantum efficiency and the rapid appearance of biochemical limitations to photosynthesis concomitant with reduced diffusional limitations. However, the combination of stresses did not result in additive effects on total plant growth or seed yield compared to reductions under either stress independently.     

Overaccumulation of glycine betaine enhances tolerance to drought and heat stress in wheat leaves in the protection of photosynthesis

We investigated the different responses of wheat (Triticum aestivum L.) plants to drought- (DS) and heat stress (HS), and analyzed the physiological mechanisms of glycine betaine (GB) involved in the improvement of wheat tolerance to the combination of these stresses. The transgenic wheat T6 line was generated by introducing a gene encoding betaine aldehyde dehydrogenase (BADH) into the wild-type (WT) Shi4185 line. The gene was cloned from the Garden Orache plant (Atriplex hortensis L.). Wheat seedlings were subjected to drought stress (30%, PEG-6000), heat stress (40°C), and their combination. Photosynthetic gas exchange, water status and lipid peroxidation of wheat leaves were examined under different stresses. When subjected to a combination of drought and heat, the inhibition of photosynthesis was significantly increased compared to that under DS or HS alone. The increased inhibition of photosynthesis by the combined stresses was not simply the additive stress effect of separate heat- and drought treatments; different responses in plant physiology to DS and HS were also found. HS decreased the chlorophyll (Chl) content, net photosynthetic rate (P _N), carboxylation efficiency (CE) and apparent quantum yield (AQY) more than DS but DS decreased the transpiration rate (E), stomata conductance (g _s) and intercellular CO₂ concentration (C _i) more than HS. GB over-accumulation led to increased photosynthesis not only under individual DS or HS but also under their combination. The enhancement of antioxidant activity and the improvement of water status may be the mechanisms underlying the improvement of photosynthesis by GB in wheat plants.     

Photosynthetic limitations in response to water stress and recovery in Mediterranean plants with different growth forms

* Whether photosynthesis is limited during water stress and recovery because of diffusive or biochemical factors is still open to debate, and apparent contradictions appear when various studies on species with different growth forms are compared. * Ten Mediterranean species, representing different growth forms, were subjected to different levels of water stress, the most severe followed by rewatering. A quantitative limitation analysis was applied to estimate the effects of water stress on stomatal (S(L)), mesophyll conductance (MC(L)) and biochemical limitations (B(L)). * Results confirmed a general pattern of photosynthetic response to water stress among C(3) plants when stomatal conductance (g(s)) is used as a reference parameter. As g(s) values decreased from a maximum to approx. 0.05 mol H(2)O m(-2) s(-1), the total photosynthetic limitation rose from 0 to approx. 70%, and this was caused by a progressive increase of both S(L) and MC(L) limitations, while B(L) remained negligible. When lower values of g(s) were achieved (total photosynthetic limitation increased from 70 to 100%), the contribution of S(L) declined, while MC(L) still increased and B(L) contributed significantly (20-50%) to the total limitation. * Photosynthetic recovery of severely stressed plants after rewatering showed a dominant role of MC(L), irrespective of the degree of photosynthesis recovery.     

Differences in drought sensitivities and photosynthetic limitations between co-occurring C3 and C4 (NADP-ME) Panicoid grasses

Background and Aims

The success of C₄ plants lies in their ability to attain greater efficiencies of light, water and nitrogen use under high temperature, providing an advantage in arid, hot environments. However, C₄ grasses are not necessarily less sensitive to drought than C₃ grasses and are proposed to respond with greater metabolic limitations, while the C₃ response is predominantly stomatal. The aims of this study were to compare the drought and recovery responses of co-occurring C₃ and C₄ NADP-ME grasses from the subfamily Panicoideae and to determine stomatal and metabolic contributions to the observed response.

Methods

Six species of locally co-occurring grasses, C₃ species Alloteropsis semialata subsp. eckloniana, Panicum aequinerve and Panicum ecklonii, and C₄ (NADP-ME) species Heteropogon contortus, Themeda triandra and Tristachya leucothrix, were established in pots then subjected to a controlled drought followed by re-watering. Water potentials, leaf gas exchange and the response of photosynthetic rate to internal CO₂ concentrations were determined on selected occasions during the drought and re-watering treatments and compared between species and photosynthetic types.

