Effects of salinity on photosynthesis,leaf anatomy,ion accumulation and photosynthetic nitrogen use efficiency in five Indian mangroves

Five species of mangroves (Bruguiera gymnorrhiza, Excoecaria agallocha, Heritiera fomes, Phoenix paludosa and Xylocarpus granatum) were investigated with respect to their photosynthesis rate, chlorophyll content, mesophyll conductance, specific leaf area, stomatal conductance and photosynthetic nitrogen use efficiency under saline (15–27 PPT) and non-saline (1.8–2 PPT) conditions. Some inorganic elements were estimated from the leaf samples to compare the concentrations with change in salinity. Elevated assimilation rate coupled with increased chlorophyll content, more mesophyll and stomatal conductance and higher specific leaf area in non-saline condition indicates that these mangroves can grow well even with minimal salinity in soil. In B. gymnorrhiza, E. agallocha and P. paludosa the optimum PAR acquisition for photosynthesis was higher under salt stress, while the maximal rate of assimilation was lower even with minimal salinity. H. fomes and X. granatum followed the opposite trend, where the peak photosynthesis rate was lower under non-saline conditions even at a higher irradiance than in the saline forest. This indicates less affinity of H. fomes and X. granatum to high substrate salinity. Accumulation of Na⁺ increased in plants in saline substrate, while in most of the species, salinity imposed reduction in Ca⁺ and Mg⁺ uptake. Increased K⁺ content can be attributed to high substrate level K⁺ in non-saline soil. Trace amount of salinity induced Cu⁺⁺ detected in leaves of H. fomes may impart some toxic effects. Photosynthetic nitrogen use efficiency increased in non-saline soil that can be attributed to higher photosynthetic peak in most of the species and/or lower nitrogen accumulation in plant samples.     

Photosynthesis and antioxidative enzyme activities in five Indian mangroves with respect to their adaptability

Five typical mangroves were taken (Bruguiera gymnorrhiza, Excoecaria agallocha, Heritiera fomes, Phoenix paludosa and Xylocarpus granatum) both from Sundarbans (in situ) and grown in a mesophytic environment (ex situ, in the Institute’s premises) for 12–15 years. A comparative account of PAR utilization for maximum photosynthesis, stomatal conductance and production of two antioxidant enzymes (peroxidase and Superoxide dismutase) were done between the in situ and ex situ habitats. The present work revealed that the average net photosynthesis was slightly higher in mangroves from non-saline habitats than that of the native ones. At the same time, stomatal conductances were remarkably reduced under salinity-stressed habitats when compared with those of the mesophytic counterparts, by 25–52%. Salinity imposed increase of antioxidant enzymes was observed. Both the investigated antioxidant enzymes showed considerable increase in saline-grown individuals and proved their efficient scavenging ability to evolve reactive oxygen species (ROS), but these increases were relatively lower in Heritiera and Xylocarpus even though the net photosynthesis was higher. This might be related to their lower adaptability under increased salinity stress than those of the other three species investigated.     

Salicylic acid alleviates decreases in photosynthesis under salt stress by enhancing nitrogen and sulfur assimilation and antioxidant metabolism differentially in two mungbean cultivars

Salicylic acid (SA) is known to affect photosynthesis under normal conditions and induces tolerance in plants to biotic and abiotic stresses through influencing physiological processes. In this study, physiological processes were compared in salt-tolerant (Pusa Vishal) and salt-sensitive (T44) cultivars of mungbean and examined how much these processes were induced by SA treatment to alleviate decrease in photosynthesis under salt stress. Cultivar T44 accumulated higher leaf Na⁺ and Cl⁻ content and exhibited greater oxidative stress than Pusa Vishal. Activity of antioxidant enzymes, ascorbate peroxidase (APX) and glutathione reductase (GR) was greater in Pusa Vishal than T44. Contrarily, activity of superoxide dismutase (SOD) was greater in T44. The greater accumulation of leaf nitrogen and sulfur through higher activity of their assimilating enzymes, nitrate reductase (NR) and ATP-sulfurylase (ATPS) increased reduced glutathione (GSH) content more conspicuously in Pusa Vishal than T44. Application of 0.5 mM SA increased nitrogen and sulfur assimilation, GSH content and activity of APX and GR. This resulted in the increase in photosynthesis under non-saline condition and alleviated the decrease in photosynthesis under salt stress. It also helped in restricting Na⁺ and Cl⁻ content in leaf, and maintaining higher efficiency of PSII, photosynthetic N-use efficiency (NUE) and water relations in Pusa Vishal. However, application of 1.0 mM SA resulted in inhibitory effects. The effect of SA was more pronounced in Pusa Vishal than T44. These results indicate that SA application alleviates the salt-induced decrease in photosynthesis mainly through inducing the activity of NR and ATPS, and increasing antioxidant metabolism to a greater extent in Pusa Vishal than T44.     

