Salt Stress Increases the Level of Translatable mRNA for Phosphoenolpyruvate Carboxylase in Mesembryanthemum crystallinum

Mesembryanthemum crystallinum responds to salt stress by switching from C₃ photosynthesis to Crassulacean acid metabolism (CAM). During this transition the activity of phosphoenolpyruvate carboxylase (PEPCase) increases in soluble protein extracts from leaf tissue. We monitored CAM induction in plants irrigated with 0.5 molar NaCl for 5 days during the fourth, fifth, and sixth week after germination. Our results indicate that the age of the plant influenced the response to salt stress. There was no increase in PEPCase protein or PEPCase enzyme activity when plants were irrigated with 0.5 molar NaCl during the fourth and fifth week after germination. However, PEPCase activity increased within 2 to 3 days when plants were salt stressed during the sixth week after germination. Immunoblot analysis with anti-PEPCase antibodies showed that PEPCase synthesis was induced in both expanded leaves and in newly developing axillary shoot tissue. The increase in PEPCase protein was paralleled by an increase in PEPCase mRNA as assayed by immunoprecipitation of PEPCase from the in vitro translation products of RNA from salt-stressed plants. These results demonstrate that salinity increased the level of PEPCase in leaf and shoot tissue via a stress-induced increase in the steady-state level of translatable mRNA for this enzyme.     

The effects of salt on the pattern of protein synthesis in barley roots

The effect of salt stress on the incorporation of [³⁵S]methionine into protein was examined in roots of barley (Hordeum vulgare L. cv California Mariout 72). Plants were grown in nutrient solution with or without 200 millimolar NaCl. Roots of intact plants were labeled in vivo and proteins were extracted and analyzed by fluorography of two-dimensional gels. Although the protein patterns for control and salt-stressed plants were qualitatively similar, the net synthesis of a number of proteins was quantitatively changed. The most striking change was a significant increase of label in two protein pairs that had pIs of approximately 6.3 and 6.5. Each pair consisted of proteins of approximately 26 and 27 kilodaltons (kD). In roots of control plants, the 27-kD proteins were more heavily labeled in the microsomal fraction relative to the 26-kD proteins, whereas the 26-kD proteins were enriched in the post 178,000 g supernatant fraction; in roots of salt treated plants, the 26- and 27-kD proteins were more intensely labeled in both fractions. Labeling of the 26- and 27-kD proteins returned to control levels when salt-stressed plants were transferred to nutrient solution without NaCl. No cross-reaction was detected between the antibody to the 26-kD protein from salt-adapted tobacco cells and the 26- and 27-kD proteins of barley.     

Synthesis and degradation of barley nitrate reductase

Nitrate and light are known to modulate barley (Hordeum vulgare L.) nitrate reductase activity. The objective of this investigation was to determine whether barley nitrate reductase is regulated by enzyme synthesis and degradation or by an activation-inactivation mechanism. Barley seedling nitrate reductase protein (cross-reacting material) was determined by rocket immunoelectrophoresis and a qualitative immunochemical technique (western blot) during the induction and decay of nitrate reductase activity. Nitrate reductase cross-reacting material was not detected in root or shoot extracts from seedlings grown without nitrate. Low levels of nitrate reductase activity and cross-reacting material were observed in leaf extracts from plants grown on nitrate in the dark. Upon nitrate induction or transfer of nitrate-grown etiolated plants to the light, increases in nitrate reductase activity were positively correlated with increases in immunological cross-reactivity. Root and shoot nitrate reductase activity and cross-reacting material decreased when nitrate-induced seedlings were transferred to a nitrate-free nutrient solution or from light to darkness. These results indicate that barley nitrate reductase levels are regulated by de novo synthesis and protein degradation.     