Key Results

Leaves of C₄ species of grasses maintained their photosynthetic advantage until water deficits became severe, but lost their water-use advantage even under conditions of mild drought. Declining C₄ photosynthesis with water deficit was mainly a consequence of metabolic limitations to CO₂ assimilation, whereas, in the C₃ species, stomatal limitations had a prevailing role in the drought-induced decrease in photosynthesis. The drought-sensitive metabolism of the C₄ plants could explain the observed slower recovery of photosynthesis on re-watering, in comparison with C₃ plants which recovered a greater proportion of photosynthesis through increased stomatal conductance.

Conclusions

Within the Panicoid grasses, C₄ (NADP-ME) species are metabolically more sensitive to drought than C₃ species and recover more slowly from drought.     

Differences in the physiological state between triticale and maize plants during drought stress and followed rehydration expressed by the leaf gas exchange and spectrofluorimetric methods

The studies were carried out in order to estimate differences in the physiological state between triticale and maize plants subjected to drought stress followed by rehydration. The physiological state of the plants was evaluated by measurements of leaf water potential, net photosynthesis, transpiration and stomatal conductance. Spectrofluorimetric methods for the study of blue, green and red fluorescence were applied. We observed that the soil drought induced a greater water loss in triticale leaves than in maize and consequently caused greater injuries to the photosynthetic apparatus. Moreover, triticale plant recovery was slower than in maize plants during the rehydration phase. The effect was probably connected with the higher functional and structural disorganisation of the photosynthetic apparatus observed during drought stress in triticale. Water stress is responsible for damages to photosystem PS II. The worst light utilisation in photosynthetic light conversion was recorded as an increase in the intensity of red fluorescence. Drought stress induced a strong increase in the intensity of blue and green fluorescence in the studied species and it was still high in maize plants during the first day of rehydration. Increase in the intensity of blue and green fluorescence in maize seems to be the effect of the photoprotection mechanism which prevents damage to PS II through utilisation of excess energy.     

水分胁迫对冬小麦CO_2同化作用的影响

比较了两个小麦品种陕合6号和郑引1号经受不同程度的水分胁迫处理后,叶片多种光合参数:净光合速率(P_n)、气孔导度(G_s)、细胞间隙CO_2浓度(G_i)、表观量子需要量和羧化效率以及Rubis CO蛋白量与活性等的变化。在轻度水分胁迫下,叶片光合速率降低的根本原因在于气孔导度的下降;而在严重胁迫下,非气孔因素起主要作用。     

Drought stress memory in the photosynthetic mechanisms of an invasive CAM species, <Emphasis Type="Italic">Aptenia cordifolia</Emphasis>

Plants are known for their high capacity to acclimatise to fluctuating environmental conditions. A wide range of environmental conditions can lead to suboptimal physiological efficiency. However, recent studies have shown that plants can withstand repeated periods of stress. To find out how they do it, we studied photosynthetic adjustments to repeated water stress in Aptenia cordifolia: a facultative, invasive CAM species. Plants were subjected to three cycles of water deficit, and photosynthetic parameters and chloroplast antioxidants were quantified to gain an understanding of the mechanisms by which they cope with repeated stress periods. Significant modification of the photosystems’ antenna and reaction centres was observed in plants subjected to previous water stress cycles, and this led to higher PSII efficiency than in plants challenged with drought for the first time. These findings underline the biological significance of stress memory and show how plants can adjust their photosynthetic apparatus to fluctuating environmental conditions and thus optimise photosynthesis and photoprotection under drought conditions.     

干旱胁迫对夏蜡梅光合特性的影响

以2年生夏蜡梅(Sinocalycanthus chinensis)苗木为材料,通过盆栽试验,研究土壤干旱胁迫对夏蜡梅叶片光合特性的影响,结果显示:随着干旱胁迫程度的加重,夏蜡梅叶片净光合速率、蒸腾速率、气孔导度显著降低;胞间CO2浓度在轻度和中度胁迫下显著低于对照,但在重度胁迫下显著高于对照;水分利用效率在胁迫下提高,且以中度胁迫为最大.温度升高使叶片净光合速率和水分利用效率降低、蒸腾速率升高,加重了干旱对光合作用的不利影响.对照及轻度和中度干旱胁迫处理的净光合速率、气孔导度和水分利用效率日变化曲线均为双峰型,但重度胁迫下净光合速率和气孔导度日变化转变为峰值很小的单峰型;各处理的蒸腾速率日变化曲线为单峰型并以午间最高.表明夏蜡梅光合作用对干旱胁迫有一定的适应能力,但重度胁迫对其造成严重影响.     