Genetic variation and plasticity of Plantago coronopus under saline conditions

Phenotypic plasticity may allow organisms to cope with variation in the environmental conditions they encounter in their natural habitats. Salt adaptation appears to be an excellent example of such a plastic response. Many plant species accumulate organic solutes in response to saline conditions. Comparative and molecular studies suggest that this is an adaptation to osmotic stress. However, evidence relating the physiological responses to fitness parameters is rare and requires assessing the potential costs and benefits of plasticity. We studied the response of thirty families derived from plants collected in three populations of Plantago coronopus in a greenhouse experiment under saline and non-saline conditions. We indeed found a positive selection gradient for the sorbitol percentage under saline conditions: plant families with a higher proportion of sorbitol produced more spikes. No effects of sorbitol on fitness parameters were found under non-saline conditions.Populations also differed genetically in leaf number, spike number, sorbitol concentration and percentages of different soluble sugars. Salt treatment led to a reduction of vegetative biomass and spike production but increased leaf dry matter percentage and leaf thickness. Both under saline and non-saline conditions there was a negative trade-off between vegetative growth and reproduction. Families with a high plasticity in leaf thickness had a lower total spike length under non-saline conditions. This would imply that natural selection under predominantly non-saline conditions would lead to a decrease in the ability to change leaf morphology in response to exposure to salt. All other tests revealed no indication for any costs of plasticity to saline conditions.     

Photosynthetic and wood anatomical responses of Tamarix africana Poiret to water level reduction after short-term fresh- and saline-water flooding

In the Mediterranean basin, Tamarix spp. constitute important populations along rivers and sea coasts, and might be primarily subjected to water level fluctuations and salinization, as a consequence of global climate change. Here, we analyze leaf gas exchange and xylem anatomy during a water level decrease below the soil surface after short-term flooding with fresh- and saline-water (200?mM) in order to predict Tamarix africana Poiret responses under future environmental conditions. Fresh-water level reduction negatively affected stomatal conductance (?56.3?%), but only when water decreased to the lowest level (15?cm below the soil surface). No effects on assimilation rates and xylem vessel dimensions occurred. Under saline conditions, the rate of the water level decrease was lower compared to the non-saline treatment, as stomatal conductance was negatively affected by salinity (?59.5?%) and significantly declined over time. Moreover, decreases in mean xylem vessel area (?51.3?%), assimilation rates (?52.2?%) and stomatal conductance (?76.0?%) were also observed compared to the control, indicating both an osmotic stress and a toxic effect of NaCl on leaf gas exchange. These leaf responses were probably induced by greater belowground-root salt absorption and transport compared to previous flooding conditions, as confirmed by the increase in salt excretion (+473.2?%). The results emphasize the survival risk of Tamarix spp. to water level variation under both saline and non-saline conditions, and the need of management practices focused on the conservation of these populations.     

