Exogenous ornithine is an effective precursor and the δ-ornithine amino transferase pathway contributes to proline accumulation under high N recycling in salt-stressed cashew leaves.

The role of the δ-ornithine amino transferase (OAT) pathway in proline synthesis is still controversial and was assessed in leaves of cashew plants subjected to salinity. The activities of enzymes and the concentrations of metabolites involved in proline synthesis were examined in parallel with the capacity of exogenous ornithine and glutamate to induce proline accumulation. Proline accumulation was best correlated with OAT activity, which increased 4-fold and was paralleled by NADH oxidation coupled to the activities of OAT and Δ(1)-pyrroline-5-carboxylate reductase (P5CR), demonstrating the potential of proline synthesis via OAT/P5C. Overall, the activities of GS, GOGAT and aminating GDH remained practically unchanged under salinity. The activity of P5CR did not respond to NaCl whereas Δ(1)-pyrroline-5-carboxylate dehydrogenase was sharply repressed by salinity. We suggest that if the export of P5C from the mitochondria to the cytosol is possible, its subsequent conversion to proline by P5CR may be important. In a time-course experiment, proline accumulation was associated with disturbances in amino acid metabolism as indicated by large increases in the concentrations of ammonia, free amino acids, glutamine, arginine and ornithine. Conversely, glutamate concentrations increased moderately and only within the first 24h. Exogenous feeding of ornithine as a precursor was very effective in inducing proline accumulation in intact plants and leaf discs, in which proline concentrations were several times higher than glutamate-fed or salt-treated plants. Our data suggest that proline accumulation might be a consequence of salt-induced increase in N recycling, resulting in increased levels of ornithine and other metabolites involved with proline synthesis and OAT activity. Under these metabolic circumstances the OAT pathway might contribute significantly to proline accumulation in salt-stressed cashew leaves.

Photosynthetic changes and protective mechanisms against oxidative damage subjected to isolated and combined drought and heat stresses in Jatropha curcas plants.

Photosynthetic changes and protective mechanisms against oxidative damage were evaluated in Jatropha curcas leaves subjected to drought and heat stresses, both individually and combined, in order to elucidate the synergistic and antagonistic mechanisms involved with these abiotic factors. Both the drought and heat stresses caused significant damage to the leaf membrane integrity and lipid peroxidation, and the combination of these stresses greatly enhanced these physiological disturbances. The leaf CO(2) assimilation rate, stomatal conductance and instantaneous carboxylation efficiency (P(N)/C(I)) were significantly decreased in all plants subjected to stressful conditions in comparison to unstressed plants (reference). In contrast, a reduction in photochemical activity was observed only in plants exposed to drought and drought+heat conditions. Catalase (CAT), ascorbate peroxidase (APX) and superoxide dismutase (SOD) activities were stimulated only under heat stress, whereas APX activity was increased in all treated plants in comparison to the references. Moreover, the leaf H(2)O(2) content was increased similarly under all studied stresses. However, the balance of reduced and oxidized ascorbate did not show significant differences between reference and stressed plants. Although J. curcas plants acclimated to the studied stresses, they did not present an efficient mechanism for protection against drought-induced oxidative stress, especially when at high temperatures. However, heat-treated plants triggered an efficient enzymatic antioxidant system of reactive oxygen species scavenging and an effective protection against photochemical damages. The combination of drought and heat most significantly impaired the photosynthetic assimilation of CO(2) and the photochemical activity. These results indicate that drought greatly disturbs photosystem II activity and oxidative metabolism and that these negative effects are strongly stimulated by heat stress. The data also evidence that the combination of heat and drought triggers an intricate response involving antagonistic and synergistic interactions.

Source-sink regulation of cotyledonary reserve mobilization during cashew (Anacardium occidentale) seedling establishment under NaCl salinity.

