Assimilação de nitrogênio e acumulação de solutos em plantas de cajueiro (Anacardium occidentale L.) em resposta ao estresse salino

Abstract

In order to evaluate effects and mechanism of salinity (NaCI) in nitrogen assimilation and solute accumulation, 40 day-old cashew plants (Anacardium occidenta/e l.), clone CCP 1001, were grown in a salinity medium, under green house condition with internal day temperature changing from 28 to 36°C and night temperature varying from 24 to 27°C and reiative humidity of 40% (day) and 80% (night). The maximal photosinthetic active radiation (PAR) close to the leaves was 1,000 umol.rnê.s'. The results showed that decline of fresh mass was dependent on intensity and time of exposure to NaCI. With regard to decrease in nitrate content on salt treated plants there was compatibility among it and fresh mass production, but nitrate content did not seem to be induced, at least in the earliest stage of salinity treatment, by the substantial fali in transpiration. Contrarily, reduction in transpiration was found to be the main factor responsible for the decrease of in vivo nitrate reductase activity (NR) both in leaf and root. Substantiated by experiments performed with NR enzymatic extract ( in vitro NR) may be suggested that the direct effect of Na+ and cr ions upon NR activity is poor. In relation to GSactivity it was seen a different behavior. At about the same time in which GS activity increased in the leaf, it decreased in the root. It is interesting to emphasizethat increaseand decreaseof GSactivity in leaf and root, respectively, were followed by the concentrations of free amino acíds, particularly proline, and total soluble proteins. Proline in leaf of plants grown upon 100 mrnol.L" NaCl reached concentration about 17 fold higher than in controlo There, . occurred a marked augmentation of free NH4+ and a decrease of total N. On the other hand, increasing of N compounds in leaf, particularly proline, was not able to promote osmotic adjustment in cashew plants under the conditions of this study. Thus, in spite of augmentation of nitrogenous compounds concentration in leaf of cashew plants, they showed toxicity symptoms, probably due to accumulation of a+ and Cl' ions at a toxic leveI in that part of the plant. It could be taken to explain, at least in part, the pronounced sensitivity to NaCI exhibited by cashew plants. In addition, in salt treated plants there was considerable reduction of K+ content, mainly in the root. This fact points out to an enhancement of K+ efflux probably due to increase of permeability of root plasma membrane. Finally, the results presented in this study led to suggest that augmentation of leaf GSactivity in cashew, growing under condition of salinity, is induced by an additional supply of ammonium and glutamate, which, in turns, seemsto be provided by increase in amino acids and protein catabolism and also from increase in photorespiratory activity and from earlier induced leaf senescence.