盐胁迫对不同生境白榆生理特性与耐盐性的影响

以中强度盐土、轻度盐土和非盐土3种生境白榆种子实生苗为试验材料,研究了不同程度盐胁迫(CK、2、4、6、8和10 g·kg-1)条件下3种生境白榆幼苗的耐盐阈值及生理特性.结果表明:随着土壤盐浓度的增加,中强度和轻度盐土生境白榆幼苗叶片的细胞膜透性、Na+含量和Na+/K+增幅低于非盐土生境;叶片的脯氨酸、可溶性糖和K+含量增幅高于非盐土生境;叶片淀粉含量、净光合速率、蒸腾速率、胞间CO2浓度和气孔导度降幅小于非盐土生境.不同生境白榆耐盐性的强弱顺序为:中强度盐土生境(7.76 g·kg-1)＞轻度盐土生境(7.37g·kg-1)＞非盐土生境(6.95 g·kg-1).与非盐土生境相比,中强度和轻度盐土生境白榆各项生理指标对盐土环境的适应能力更强.     

Effects of addition of external nitric oxide on the allocation of photosynthetic electron flux in Rumex K-1 leaves under osmotic shock

Photosynthetic electron flux allocation, stomatal conductance, and the activities of key enzymes involved in photosynthesis were investigated in Rumex K-1 leaves to better understand the role of nitric oxide (NO) in photoprotection under osmotic stress caused by polyethylene glycol. Gas exchange and chlorophyll fluorescence were measured simultaneously with a portable photosynthesis system integrated with a pulse modulated fluorometer to calculate allocation of photosynthetic electron fluxes. Osmotic stress decreased stomatal conductance, photosynthetic carbon assimilation, and nitrate assimilation, increased Mehler reaction, and resulted in photoinhibition. Addition of external NO enhanced the stomatal conductance, photosynthetic rate, activities of glutamine synthetase and nitrate reductase, and reduced Mehler reaction and photoinhibition. These results demonstrated that osmotic stress reduced CO₂ assimilation, decreasing the use of excited energy via CO₂ assimilation which caused significant photoinhibition. Improving stomatal conductance by the addition of external NO enhanced the use of excited energy via CO₂ assimilation. As a result, less excited energy was allocated to Mehler reaction, which reduced production of reactive oxygen species via this pathway. We suppose that Mehler reaction is not promoted unless photosynthesis and nitrogen metabolism are prominently inhibited.     

Modulation in growth,photosynthetic efficiency,activity of antioxidants and mineral ions by foliar application of glycinebetaine on pea (Pisum sativum L.) under salt stress

A pot experiment was carried out to explore the role of glycinebetaine (GB) as foliar spray foliar on two pea (Pisum sativum L.) varieties (Pea 09 and Meteor Fsd) under saline and non-saline conditions. Thirty-two-day-old plants were subjected to two levels 0 and 150 mM of NaCl stress. Salt treatment was applied in full strength Hoagland’s nutrient solution. Three levels 0, 5 and 10 mM of GB were applied as foliar treatment on 34-day-old pea plants. After 2 weeks of foliar treatment with GB data for various growth and physiochemical attributes were recorded. Rooting-medium applied salt (150 mM NaCl) stress decreased growth, photosynthesis, chlorophyll, chlorophyll fluorescence and soluble protein contents, while increasing the activities of enzymatic (POD and CAT) and non-enzymatic (ascorbic acid and total phenolics) antioxidant enzymes. Foliar application of GB decreased root and shoot Na⁺ under saline conditions, while increasing shoot dry matter, root length, root fresh weight, stomatal conductance (g _s), contents of seed ascorbic acid, leaf phenolics, and root and shoot Ca²⁺ contents. Of three GB (0, 5, 10 mM) levels, 10 mM proved to be more effective in mitigating the adverse effects of salinity stress. Overall, variety Pea 09 showed better performance in comparison to those of var. Meteor Fsd under both normal and salinity stress conditions. GB-induced modulation of seed ascorbic acid, leaf phenolics, g _s, and root Ca²⁺ values might have contributed to the increased plant biomass, reduction of oxidative stress, increased osmotic adjustment and better photosynthetic performance of pea plants under salt stress.     