Seedling establishment is a critical process to crop productivity, especially under saline conditions. This work was carried out to investigate the hypothesis that reserve mobilization is coordinated with salt-induced inhibition of seedling growth due to changes in source-sink relations. To test this hypothesis, cashew nuts (Anacardium occidentale) were sown in vermiculite irrigated daily with distilled water (control) or 50mM NaCl and they were evaluated at discrete developmental stages from the seed germination until the whole seedling establishment. The salt treatment coordinately delayed the seedling growth and the cotyledonary reserve mobilization. However, these effects were more pronounced at late seedling establishment than in earlier stages. The storage protein mobilization was affected by salt stress before the lipid and starch breakdown. The globulin fraction represented the most important storage proteins of cashew cotyledons, and its mobilization was markedly delayed by NaCl along the seedling establishment. Free amino acids were mostly retained in the cotyledons of salt-treated seedlings when the mobilization of storage proteins, lipids and starch was strongly delayed. Proline was not considerably accumulated in the cotyledons of cashew seedlings as a response to NaCl salinity. According to these results it is noteworthy that the salt-induced inhibition of seedling growth is narrowly coordinated with the delay of reserve mobilization and the accumulation of hydrolysis products in cotyledons. Also, it was evidenced that free amino acids, especially those related to nitrogen transport, are potential signals involved in the regulation of storage protein hydrolysis during cashew seedling establishment under NaCl salinity.

Roots and leaves display contrasting oxidative response during salt stress and recovery in cowpea.

In this study, we compare some antioxidative responses of leaves and roots associated to growth reduction in cowpea plants (Vigna unguiculata) during short-term salt stress and recovery. The salt treatment was imposed (200 mM NaCl) for six consecutive days and the salt withdrawal after 3 d. The salt treatment caused an almost complete cessation in the relative growth rate of both leaves and roots. Although NaCl withdrawal has induced an intense reduction in the Na(+) content from the leaves and roots, the growth recovery was slight, after 3 d. The leaf lipid peroxidation was increased in salt-stressed plants and slightly reduced in recovered plants after 3 d. Surprisingly, in the salt-stressed roots it decreased markedly after 3 d treatment and in the pre-stressed/recovered roots it was restored to levels near to the control. In leaves, catalase (CAT) activity showed a rapid and prominent decrease after 1 d of NaCl treatment and salt withdrawal had no effect on its recovery. In contrast, the root CAT activity was not changed by effects of both NaCl and salt withdrawal, over time interval. Leaf superoxide dismutase (SOD) activity did not change in all treatments, whereas in roots it significantly decreased after 3 d of salt treatment and recovered after NaCl withdrawal. Contrasting to the other enzymes, the guaiacol-peroxidase activity increased in leaves and roots, reaching almost 200% of control values and it significantly decreased in both organs from the pre-stressed/recovered plants. In conclusion, cowpea roots and leaves present distinct mechanisms of response to lipid peroxidation and CAT and SOD activities during salt stress and recovery. However, these responses and/or the oxidative damages caused by reactive oxygen species were not related with the growth reduction.

Superoxide dismutase, catalase and peroxidase activities do not confer protection against oxidative damage in salt-stressed cowpea leaves.

• The aim of this study was to determine whether guaiacol peroxidase (POX), superoxide dismutase (SOD) and catalase (CAT) activities are effective in the protection and recovery of cowpea (Vigna unguiculata (L.) Walp.) leaves exposed to a salt-induced oxidative stress. The salt treatment (200 mm NaCl) was imposed during six consecutive days and the salt withdrawal after 3 d (recovery treatment). Control plants received no NaCl treatment. • The salt treatment caused almost complete cessation of leaf relative growth rate in parallel with the transpiration rate. The restriction in leaf growth was associated with a progressive increase in membrane damage, lipid peroxidation and proline content. Salt withdrawal induced a significant recovery in both leaf growth rate and transpiration. Surprisingly, these prestressed/recovered plants showed only a slight recovery in leaf lipid peroxidation and membrane damage. • Leaf CAT activity experienced a twofold decrease only after 1 d NaCl treatment, and salt withdrawal had no effect on its recovery. SOD activity did not change compared with control plants. By contrast, POX activity significantly increased after 1 d NaCl treatment and showed a significant recovery to levels near to those of control. • In conclusion, it appears that the ability of cowpea plants to survive under high levels of salinity is not caused by an operating antioxidant system involving SOD, POX and CAT activities in mature leaves.