土壤水分梯度对白三叶(Trifolium repens)光合作用和根系分布的影响

通过对两个品种白三叶Trifolium repens cv.Haifa(海发)和Trifolium repens cv.Rivendel(瑞文德)盆栽试验,模拟3种不同的土壤水分状况(无水分胁迫:保持植株良好的水分供应;轻度胁迫:表层0～20cm土壤处于干旱状态;重度胁迫:表层0～20cm土壤处于极干旱状态,20～40cm土壤处于干旱状态)对白三叶光合作用和根系生长的影响.结果表明,当植株未遭受水分胁迫时,两个品种白三叶的光合作用和根系生长状况没有明显差异;当表层0～20cm处于干旱状态时,'海发'在处理后期的净光合速率和水分利用效率升高,根系生长量增大,表现出促进作用,'瑞文德'受到的影响不显著;当表层0～20cm处于极干旱、20～40cm处于干旱状态时,'海发'在处理前期受到轻微影响,随后恢复正常状态,'瑞文德'则受到较严重的影响.随着干旱程度的加深和时间的延长,白三叶的根冠比逐渐增大.与'瑞文德'相比,在相同时期相同胁迫程度下,'海发'的根冠比没有显著差异,但深根数量大大超过'瑞文德',因而,'海发'的耐旱能力强于'瑞文德'.     

小麦籽粒发育初期土壤水分亏缺对植株各部位光合作用的影响

小麦开花后5～16天内给予不同程度的土壤干旱处理。无论轻、中或重度干旱均不同程度地释低了植株的净光合率,但不同器官或部位对干旱的反应不同,上部器官的光合受影响较小,故随着干旱的加重在全株光合中所占比例逐渐增加。恢复灌水后,曾受轻度及中度干旱的植株,中、上部器官的光合有补偿性增强,因而可能减轻产量的损失。     

Photosynthesis in Encelia farinosa Gray in Response to Decreasing Leaf Water Potential

Photosynthetic responses of intact leaves of the desert shrub Encelia farinosa were measured during a long term drought cycle in order to understand the responses of stomatal and nonstomatal components to water stress. Photosynthetic rate at high irradiance and leaf conductance to water vapor both decreased linearly with declining leaf water potential. The intercellular CO₂ concentration (c_i) remained fairly constant as a function of leaf water potential in plants subjected to a slow drought cycle of 25 days, but decreased in plants exposed to a 12-day drought cycle. With increasing water stress, the slope of the dependence of photosynthesis on c_i (carboxylation efficiency) decreased, the maximum photosynthetic rates at high c_i became saturated at lower values, and water use efficiency increased. Both the carboxylation efficiency and photosynthetic rates were positively correlated with leaf nitrogen content. Associated with lower leaf conductances, the calculated stomatal limitation to photosynthesis increased with water stress. However, because of simultaneous changes in the dependence of photosynthesis on c_i with water stress, increased leaf conductance alone in water-stressed leaves would not result in an increase in photosynthetic rates to prestressed levels. Both active osmotic adjustment and changes in specific leaf mass occurred during the drought cycle. In response to increased water stress, leaf specific mass increased. However, the increases in specific leaf mass were associated with the production of a reflective pubescence and there were no changes in specific mass of the photosynthetic tissues. The significance of these responses for carbon gain and water loss under arid conditions are discussed.     

干旱胁迫对华北绣线菊和金山绣线菊光合能力的影响

分别采取轻度、中度、重度干旱胁迫和复水处理,研究华北绣线菊和金山绣线菊的光合能力动态变化;利用二维双向电泳与质谱鉴定等技术,分析鉴定干旱胁迫前后2种绣线菊蛋白质的差异表达,以及引起其光合能力改变的生理机制.结果表明:干旱胁迫处理显著影响了2种绣线菊的光合能力,最大光合速率、光补偿点和光饱和点逐渐下降,其干旱胁迫反应为渐进效应.轻度和中度干旱胁迫后,2种绣线菊的恢复能力较强;而重度干旱胁迫后的恢复能力较弱.经干旱胁迫诱导后,抗旱能力弱的金山绣线菊有6处蛋白点消失、11处新增蛋白点、13处蛋白点上调表达、4处蛋白点下调表达,均为低分子量酸性蛋白;其中由干旱诱导表达的3种差异蛋白分别为放氧增强蛋白因子1、2和1,5-二磷酸核酮糖羧化酶/加氧酶大亚基的降解片断.绣线菊抗旱能力的差异与干旱胁迫期间光合能力的变化有关.