Modelling C3 and C4 photosynthesis under water-stressed conditions

Despite the observed impact of water stress on photosynthesis, some of the most used models of CO₂ assimilation in C₃ and C₄ functional types do not directly account for it. We discuss an extension of these models, which explicitly includes the metabolic and diffusive limitations due to water stress on photosynthesis. Functional relationships describing the photosynthetic processes and CO₂ diffusion inside leaves are modified to account for leaf water status on the basis of experimental results available in the literature. Extensive comparison with data shows that the model is suitable to describe the reduction in CO₂ assimilation rate with decreasing leaf water potentials in various species. A simultaneous analysis of photosynthesis, transpiration and soil moisture dynamics is then carried out to explore the actual impact of drought on different photosynthesis processes and on the overall plant activity. The model well reproduces measured CO₂ assimilation rate as a function of soil moisture and could be useful to formulate hypotheses for detailed experiments as well as to simulate in detail transpiration and photosynthesis dynamics under water stress.     

Photosynthetic response of three climber plant species to osmotic stress induced by polyethylene glycol (PEG) 6000

We examined the photosynthesis response to osmotic stress in three climber plant species, Pharbitis nil (Linn.) Choisy, Lonicera japonica Thunb, and Parthenocissus tricuspidata (Sieb.et Zucc.) Planch. All climber plants were exposed to osmotic stress induced by polyethylene glycol (PEG) 6000 at 4 levels (slight, moderate, severe osmotic and the control) for 30?days. Photosynthesis response was determined by measuring leaf photosynthesis, chlorophyll fluorescence, carbonic anhydrase activity and stable carbon isotope ratios. P. nil maintained high photosynthetic activity under long-term moderate osmotic stress due to both stable photosystem II photochemical efficiency and high carbonic anhydrase activity. L. japonica maintained high photosynthetic activity under long-term moderate stress due to high carbonic anhydrase activity rather than photosystem II photochemical efficiency. P. tricuspidata tolerated only short-term moderate osmotic stress and long-term slight osmotic stress because its response was mainly stomatal limitation, with the lowest photosynthetic activity and hardly any carbonic anhydrase activity. Carbonic anhydrase activity was inversely correlated with stable carbon isotope ratios. The regulation by carbonic anhydrase was probably the reason for P. nil and L. japonica to tolerate long-term moderate osmotic stress. The selection on the species should consider the differential adaptation mechanism to osmotic stress during the development of drought-resistant plants.     

Gas-Exchange Properties of Salt-Stressed Olive (Olea europea L.) Leaves

The effects of two levels of salinity on photosynthetic properties of olive (Olea europea L.) leaves were observed either in low or in high H₂O vapor pressure deficit (vpd). Under moderate salt stress, stomata were found to be less open and responsive both to light and vpd, but the predominant limitation of photosynthesis was due to the mesophyll capacity of CO₂ fixation. We elaborate a procedure to correlate mesophyll capacity and liquid phase diffusive conductance. The estimated liquid phase diffusive conductance was reduced by salt and especially by high vpd; morphological and physiological changes could be responsible for this reduction. As a result, the chloroplast CO₂ partial pressure was found to decrease both under salt and vpd stress, thus resulting in a ribulose-1,5-bisphosphate carboxylase limitation of assimilation. However, under combined salt and vpd stress, O₂ sensitivity of assimilation increased, as would be expected under conditions of limiting ribulose 1,5-bisphosphate regeneration. Fluorescence induction measurements indicated that, under these conditions, energy supply may become limiting. When Cl⁻ concentration exceeded 80 millimolar in tissue water, zero growth and 50% leaf drop was observed. Fluorescence induction showed irreversible damage at Cl⁻ levels higher than 200 millimolar and basal leaves reached this concentration earlier than the apical ones.     

Responses of leaf growth and gas exchanges to salt stress during reproductive stage in wild wheat relative Aegilops geniculata Roth. and wheat (Triticum durum Desf.)

In order to investigate the effect of salinity on the growth and photosynthesis of the wild wheat and wheat, three accessions of Aegilops geniculata from Ain Zana, Zaghouan and Sbitla and one variety of durum wheat (Triticum durum) were grown in the INRAT greenhouse and treated with different salinity levels. The growth of leaves, water status and gas exchange parameters have been measured at the reproductive stage. The flag leaf length, total leaf dry weight, water status, CO₂ assimilation rate, stomatal conductance, intercellular CO₂ and transpiration for the three Ae. geniculata accessions and wheat variety significantly decreased with increasing salt. The decline in photosynthesis measured in response to salt stress was proportionally greater than the declines in transpiration, resulting in a reduction of water-use efficiency, at both the leaf and whole-plant levels. Among the factors inhibiting photosynthetic activity, those of a stomatal nature had a greater effect. This study has shown a high degree of variation of these characters mainly related to geographical origin. It was observed also that Sbitla accession was less affected by the imposed salt stress than all the others while Ain Zana was the most affected one.     