Fatty acid composition of lipids in vegetative organs of the halophyte Suaeda altissima under different levels of salinity

Qualitative and quantitative composition of fatty acids (FA) in the lipids of vegetative organs of the halophyte Suaeda altissima (L.) Pall. grown at different NaCl concentrations in nutrient solution was studied. Along with this, the biomass of these organs, the content of water and Na⁺, Cl^?, and K⁺ ions in them, and the ultrastructure of root and leaf cells were determined. At both low (1 mM) and high (750 mM) NaCl concentrations in nutrient solution, plants could maintain growth and water content in organs, demonstrating a noticeable increase in the dry weight and a slight increase in the water content at 250 mM NaCl. At all NaCl concentrations in nutrient solution, S. altissima tissues contained a relatively high K⁺ amount. Under salinity, Na⁺ and Cl^? ions contributed substantially into the increase in the cell osmotic pressure, i.e., a decrease in their water potential; in the absence of salinity, K⁺ fulfilled this function. In the cells of both roots and leaves, NaCl stimulated endo- and exocytosis, supposedly involved in the vesicular compound transport. 750 mM NaCl induced plasmolysis and changes in the membrane structure, which can be interpreted as degradation processes. Under optimal NaCl concentration in medium (250 mM), the content of lipids in plant aboveground organs per fresh weight was more than 2.5-fold higher than under 1 or 750 mM NaCl, whereas in the roots opposite patten was observed. When plants were grown under non-optimal conditions, substantial changes occurred in the qualitative and quantitative FA composition in lipids of both aboveground organs and roots. Observed changes are discussed in relation to processes underlying S. altissima salt tolerance and those of disintegration occurring at the high external NaCl concentration (750 mM).     

Interaction between Light Intensity and NaCl Salinity and Their Effects on Growth, CO(2) Assimilation, and Photosynthate Conversion in Young Broad Beans

Seedings of Vicia faba were grown for four weeks at two different light intensities (55 and 105 watts per square meter) in a saline (50 millimolar NaCl) and nonsaline nutrient solution. NaCl salinity depressed growth and restricted protein formation, CO₂ assimilation, and especially the incorporation of photosynthates into the lipid fraction. Conversion of photosynthates in leaves was much more affected by salinity than was photosynthate turnover in roots. The detrimental effect of NaCl salinity on growth, protein formation, and CO₂ assimilation was greater under low than under high light conditions. Plants of the high light intensity treatment were more capable of excluding Na⁺ and Cl⁻ and accumulating nutrient cation species (Ca²⁺, K⁺, Mg²⁺) than plants grown under low light intensity. It is suggested that the improved ionic status provided better conditions for protein synthesis, CO₂ assimilation, and especially for the conversion of photosynthates into lipids.     

Chlorate Toxicity and Nitrate Reductase Activity in Tomato Plants

Chlorate damage was studied in tomato plants ( Lycopersicum esculentum cv. Moneymaker) that were supplied with a nitrogen-free nutrient solution or with a nutrient solution, containing either nitrate or ammonium as a nitrogen source. Damage was low in ammonium-fed plants and high in nitrate-fed plants and in nitrogen-less plants. Nitrate reductase activity could be detected in all treatments, although the activity was highest in the nitrate-fed plants.
The hypothesis that chlorate can be used as a substrate by the enzyme nitrate reductase in higher plants, was studied and proved to be true for the tomato plants, as was found earlier for Escherichia and Chlorella . The affinity of the enzyme for chlorate was lower than for nitrate, the K _m being 4 m M and 0.15 m M respectively. Induction of the enzyme by chlorate could not be detected. The enzyme activity was lowered in leaf discs after a 7 h treatment with chlorate and the inhibition was proportional to the chlorate concentration of the medium.
The results were discussed in terms of competition between nitrate and chlorate at the uptake and the enzyme site and with regard to a possible influence of chlorate on synthesis and breakdown of the enzyme.     

