Photosynthesis and ion content of leaves and isolated chloroplasts of salt-stressed spinach

Spinach (Spinacia oleracea) plants were subjected to salt stress by adding NaCl to the nutrient solution in increments of 25 millimolar per day to a final concentration of 200 millimolar. Plants were harvested 3 weeks after starting NaCl treatment. Fresh and dry weight of both shoots and roots was decreased more than 50% compared to control plants but the salt-stressed plants appeared healthy and were still actively growing. The salt-stressed plants had much thicker leaves. The salt-treated plants osmotically adjusted to maintain leaf turgor. Leaf K⁺ was decreased but Na⁺ and Cl⁻ were greatly increased.

The potential photosynthetic capacity of the leaves was measured at saturating CO₂ to overcome any stomatal limitation. Photosynthesis of salt-stressed plants varied only by about 10% from the controls when expressed on a leaf area or chlorophyll basis. The yield of variable chlorophyll a fluorescence from leaves was not affected by salt stress. Stomatal conductance decreased 70% in response to salt treatment.

Uncoupled rates of electron transport by isolated intact chloroplasts and by thylakoids were only 10 to 20% below those for control plants. CO₂-dependent O₂ evolution was decreased by 20% in chloroplasts isolated from salt-stressed plants. The concentration of K⁺ in the chloroplast decreased by 50% in the salt-stressed plants, Na⁺ increased by 70%, and Cl⁻ increased by less than 20% despite large increases in leaf Na⁺ and Cl⁻.

It is concluded that, for spinach, salt stress does not result in any major decrease in the photosynthetic potential of the leaf. Actual photosynthesis by the plant may be reduced by other factors such as decreased stomatal conductance and decreased leaf area. Effective compartmentation of ions within the cell may prevent the accumulation of inhibitory levels of Na⁺ and Cl⁻ in the chloroplast.

     

Maintenance of photosynthesis at low leaf water potential in wheat : role of potassium status and irrigation history

The interaction of low water potential effects on photosynthesis, and leaf K⁺ levels in wheat (Triticum aestivum L.) plants was studied. Plants were grown at three K⁺ fertilization levels; 0.2, 2, and 6 millimolar. With well watered plants, 2 millimolar K⁺ supported maximal photosynthetic rates; 0.2 millimolar K⁺ was inhibitory, and 6 millimolar K⁺ was superoptimal (i.e. rates were no greater than at 2 millimolar K⁺). Photosynthesis was monitored at high (930 parts per million) and low (330 parts per million) external CO₂ throughout a series of water stress cycles. Plants subjected to one stress cycle were considered nonacclimated; plants subjected to two successive cycles were considered acclimated during the second cycle. Sensitivity of photosynthesis to declining leaf water potential was affected by K⁺ status; 6 millimolar K⁺ plants were less sensitive, and 0.2 millimolar K⁺ plants were more sensitive than 2 millimolar K⁺ plants to declining water potential. This occurred with nonacclimated and acclimated plants at both high and low assay CO₂. It was concluded that the K⁺ effect on photosynthesis under stress was not mediated by treatment effects on stomatal resistance. Differences between the K⁺ treatments were much less pronounced, however, when photosynthesis of nonacclimated and acclimated plants was plotted at a function of declining relative water content during the stress cycles. These results suggest that K⁺ effects on the relationship between relative water content and water potential in stressed plants was primarily responsible for the bulk of the K⁺-protective effect on photosynthesis in stressed plants. In vitro experiments with chloroplasts and protoplasts isolated from 2 millimolar K⁺ and 6 millimolar K⁺ plants indicated that upon dehydration, K⁺ efflux from the chloroplast stroma into the cytoplasm is less pronounced in 6 millimolar K⁺ protoplasts.     