Photosynthetic and growth responses of two mustard cultivars differing in phytocystatin activity under cadmium stress

Cadmium inhibits photosynthetic capacity of plants by disturbing protein conformations, whereas phytocystatins prevent degradation of target proteins and are involved in abiotic stress tolerance. Two mustard (Brassica juncea L.) cultivars, Ro Agro 4001 and Amruta, were grown with Cd (50 µM) in order to study physiological and biochemical basis of differences in Cd tolerance. Amruta accumulated higher Cd and H₂O₂ concentrations in leaves than that of Ro Agro 4001. Cd significantly decreased photosynthesis and growth of plants in both cultivars by reducing a chlorophyll content, gas exchange parameters, and activity of Rubisco; the effects were more prominent in Amruta than those in Ro Agro 4001. The greater photosynthesis and growth of Ro Agro 4001 under Cd stress might be attributed to its higher phytocystatin activity together with greater ascorbate peroxidase activity, photosynthetic nitrogen-use efficiency, sulphur assimilation (ATP-sulphurylase activity and S content), and contents of cysteine and reduced glutathione compared to Amruta. In contrast, the activity of superoxide dismutase (SOD) was higher in Amruta than that of Ro Agro 4001 under control conditions, whereas the Cd treatment increased significantly the SOD activity in both cultivars with the greater increase in Ro Agro 4001. The fluorescence spectra of phytocystatin showed a lesser change in Ro Agro 4001 under Cd stress than that in Amruta suggesting higher resistance of Ro Agro 4001 to Cd. The higher phytocystatin activity under Cd stress in Ro Agro 4001 compared to Amruta enabled the plants to protect their proteins more efficiently. This resulted in a greater increase of photosynthetic capacity in Ro Agro 4001 than that of Amruta. Thus, the phytocystatin activity may be considered as a physiological parameter for augmenting photosynthesis and growth of mustard under Cd stress.     

Arbuscular mycorrhizal symbiosis alleviates detrimental effects of saline reclaimed water in lettuce plants

The present study evaluated the effects of inoculation with arbuscular mycorrhizal fungi (AMF; Glomus iranicum var. tenuihypharum sp. nova) on the physiological performance and production of lettuce plants grown under greenhouse conditions and supplied with reclaimed water (RW; urban-treated wastewater with high electrical conductivity; 4.19 dS m^?1). Four treatments, fresh water, fresh water plus AMF inoculation, RW and RW plus AMF inoculation, were applied and their effects, over time, analyzed. Root mycorrhizal colonization, plant biomass, leaf-ion content, stomatal conductance and net photosynthesis were assessed. Overall, our results highlight the significance of the AMF in alleviation of salt stress and their beneficial effects on plant growth and productivity. Inoculated plants increased the ability to acquire N, Ca, and K from both non-saline and saline media. Moreover, mycorrhization significantly reduced Na plant uptake. Under RW conditions, inoculated plants also showed a better performance of physiological parameters such as net photosynthesis, stomatal conductance and water-use efficiency than non-mycorrhizal plants. Additionally, the high concentration of nutrients already dissolved in reclaimed water suggested that adjustments in the calculation of the fertigation should be conducted by farmers. Finally, this experiment has proved that mycorrhization could be a suitable way to induce salt stress resistance in iceberg lettuce crops as plants supplied with reclaimed water satisfied minimum legal commercial size thresholds. Moreover, the maximum values of Escherichia coli in the reclaimed water were close to but never exceeded the international thresholds established (Spanish Royal Decree 1620/2007; Italian Decree, 2003) and hence lettuces were apt for sale.     