Water and salt stresses, kinetin and protein synthesis in tobacco leaves

The capacity of tobacco (Nicotiana rustica) leaf discs to incorporate l-leucine ¹⁴C into proteins was measured. Leaf discs were obtained from plants which experienced soil water depletion, or which were exposed to a saline or osmotic stress in the root medium. The stresses were brief of relatively short duration and water potential did not decrease below 4 bars in the root media. Leaf discs were sampled 2 hours after stress removal, achieved by reirrigation, or replacement of saline and osmotic solutions with normal nutrient solution. Plants were always turgid when leaves were sampled.     

Combined effect of NaCl-salinity and hypoxia on growth,photosynthesis, water relations and solute accumulation in Phragmites australis plants

Aim of the present study was to investigate the effects of two key environmental factors of estuarine ecosystems, salinity and hypoxia, on the physiological attributes in reed plants (Phragmites australis (Cav.) Trin. ex Steudel). Growth, leaf gas exchange, water (and ion) relations, and osmotic adjustment were determined in hydroponically grown plants exposed to hypoxia at varying NaCl-salinity concentrations (0, 50, 100, and 200 mM). Plants grew well under hypoxia treatment with standard nutrient solution without added salt and at NaCl concentrations up to 100 mM. Reed plants were able to produce and allocate phytomass to all their organs even at the highest salt level (200 mM NaCl). In plants subjected to hypoxia at various water potentials no clear relationships were found between growth and photosynthetic parameters except for g_s, whereas growth displayed a highly significant correlation with plant–water relations. A and g_s of reed plants treated with hypoxia at varying water potential of nutrient solutions were positively correlated and the former variable also had a strong positive relationship with E. Leaf Ψ_w and Ψ_π followed a similar trend and declined significantly as water potential of watering solutions was lowered. Highly significant positive correlations were identified between leaf Ψ_w and photosynthetic parameters. At all NaCl concentrations, the increase in total inorganic ions resulted from increased Na⁺ and Cl⁻ while K⁺, Ca²⁺, and Mg²⁺ concentrations decreased with increasing osmolality of nutrient solutions. Common reed has an efficient mechanism of Na⁺ exclusion from the leaves and exhibited a high leaf K⁺/Na⁺ selectivity ratio over a wide range of salinities under hypoxia treatment. In Phragmites australis grown in 200 mM NaCl, K⁺ contributed 17% toΨ_π, whereas Na⁺ and Cl⁻ accounted for only 11% and 6%, respectively. At the same NaCl concentration, the estimated contribution of proline to Ψ_π was less than 0.2%. Changes in leaf turgor occurred with a combined effect of salinity and hypoxia, suggesting that reed plants could adjust their water status sufficiently.     

Effect of Ethephon on Protein Degradation and the Accumulation of ;Pathogenesis-Related' (PR) Proteins in Tomato Leaf Discs

The effect of ethephon (2-chloroetylphosphonic acid) on the degradation of proteins and on the induction of Lycopersicon esculentum pathogenesis-related (PR) proteins was studied in tomato leaf discs. The rate of ribulose, -1,5-bisphosphate carboxylase/oxygenase (Rubisco) degradation was maximal in discs after 48 hours of incubation with 1 millimolar ethephon, leading to complete disappearance of Rubisco after 96 hours. This effect was correlated with an increase in PR protein synthesis and the induction of the previously reported alkaline proteolytic enzyme PR-P69 (P Vera, V Conejero [1988] Plant Physiol 87: 58-63). In vivo pulse-chase experiments demonstrated that ethephon not only affected Rubisco content but that of many other ³⁵S-labeled proteins as well, indicating that ethylene activates a general and nonspecific mechanism of protein degradation. This effect was partially inhibited in vivo by the action of pCMB, a selective inhibitor of cysteine-proteinases such as P69. These data reinforce the hypothesis that P69 and perhaps other PR proteins are involved in the mechanism of accelerated protein degradation activated by ethylene.     