Silicon nutrition alleviates physiological disorders imposed by salinity in hydroponically grown canola (Brassica napus L.) plants

The effects of Si nutrition on transpiration, leaf anatomy, accumulation of Na⁺, K⁺, Cl^?, P, Fe and B and some reactive oxygen species related parameters were investigated in canola plants under salinity. Plants were grown hydroponically in growth chamber under controlled conditions at 0 and 100?mM NaCl each supplied with or without 1.7?mM silicon (Si) as sodium silicate. Salinity imposed significant reduction in growth parameters of plants like fresh weights of roots and shoots and leaf area. It also led to accumulation of Na⁺ and Cl^? and a decrease in the concentration of K⁺, P, B and Fe. Reduction of transpiration, stomatal density and specific leaf area in leaves and an increase in leaf thickness were amongst other symptoms in salt-affected plants. Salinity led to higher concentration of hydrogen peroxide, increased lipid peroxidation and decrease of catalase and peroxidase activity, which suggests the induction of oxidative stress in plants. Silicon nutrition could prevent toxic ions (Na⁺ and Cl^?) accumulation while higher levels of essential minerals like K⁺, P and Fe were maintained in plants. Consequently, silicon nutrition decreased oxidative stress in plants, evidenced by increase in antioxidant enzyme activity, reduction in hydrogen peroxide and lipid peroxidation.     

Effect of water stress on the chloroplast antioxidant system: I. Alterations in glutathione reductase activity

The effect of water stress on glutathione reductase and catalase activities was evaluated in leaf blades of field-grown winter wheat (Triticum aestivum L.). Wheat was sown at two seeding rates under both irrigated and dryland conditions. Flag leaves from dryland plants sown at 60 kilograms/hectare showed no change in either glutathione reductase or catalase activities per unit leaf area, while leaves from the basal portion of the canopy exhibited a 273% increase in glutathione reductase activity and a 60% increase in catalase activity. Glutathione reductase activity in dryland plants sown at 120 kilograms/hectare increased 25% in flag leaves and 225% in basal leaves. No change in catalase activity was observed in either flag or basal leaves from these same plants. The increase in glutathione reductase activity in response to water stress was observed when activity was expressed on either a per unit leaf area, protein, or chlorophyll basis. No change in catalase activity was detected when enzyme activity was expressed on a protein basis.     

Water relations and stomatal characteristics of Mediterranean plants with different growth forms and leaf habits: responses to water stress and recovery

The aim of this study was to extent the range of knowledge about water relations and stomatal responses to water stress to ten Mediterranean plants with different growth forms and leaf habits. Plants were subjected to different levels of water stress and a treatment of recovery. Stomatal attributes (stomatal density, StoD), stomatal conductance (g _s), stomatal responsiveness to water stress (SR), leaf water relations (pre-dawn and midday leaf water potential and relative water content), soil to leaf apparent hydraulic conductance (K _L) and bulk modulus of elasticity (ε) were determined. The observed wide range of water relations and stomatal characteristics was found to be partially depended on the growth form. Maximum g _s was related to StoD and the stomatal area index (SAI), while g _s evolution after water stress and recovery was highly correlated with K _L. Relationships between SR to water deficit and other morphological leaf traits, such as StoD, LMA or ε, provided no general correlations when including all species. It is concluded that a high variability is present among Mediterranean plants reflecting a continuum of leaf water relations and stomatal behaviour in response to water stress.     