Photosynthetic responses to soil water stress in summer in two Japanese urban landscape tree species (<Emphasis Type="Italic">Ginkgo biloba</Emphasis> and <Emphasis Type="Italic">Prunus yedoensis</Emphasis>): effects of pruning mulch and irrigation management

Key message

Stomatal regulation involves beneficial effects of pruning mulch and irrigation on leaf photosynthesis in Prunus yedoensis and Ginkgo biloba under moderate drought. G. biloba showed conservative water use under drought.

Abstract

Leaf photosynthesis is highly sensitive to soil water stress via stomatal and/or biochemical responses, which markedly suppress the growth of landscape trees. Effective irrigation management to maintain leaf photosynthesis and information on species-specific photosynthetic responses to soil water stress are essential for the sustainable management of landscape trees in Japan, in which summer drought often occurs. In order to investigate effective irrigation management, we used plants with moderate soil water stress as controls, and examined the effects of daily irrigation and pruning mulch on leaf photosynthesis in container-grown Ginkgo biloba and Prunus yedoensis, which are the first and second main tall roadside trees in Japan. Stomatal conductance was significantly increased by pruning mulch and daily irrigation, with similar increases in leaf photosynthesis being observed in P. yedoensis and G. biloba. In order to obtain information on species-specific photosynthetic responses to soil water stress, we compared the responses of leaf photosynthesis and leaf water status to reductions in soil water content (SWC) between the two species. G. biloba maintained a constant leaf water potential, leaf water content, maximum carboxylation rate, and electron transport rate with reductions in SWC, whereas reductions were observed in P. yedoensis. We concluded that pruning mulch and irrigation effectively offset the negative impact of moderate water stress on leaf photosynthesis in summer in P. yedoensis and G. biloba via stomatal regulation, and also that G. biloba maintained its photosynthetic biochemistry and leaf water status better than P. yedoensis under severe water stress.

     

Phosphoglycerate dehydrogenase from soybean nodules : partial purification and some kinetic properties

The relationship between single leaf photosynthesis and conductance was examined in cotton (Gossypium hirsutum L.) across a range of environmental conditions. The purpose of this research was to separate and define the degree of stomatal and nonstomatal limitations in the photosynthetic process of field-grown cotton.

Photosynthetic rates were related to leaf conductance of upper canopy leaves in a curvilinear manner. Increases in leaf conductance of CO₂ in excess of 0.3 to 0.4 mole per square meter per second did not result in significant increases in gross or net photosynthetic rates. No tight coupling between environmental influences on photosynthetic rates and those affecting conductance levels was evident, since photosynthesis per unit leaf conductance did not remain constant. Slowly developing water stress caused greater reductions in photosynthesis than in leaf conductance, indicating nonstomatal limitations of photosynthesis.

Increases in external CO₂ concentration to levels above ambient did not produce proportional increases in photosynthesis even though substomatal or intercellular CO₂ concentration increased. The lack of a linear increase in photosynthetic rate in response to increases in leaf conductance and in response to increases in external CO₂ concentration demonstrated that nonstomatal factors are major photosynthetic rate determinants of cotton under field conditions.

     

Ear of durum wheat under water stress: water relations and photosynthetic metabolism

The photosynthetic characteristics of the ear and flag leaf of well-watered (WW) and water-stressed (WS) durum wheat (Triticum turgidum L. var. durum) were studied in plants grown under greenhouse and Mediterranean field conditions. Gas exchange measurements simultaneously with modulated chlorophyll fluorescence were used to study the response of the ear and flag leaf to CO₂ and O₂ during photosynthesis. C₄ metabolism was identified by assessing the sensitivity of photosynthetic rate and electron transport to oxygen. The presence of CAM metabolism was assessed by measuring daily patterns of stomatal conductance and net CO₂ assimilation. In addition, the histological distribution of Rubisco protein in the ear parts was studied by immunocytochemical localisation. Relative water content (RWC) and osmotic adjustment (osmotic potential at full turgor) were also measured in these organs. Oxygen sensitivity of the assimilation rate and electron transport, the lack of Rubisco compartmentalisation in the mesophyll tissues and the gas-exchange pattern at night indicated that neither C₄ nor CAM metabolism occurs in the ear of WW or WS plants. Nevertheless, photosynthetic activity of the flag leaf was more affected by WS conditions than that of the ear, under both growing conditions. The lower sensitivity under water stress of the ear than of the flag leaf was linked to higher RWC and osmotic adjustment in the ear bracts and awns. We demonstrate that the better performance of the ear under water stress (compared to the flag leaf) is not related to C₄ or CAM photosynthesis. Rather, drought tolerance of the ear is explained by its higher RWC in drought. Osmotic adjustment and xeromorphic traits of ear parts may be responsible.     