Chromium-induced inhibition of ethylene evolution in bean (Phaseolus vulgaris) leaves

The influence of chromium concentration on ethylene production in bean plants ( Phaseolus vulgaris L. cv. Contender) was investigated. A Cr ion-induced inhibition of ethylene synthesis from endogenous 1-aminocyclopropane-1-carboxylic acid (ACC) was observed within both leaf discs floated on 2 m M CrO²⁻₄ or Cr³⁺ and leaf discs from plants cultured in nutrient solutions containing 10, 20 or 40 μ M CrO²⁻₄. However, Cr ions supplied either to plants with the nutrient solution or to discs with the incubation medium rather increased the conversion of exogenous ACC to ethylene. Primary leaves of plants exposed to CrO²⁻₄-containing nutrient solutions showed a statistically insignificant decrease of ACC-synthase activity. In the trifoliolate leaves of plants exposed to 10 μ M CrO²⁻₄, in which a significant decrease of ethylene production from endogenous ACC was observed, a substantial increase of ACC synthase was found. These results indicate that Cr ion-induced inhibition of ethylene production is not due to a breakdown of membrane integrity, which is necessary for ethylene forming enzyme activity, but caused by metabolic alterations leading to decreased ACC availability. Chromium ions may act by inhibiting ACC synthase activity or by diverting a metabolic step prior to the ACC synthase catalyzed reaction.     

Effect of Light and NO(3) on Wheat Leaf Phosphoenolpyruvate Carboxylase Activity: Evidence for Covalent Modulation of the C(3) Enzyme

Phosphoenolpyruvate carboxylase (PEPcase) activity was studied in excised leaves of wheat (Triticum aestivum L.) in the dark and in the light, in presence of either N-free (low-NO₃⁻ leaves) or 40 millimolar KNO₃ (high-NO₃⁻ leaves) nutrient solutions. PEPcase activity increased to 2.7-fold higher than that measured in dark-adapted tissue (control) during the first 60 minutes and continued to increase more slowly to 3.8-fold that of the control. This level was reached after 200 minutes exposure of the leaves to light and high NO₃⁻. In contrast, the lower rate of increase recorded for low-NO₃⁻ leaves ceased after 60 minutes of exposure to light at 2.3-fold the control level. The short-term NO₃⁻ effect increased linearly with the level of NO₃⁻ uptake. In immunoprecipitation experiments, the antibody concentration for PEPcase precipitation increased with the protein extracts from the different treatments in the order: control, illuminated low-NO₃⁻ leaves, illuminated high-NO₃⁻ leaves. This order also applied with regard to a decreasing sensitivity to malate and an increasing stimulation by okadaic acid (an inhibitor of P-protein phosphatases). Following these studies, ³²P labeling experiments were carried out in vivo. These showed that the light-induced change in the properties of the PEPcase was due to an alteration in the phosphorylation state of the protein and that this effect was enhanced in high-NO₃⁻ conditions. Based on the responses of PEPcase and sucrose phosphate synthase in wheat leaves to light and NO₃⁻, an interpretation of the role of NO₃⁻ as either an inhibitor of P-protein phosphatase(s) or activator of protein kinase(s) is inferred. In the presence of NO₃⁻, the phosphorylation state of both PEPcase and sucrose phosphate synthase is increased. This causes activation of the former enzyme and inhibition of the latter. We suggest that NO₃⁻ modulates the relative protein kinase/protein phosphatase ratio to favor increased phosphorylation of both enzymes in order to redirect carbon flow away from sucrose synthesis and toward amino acid synthesis.     

Chloride salinity reduces cadmium accumulation by the Mediterranean halophyte species Atriplex halimus L.