外源SA和NO对NaCl胁迫下番茄幼苗生长、光合及离子分布的影响

以番茄(Lycopersicon esculentum Mill.)品种‘秦丰保冠’为试材,采用营养液培养法,研究单独和复配施用外源水杨酸(SA)、一氧化氮(NO)供体硝普钠(SNP)对100mmol/L NaCl胁迫下番茄幼苗生长、光合及离子分布的影响。结果表明:(1)单独和复配外施SA、SNP均能有效抑制NaCl胁迫下番茄幼苗叶片光合色素(Chla、Chlb、Chla+b和Car)含量、Chla/b值、净光合速率(Pn)、蒸腾速率(Tr)、气孔导度(Gs)、瞬时水分利用效率(WUEt)、表观光能利用效率(LUEapp)和表观CO2利用效率(CUEapp)的下降及Car/Chla+b值和胞间CO2浓度(Ci)的升高,并以SA和SNP复配处理效果最明显。(2)NaCl胁迫下,外源SA、SNP单独和复配处理的番茄幼苗各器官(叶、茎和根)中Cl-、Na+含量和Na+/K+、Na+/Ca2+、Na+/Mg2+值显著降低,而K+、Ca2+和Mg2+的含量却不同程度提高,其中以SA和SNP复配处理效果最好。(3)单独和复配外施SA、SNP均能有效减轻NaCl胁迫对番茄幼苗生长的抑制作用,并促进各器官生物量的积累和壮苗的形成,且以SA和SNP复配处理效果更佳。研究表明,复配外施SA和SNP在诱导番茄幼苗提高抗(耐)盐能力方面具有协同增效作用。     

Abaxial and Adaxial Stomatal Density, Stomatal Conductances and Water Status of Bean Primary Leaves as Affected by Paclobutrazol

The plant growth retardant, paclobutrazol at 8.5 or 17.0 μM concentrations effectively inhibited the stem elongation and primary leaf expansion of bean seedlings. Although the retardant reduced the relative water content in well-watered plants, the water and pressure potentials remained high in the primary leaves. K⁺, Na⁺, Mg²⁺ and Ca²⁺ contents in the primary leaves of the paclobutrazol-treated plants were not significantly different from those in the control. The stomatal density increased on both surfaces but the length of guard cells was not reduced significantly on the adaxial epidermes of the paclobutrazol-treated primary leaves. The inhibitory effect of paclobutrazol on the abaxial stomatal conductances became more pronounced with time during the light period but the adaxial surfaces displayed similar or slightly higher conductances than those of the control. The transpiration rate on a unit area basis did not change significantly or increased in the treated leaves thus the reduced water loss of paclobutrazol-treated plants was due to the reduced leaf area. Stomatal conductances of the adaxial surfaces responded more intensively to exogenous abscisic acid and the total leaf conductance decreased faster with increasing ABA concentration in the control than in the paclobutrazol-treated leaves. Paclobutrazol, an effective inhibitor of phytosterol biosynthesis, not only amplified the stomatal differentiation but increased the differences between the adaxial and abaxial stomatal conductances of the primary leaves.     

Variability of leaf photosynthetic characteristics in rice and its relationship with resistance to water stress under different nitrogen nutrition regimes

The negative effects of water stress on rice can be alleviated by NH₄⁺ nutrition. However, the effects of mixed nitrogen (N) nutrition (NO₃^? + NH₄⁺) on resistance to water stress are still not well known. To investigate the response of rice growth to water stress and its relationship with photosynthetic characteristics, a hydroponic experiment supplying different N forms was conducted. Compared with NO₃^? nutrition, mixed‐N and NH₄⁺ nutrition greatly alleviated the reduction of leaf area, chlorophyll content, and photosynthesis under water stress, whilst subsequently maintaining higher biomass. In contrast, water stress inhibited the root‐shoot ratios in NH₄⁺‐ and mixed‐N‐supplied plants, indicating reduced root growth and higher photosynthate availability to shoots. The following key observations were made: (1) a similar stomatal limitation and low proportion of activated Rubisco were observed among the three different N nutrition regimes; (2) increased mesophyll conductance in NH₄⁺‐ and mixed‐N‐supplied plants simultaneously stimulated leaf photosynthesis and improved the water use efficiency and (3), the maximum carboxylation rate and actual photochemical efficiency of photosystem II in NH₄⁺‐ and mixed‐N‐supplied plants were significantly higher than that in NO₃^?‐supplied plants, thus resulting in higher photochemical efficiency under water stress. In conclusion, mixed‐N and NH₄⁺ nutrition may be used to develop strategies for improved water stress resistance and stimulated biomass production under conditions of osmotic stress and possibly drought.     