Tolerance and physiological responses of Phragmites australis to water deficit

The water stress tolerance of Phragmites australis (Cav.) Trin ex. Steud. grown in the laboratory were investigated by examining effects of different levels of imposed water deficits on growth, photosynthesis and various physiological traits related to water stress. Individual plants were grown under conditions of unrestricted water supply and compared with groups of plants receiving 60, 30, 15 or 5% of previous daily water requirements, respectively.Water deficit was found to reduce the leaf area and the leaf biomass per plant due to decreased production of new leaves, increased leaf shedding and reduced average leaf size. Leaf production and leaf expansion growth were very sensitive to water availability and were reduced when plants were subjected to fairly mild water deficit. Osmolality in sap expressed from leaves and the concentration of proline in leaves were only significantly increased in severely stressed plants, indicating that osmotic adjustment was of minor importance until a critical stress level was reached. Photosynthetic parameters were rather unaffected until the water availability was very low and led to the assertion that reduced CO₂ assimilation was mainly due to stomatal closure and not biochemical changes. Water stress had no effect on the activity of Rubisco. The CO₂ assimilation rate and stomatal conductance decreased in such a way that the intrinsic water use efficiency (A/g_s) increased, indicating efficient CO₂ utilization in water stressed plants. The apparent quantum yield (φ_i) was reduced in leaves of the most stressed plants, probably due to a decrease in the CO₂ molar fraction in the chloroplasts following stomatal closure.The initial response of P. australis to water deficit is a reduction in leaf area, the remaining leaves staying physiological rather well functioning until they are severely stressed. A high intrinsic water use efficiency and the ability to maintain some capacity for photosynthesis under severe water stress can undoubtedly contribute to the survival of P. australis under dry conditions. Taken together with its well-developed adaptations to flooding, P. australis seems very well adapted to grow in wetland areas with a widely fluctuating hydroperiod. P. australis grows very well in rather deep water, but can also tolerate extensive periods of drought with reduced availability of water.     

24-epibrassionlide improves photosynthetic response of Rhododendron delavayi to drought

Rhododendron delavayi is an alpine evergreen ornamental plant with strong tolerance to drought stress. Brassinosteroids are promising agents for alleviating the negative effects of drought on plants, but the mechanism by which BRs induce plant resistance to drought is not well understood. The present study investigated the effects of exogenous spray of 24-epibrassionlide (EBR) at different concentrations (0~1 mg l⁻¹) on the physiological response of R. delavayi to drought caused by no watering for 10 days. With the increase in EBR concentration, net photosynthetic rate, stomatal conductance, transportation rate, light saturated photosynthetic rate, light compensation point, light saturation point, excitation energy capture efficiency of reaction center, actual photochemical efficiency of photosystem II (PSII), photochemical quenching and electron transport rate significantly increased, but there were no significant effects on photosynthetic pigment content. These results suggested that the EBR-induced improvement in CO₂ assimilation under drought was mainly related to stomatal and non-stomatal factors, and partially attributed to the increased photochemical efficiency of PSII. In addition, the leaf water potential increased with the increase in EBR concentration, while the malondialdehyde, superoxide dismutase, catalase, proline and soluble protein decreased. The results suggested EBR application partially alleviated the negative effect of drought on R. delavayi by improving water relations and decreasing lipid peroxidation and reactive oxygen species production. We concluded that exogenous application of EBR improved photosynthesis and alleviated the negative effects of drought-induced membrane peroxidation and severe oxidative stress.