Soil salinity usually increases bioavailability of Cd on heavy metal polluted soils but its impact on Cd absorption and accumulation by plants remains largely unknown. Plants from the halophyte species Atriplex halimus were therefore exposed for 12 and 14 days to nutrient solution containing 50 μM CdCl₂ in the presence of NaCl, KCl or NaNO₃ 50 mM. Most Cd present in solution remained as Cd–EDTA and salinity had no impact on Cd speciation. Chloride salinity (NaCl and KCl) reduced Cd accumulation in shoots and roots while NaNO₃ increased Cd accumulation in leaves. More than 30% of accumulated Cd was found at the leaf surface and accumulated in trichomes but all tested salts decreased the proportion of excreted Cd. Cadmium induced a decrease in the leaf water content. External NaCl and KCl mitigated the deleterious impact of Cd by inducing osmotic adjustment while NaNO₃ and synthesis of protecting compounds such as soluble sugars and glycinebetaine. Free polyamines (putrescine, spermidine and spermine) increased in response to Cd, Cd + NaCl and Cd + KCl while only putrescine increased in response to Cd + NaNO₃. Proline exhibited maximal concentration in the leaves of Cd + NaCl and Cd + KCl-treated plants and was correlated with osmotic adjustment. Our results suggest that chloride salinity improved the resistance of A. halimus to Cd toxicity both by decreasing the absorption of heavy metal and by improving tissular tolerance through an increase in the synthesis of osmoprotective compounds.     

Role of nitrogen metabolism in the development of salt tolerance in barley plants

The 7- to 8-day-old barley (Hordeum vulgare L.) seedlings grown in KNO3 solutions (1-40 mM) were characterized by the substrate activation of nitrate reductase (NR) in the apical leaf segments (1–2 cm in length), as well as by stimulated growth, broadened leaf blades, and by vigorously developed system of shortened roots. When the seedlings were grown in the presence of 20 mM KNO₃, the ability of leaf segments to generate superoxide anion radical remained at the level typical of control plants grown in water. The content of 5-aminolevulinic acid (ALA) in plants grown in the presence of 20 mM KNO₃ was 2.2–2.4 times higher than in control plants. The plants grown in the presence of nitrate had an elevated content of chlorophylls a and b, heme, and protein (by 42%). At the same time, the proline content was almost twofold lower than in control plants, which was due to substantial reduction (by 40%) in activity of Δ¹-pyrroline-5-carboxylate synthetase (P5CS). It is concluded that the substrate activation of NR by KNO₃ under normal growth conditions results in predominant utilization of glutamic acid (the primary product of inorganic nitrogen assimilation) for biosynthesis of tetrapyrroles and protein amino acids at the expense of inhibition of proline synthesis. When barley seedlings were grown in 150 mM NaCl solution, the plant growth and the root system development were suppressed to the levels of 63 ± 6% and 61 ± 11% of the control values, respectively. In the apical leaf tissues of plants adapted to NaCl, there was a slight decrease in the total NR activity (by 10%), a significant reduction in protein content (by 32%), and a parallel increase in the content of ALA (by a factor of 4.3), chlorophylls, heme, carotenoids, proline (2.2-fold) and P5CS (1.6-fold) with respect to the control values. It is proposed that the accumulation of ALA and proline under salinity-induced suppression of nitrogen assimilation results from the predominant allocation of glutamate for biosyntheses of ALA and proline at the expense of inhibition of growth-related processes requiring intense protein synthesis. The substrate activation of NR by KNO₃ under salinity conditions was associated with prevailing allocation of the assimilated nitrogen for synthesis of proline and protein amino acids, which reinforced plant cell protection against salinity and stimulated plant growth.     

Stachyose Synthesis in Source Leaf Tissues of the CAM Plant Xerosicyos danguyi H. Humb

Leaf tissues from Xerosicyos danguyi H. Humb., a succulent member of the Cucurbitaceae, were found to possess both galactinol synthase activity and the capacity for photosynthetic production of stachyose, the phloem transport oligosaccharide common to other nonsucculent cucurbits. The amounts of stachyose isolated from leaf tissues, and the extractable activity of galactinol synthase, were somewhat higher in leaf tissues obtained from plants operating in the Crassulacean acid metabolism (CAM) mode (well watered plants) compared to leaf tissues from plants operating in the CAM-idling mode (water-stressed plants). In contrast, in leaf discs, the photosynthetic incorporation of label into stachyose following pulse labeling with ¹⁴CO₂ was similar for stressed and for nonstressed tissues. Stachyose could be extracted from, and was synthesized photosynthetically by, leaf discs which contained no vascular tissues, indicating that synthesis of stachyose can occur in photosynthetic mesophyll cells of Xerosicyos.     