Effects of 5-aminolevulinic acid on nitrogen metabolism and ion distribution of watermelon seedlings under salt stress

The effects of foliar spray application of 5-aminolevulinic acid (ALA) on the growth, nitrogen metabolism, and ion distribution of salt-stressed watermelon (Citrullus lanatus (Thunb.) Matsum. and Nakai) seedlings were investigated. Supplementation of the nutrient solution with 100 mM NaCl significantly reduced leaf and root biomass of watermelon plants. Foliar application of 1.25 mM ALA significantly alleviated the inhibition of plant growth under salt stress. Salinity induced significant accumulation of nitrate, ammonium, and soluble protein and a significant decrease in the activities of nitrate reductase (NR), nitrite reductase (NiR), glutamine synthetase (GS), glutamate synthase (GOGAT), and glutamate dehydrogenase (GDH) in watermelon plants. However, ALA significantly increased the activities of NR, GS, GOGAT, and GDH, but decreased the ammonium content and NiR activity. In addition, salt stress resulted in significant accumulation of Na⁺ and Cl^? in plants, but decreased the contents of K⁺ and Mg²⁺. Application of ALA alleviated the salt stress-induced ion toxicity, and increased the contents of K⁺ and Mg²⁺. ALA also increased soluble protein and proline contents in salt-stressed watermelon plants. These results indicated that application of ALA alleviated the accumulation of Na⁺ and Cl^? in salt-stressed watermelon plants, especially through regulating nitrogen metabolism and ion distribution, which were associated with an improvement in plant growth.     

Nitrate Reductase in Wheat Plants Grown Under Water Stress and Inoculated with Azospirillum spp.

The present investigation has been performed to evaluate nitrate reductase (NR) and nitrogenase activities as well as growth and mineral nutrition of wheat plants grown under drought stress and inoculated with different Azospirillum strains (NR⁻ and NR⁺). Fresh, dry mass and water content decreased with decreasing soil moisture content, which was accompanied with low soluble sugars and soluble protein content and increase in the total amino acids content. Azospirillum inoculation with either bacterial strain (NR⁻ and NR⁺) significantly increased the above characteristics even at 40 % moisture content. NR activity decreased in both the shoots and roots by decreasing soil moisture content. NR⁺ strain exhibited increased root NR activity compared with uninoculated plants or inoculated with NR⁻ strain. However, plants inoculated with NR⁻strain increased NR activity in the shoot more than in the root of the same plant and in the shoot of control plants. Inoculation with either NR⁻ and NR⁺ Azospirillum strains gave higher nitrogenase activity than uninoculated control plants. The low N supply (0.5 mM) did not affect nitrogenase activity. NR⁻strain was less effective than NR⁺strain in promoting total N-yield, spike numbers and their mass per pot. Azospirillum inoculation exhibited no significant changes in wheat Mg²⁺ content. However, K⁺ and Ca²⁺ have shown significantly increased values. Azospirillum beneficial effect on plant N balance and growth are most probably composed of multiple mechanisms and beneficial NR is one of them. The importance of Azospirillum NR⁺strains for increasing wheat resistance to water stress is also supported by the obtained data. This revised version was published online in July 2006 with corrections to the Cover Date.     

Physiological responses of the mangrove Rhizophora mangle grown in the absence and presence of NaCl

Abstract. Propagules of the mangrove, Rhizophora mangle L., were precultivated for 9 months in a greenhouse. The young plants were transferred into unaerated nutrient solutions without and with 200 mol m ³ NaCl and subsequently their growth, their water relations and the photosynthetic properties of their leaves were studied. Growth of the salttreated plants was significantly increased, while the control plants gradually died off after finishing the experiments. The shoot water potential and the stomatal resistance of the leaves were lowered while the chlorophyll contents and the chlorophyll a/b ratio in the leaves of salt-treated plants were increased by NaCl, the net result being an enhanced rate of CO₂ assimilation. The leaves of both sets of plants showed diurnal fluctuations in malic acid concentration which were more pronounced in the leaves of salt treated plants which, additionally, were more succulent. However, the plants showed no net CO₂ fixation at night, indicating that Rhizophora mangle is a CAM-cycling plant. After 200 d of cultivation without or with NaCl, the Na⁺, Cl and K⁺ concentrations in tissues and vacuoles were measured. Energy-dispersive X-ray microprobe analyses on root vacuoles of control plants reveal Na⁺ preference, on those of salt treated plants a strong K⁺ preference. Vacuolar K⁺ concentrations are neither affected by NaCl nor do they vary across the root radius. High vacuolar Na⁺ and Cl concentrations are found in the hypodermis followed by a stepwise decrease towards the inner root cortex cells. Ion concentrations of the photosynthetically active leaf tissues seem to be regulated by (1) radial filtration across the root cortex: (2) ion exchange of the xlem parenchyma cells: and (3) sequestration of Na⁺ and Cl in the hypodermal water storage tissue of the leaves.     

Analysis of salinity effects on basil leaf surface area, photosynthetic activity, and growth

Basil (Ocimum basilicum L.) seedlings were cultured on liquid medium in controlled conditions. Two varieties differing in leaf size were compared. When plants were 30?days old, the medium was supplemented with 50?mM NaCl. After 15?days of treatment, root, stem and leaf biomass, leaf number, and leaf surface area were measured. Ion accumulation was determined in roots, stems, and leaves. Photosynthetic parameters (CO₂ fixation rate, internal CO₂ concentration, stomatal conductance) as well as transpiration rate were determined on separate leaves. Electrolyte leakage and malondialdehyde content were used to estimate damage to membranes and lipid peroxidation, respectively. Several antioxidant enzymatic activities were used as proxies of oxidative stress. High Na⁺ concentration was reached in leaf tissues. Salt restricted whole plant biomass deposition rate by diminishing leaf number and leaf expansion, as well as photosynthetic activity were estimated from whole plant biomass production per unit leaf surface area. Diminished stomatal conductance restricted CO₂ fixation rate, and decrease in chlorophyll content presumably limited photosynthetic activity. Lipid peroxidation revealed damages to membranes. The magnitude of these responses differed between the two varieties, indicating that an intraspecific variability in salt response exists in basil.     

Ectopic expression of wheat expansin gene TaEXPA2 improved the salt tolerance of transgenic tobacco by regulating Na+/K+ and antioxidant competence

High salinity is one of the most serious environmental stresses that limit crop growth. Expansins are cell wall proteins that regulate plant development and abiotic stress tolerance by mediating cell wall expansion. We studied the function of a wheat expansin gene, TaEXPA2, in salt stress tolerance by overexpressing it in tobacco. Overexpression of TaEXPA2 enhanced the salt stress tolerance of transgenic tobacco plants as indicated by the presence of higher germination rates, longer root length, more lateral roots, higher survival rates and more green leaves under salt stress than in the wild type (WT). Further, when leaf disks of WT plants were incubated in cell wall protein extracts from the transgenic tobacco plants, their chlorophyll content was higher under salt stress, and this improvement from TaEXPA2 overexpression in transgenic tobacco was inhibited by TaEXPA2 protein antibody. The water status of transgenic tobacco plants was improved, perhaps by the accumulation of osmolytes such as proline and soluble sugar. TaEXPA2‐overexpressing tobacco lines exhibited lower Na⁺ but higher K⁺ accumulation than WT plants. Antioxidant competence increased in the transgenic plants because of the increased activity of antioxidant enzymes. TaEXPA2 protein abundance in wheat was induced by NaCl, and ABA signaling was involved. Gene expression regulation was involved in the enhanced salt stress tolerance of the TaEXPA2 transgenic plants. Our results suggest that TaEXPA2 overexpression confers salt stress tolerance on the transgenic plants, and this is associated with improved water status, Na⁺/K⁺ homeostasis, and antioxidant competence. ABA signaling participates in TaEXPA2‐regulated salt stress tolerance.     

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.     