Powdery mildew of tobacco (Erysiphe cichoracearum DC.)

Percentage germination, and growth of hyphae from single conidia of Erysiphe cichoracearum DC., were measured on leaf discs from topped and intact tobacco plants, grown in aerated nutrient solutions consisting of basal medium plus large or small amounts of potassium. The effect of supplying sodium was also studied. Discs were incubated on water and on 10% sucrose solution. Changes in free amino nitrogen and carbohydrate in comparable uninfected leaf discs, before and after incubation, were also measured. Potassium deficiency resulted in more free amino nitrogen and soluble carbohydrate and less insoluble carbohydrate, per cm.² of leaf. Spore germination was not greatly affected by treatments, though it was usually less on discs from potassium-deficient leaves. The pathogen grew slower on potassium-deficient leaf discs, whether they were incubated on water or on sucrose. Incubating discs from some leaves on sucrose, compared with water, gave greatly increased sugar content and less fungal growth; discs from other leaves had a much smaller increase in sugar, and hyphal length was similar to that on discs incubated on water. Sodium, when potassium was scarce, increased potassium deficiency symptoms, free amino nitrogen and sugar content, and resistance to powdery mildew.     

Effect of Potassium on Proline Accumulation in Maize during Wilting

Leaf discs from maize (Zea mays) grown at high as well as low level of potassium, were treated with different concentrations of KC1 for 2 h in light before exposing to wilting conditions. An enhanced accumulation of free proline effected by wilting was observed in potassium treated discs. Although proline accumulation was much less in leaf discs exposed to short-term severe wilting conditions than in those exposed to long-term mild wilting conditions, the effect of potassium was still evident. Under long-term mild wilting conditions, the enhanced effect was greater in leaf discs from plants grown at high level of potassium than in those from plants grown at low levels of potassium. Treatment with NaCl instead of KC1 did not lead to extra accumulation of proline.     

Centelloside accumulation in leaves of Centella asiatica is determined by resource partitioning between primary and secondary metabolism while influenced by supply levels of either nitrogen,phosphorus or potassium

In the present study we aimed to investigate the relevance of either N, P or K supply for herb and leaf yield and for centelloside concentrations in Centella asiatica L. Urban leaves. In this regard, we elucidated the causal relationship between assimilation rate, leaf N, P and K concentrations, herb and leaf production, and centelloside accumulation. The experiments were conducted consecutively in a greenhouse where C. asiatica was grown in hydroponic culture and fertigated with nutrient solutions at either 0, 30, 60, 100 or 150% of the N, P or K amount in a standard Hoagland solution. In general, the increase in N, P or K supply enhanced assimilation rate and herb and leaf yield. However, exceeding specific thresholds, the high availability of one single nutrient caused lower leaf N concentrations and a decline in assimilation rate and plant growth. Irrespective of N, P and K supply, the leaf centelloside concentrations were negatively associated with herb and leaf yield, which is in accordance with the assumptions of the carbon/nutrient balance and the growth differentiation balance hypotheses. Moreover, we found strong negative correlations between saponins and leaf N concentrations, while the respective sapogenins were negatively correlated with K concentrations. Using C. asiatica as model system, our experiments reveal for the first time that the accumulation of saponins and sapogenins is affected by resource allocation between primary and secondary metabolism and that besides carbon, also nutrient availability is relevant for the regulation of the centelloside synthesis. Finally, our results highlight the huge potential of optimized and carefully controlled mineral nutrition of medicinal plants for steering the bio-production of high-quality natural products.     

SALT REGULATION AND LEAF SENESCENCE IN AGING LEAVES OF JAUMEA CARNOSA (LESS.) GRAY (ASTERACEAE), A SALT MARSH SPECIES EXPOSED TO NaCl STRESS

The effects of increasing salt stress on leaf senescence and salt regulation were investigated in the halophyte Jaumea carnosa in hydroponic culture experiments. The plants were grown in Hoagland's nutrient solution plus additional NaCl salt (0, 300, 400, 500 mm NaCl). Decreases in nucleic acids, protein, and chlorophyll were used as indicators of leaf senescence. The results indicated no definitive pattern of acceleration in leaf senescence with increasing salt stress. Salt regulation was also unaffected as leaves aged under increasing NaCl concentrations. The results are consistent with those of previous studies of the halophyte which indicated that the species was very tolerant of high NaCl concentrations.     

Photosynthesis and photosynthate partitioning in n(2)-fixing soybeans

Leaf area, chlorophyll content, net CO₂ photoassimilation, and the partitioning of fixed carbon between leaf sucrose and starch and soluble protein were examined in Glycine max (L) Merr. cv Williams grown under three different nitrogen regimes. One group (Nod+/+) was inoculated with Bradyrhizobium and watered daily with a nutrient solution containing 6 millimolar NH₄NO₃. A second set (Nod+/−) was inoculated and had N₂ fixation as its sole source of nitrogen. A third group (Nod⁻) was not inoculated and was watered daily with a nutrient solution containing 6 millimolar NH₄NO₃. The mean net micromole CO₂ uptake per square decimeter per hour of the most recently matured source leaves was similar among the three groups of plants, being about 310. Mean leaf area of the source leaves, monitored for net photosynthesis was also similar. However, the mean milligram of chlorophyll per square decimeter of Nod+/− test leaves was about 50% lower than the other groups' leaves and indicated nitrogen deficiency. Thus, Nod+/− utilized their chlorophyll more efficiently for photosynthetic CO₂ uptake than the plants of the other treatments. The ratio of foliar carbohydrate:protein content was high in Nod+/− but low in the plants from the other two treatments. This inverse relationship between foliar protein and carbohydrate content suggests that more fixed carbon is diverted to the synthesis of protein when nitrogen availability is high. It was also found that Nod+/− sequestered more storage protein in their paraveinal mesophyll than plants of the other treatments. This study indicates that when inorganic nitrogen regimes are used to control photosynthate partitioning, then both leaf carbohydrate and leaf protein must be considered as end products of carbon assimilate allocation.     

Inductive responses of some organic metabolites for osmotic homeostasis in peanut (Arachis hypogaea L.) seedlings during salt stress

The salt tolerance of peanut (Arachis hypogaea L.) seedlings was evaluated by analyzing growth, nutrient uptake, electrolyte leakage, lipid peroxidation and alterations in levels of some organic metabolites under NaCl stress. The plant height, leaf area and plant biomass decreased significantly in salt-treated seedlings as compared with control. The relative water content (RWC %) of leaf decreased by 16 % at high concentrations of NaCl. There was an increase in the lipid peroxidation level and decrease in the electrolyte leakage at high concentrations of NaCl. The total free amino acid and proline contents of leaf increased by 5.5- and 43-folds, respectively in 150 mM NaCl-treated plants as compared with control. Total sugar and starch content increased significantly at high concentrations of NaCl. Chl a, Chl b, total chlorophyll and carotenoid contents decreased significantly at high salinity. Na⁺ contents of leaf, stem and root increased in dose-dependent manner. K⁺ content remained unaffected in leaf and root and decreased in stem by salinity. The results from present study reveal that the peanut plants have an efficient adaptive mechanism to tolerate high salinity by maintaining adequate leaf water status associated with growth restriction. In order to circumvent the stress resulting from high salinity, the levels of some organic metabolites such as total free amino acids, proline, total sugars and starch were elevated. The elevated levels of the organic metabolites may possibly have some role in maintenance of osmotic homeostasis, nutrient uptake and adequate tissue water status in peanut seedlings under high-salinity conditions.