Physiological and ecological characters studies on Aloe vera under soil salinity and seawater irrigation

Experiments were conducted in two Aloe vera cultivars (F0 and F50) to study the characters of physiology and ecology under salt stress. The results indicate decreases in tissue water, total soluble sugars and glucose, and increases in dry matter and membrane injury occurred both in F0 and F50 irrigated with 60% seawater. Less cell membrane injury were observed in F50. Moreover, total soluble sugars in F0 decreased obviously, however, no significant change in F50, while sucrose in plants had no significant change. Furthermore, F0 and F50 accumulated more inorganic cations in stems and roots. In addition, leaf K⁺ and Ca²⁺ contents were more in F50 than that in F0 to maintain normal plant growth though accumulation of Na⁺. F50 had a relative superiority in growth under salinity conditions due to higher K⁺/Na⁺ ratio and lower Na⁺/Ca²⁺ ratio than F0.     

Photosynthetic Response to Water Stress in Phaseolus vulgaris

Water stressed Phaseolus vulgaris L. plants were monitored to detect the relationships between net photosynthesis, transpiration, boundary layer plus stomatal resistance, mesophyll resistance, CO₂ compensation point, ribulose, 1,5-diphosphate carboxylase activity and leaf water potential. At full expansion, the first trifoliate leaves of greenhouse grown bean plants were subjected to water stress by withholding irrigation. Gas exchange and enzyme activity of the central trifoliolate leaflets were monitored as leaf water potential decreased. Although increased stomatal resistance appeared to be the primary causal factor of reduced net photosynthesis, increased mesophyll resistance and decreased ribulose 1,5-diphosphate carboxylase activity further documented the role of non-stomatal factors.     

Low CO(2) Prevents Nitrate Reduction in Leaves

The correlation between CO₂ assimilation and nitrate reduction in detached spinach (Spinacia oleracea L.) leaves was examined by measuring light-dependent changes in leaf nitrate levels in response to mild water stress and to artificially imposed CO₂ deficiency. The level of extractable nitrate reductase (NR) activity was also measured. The results are: (a) In the light, detached turgid spinach leaves reduced nitrate stored in the vacuoles of mesophyll cells at rates between 3 and 10 micromoles per milligram of chlorophyll per hour. Nitrate fed through the petiole was reduced at similar rates as storage nitrate. Nitrate reduction was accompanied by malate accumulation. (b) Under mild water stress which caused stomatal closure, nitrate reduction was prevented. The inhibition of nitrate reduction observed in water stressed leaves was reversed by external CO₂ concentrations (10-15%) high enough to overcome stomatal resistance. (c) Nitrate reduction was also inhibited when turgid leaves were kept in CO₂-free air or at the CO₂-compensation point or in nitrogen. (d) When leaves were illuminated in CO₂-free air, activity of NR decreased rapidly. It increased again, when CO₂ was added back to the system. The half-time for a 50% change in activity was about 30 min. It thus appears that there is a rapid inactivation/activation mechanism of NR in leaves which couples nitrate reductase to net photosynthesis.     

Effect of Seawater on Photosynthesis and Dry Matter Accumulation in Developing Rice Grains

To understand the physiology of rice under seawater salinity, potted rice plants were irrigated with different concentrations of Japan seawater (electrical conductivity 0.9, 5.7, 11.5, or 21.5 mS cm^-1) from 10 d after transplanting (DAT) to 35 DAT, and from 75 to 100 DAT. Seawater salinity decreased the net photosynthetic rate, stomatal conductance, intercellular CO₂ concentration, transpiration rate, leaf water and osmotic potentials, and relative water content, and increased leaf temperature. The contents of chlorophylls, carotenoids, and total sugars significantly decreased in the leaves but content of non-reducing sugars decreased only slightly. With increasing salinity the Na⁺ concentration increased, while Ca²⁺, Mn²⁺, and K⁺ concentrations decreased. Salinity decreased the contents of sugars and proteins, dry mass, and rate of dry mater accumulation in developing grains.