Assimilação de nitrogênio e ajustamento osmótico em plantas noduladas de feijão-de-corda [Vigna unguiculata (L.) Walp] submetidas ao estresse hídrico

Abstract

Despite the great importance of crop legumes in providing proteins to animais and men through atmospheric nitrogen recycling there is few works dealing with the physiology and biochemistry of nodulated plants in presence of nitrate and under water deficit. Cowpea [Vigna unguiculata (L.) Walp] nodulated plants, cv. Vita 3 and Vita 7, inoculated with Bradyrhizobium sp., CB-756strain, in presence or absence of 5 mM NO-3, were grown under greenhouse conditions in modified Leonard jars containing a 1 : 2 silica:vermiculite substrate and watered with a nutrient solution. The water stress, initiated at 28 days after emergence (DAE), was imposed by withholding water for four (experiments 1 and 2 ) or three (experiment 3 ) subsequent days followed by rehydration during two days in the case of experiments 1 and 2. Under these conditions the nodulated plants showed a good adaptation to the water deficit and an excellent recuperation after rehydration. In addition, the photosynthetic apparatus seemed to be not affected, based on chlorophyll concentrations. Under water stress nodules and leaves were more seriously affected although leaves suffered less as measured by the balance between protein mass. The stress tolerance of Vita 7 was recognized by root growth and drastic reduction of dry mass accumulation in the plant shoot. Cowpea plants showed intense and fast accumulation of free proline, particularly in roots, in response to a mild reduction in available water, probably as an osmotic adjustment. Soon after rehydration there was an abrupt decrease in proline concentration. The free amino acid and ureide concentrations increased intensively with the water deficit, whereas the relative composition and concentration of total amino acids stayed unaltered. Leaf proline biosynthesís was likely to be related to the increase in the glutamine synthetase (GS) activity induced by water deficit. The nitrate reduction process in leaves, measured through nitrate reductase activity and nitrate accumulation in plant tissues, was drastically reduced. However, 2 4 hours after rehydration, the stressed plants recovered their nitrate reductase activity very quickly and showed a significant fali in the nitrate concentrations in the root, stem and leaf. Contrary, nodule nitrate reductase activity was not affected during the water deficit which suggests the predominance of a constitutive isoform in this tissue in comparison with the enzyme present in leaves inducible by nitrate. The symbiotic fixation of nitrogen was greatly reduced under water deficit as evaluated by noclule growth, glutamine synthetase activity, leghemoglobin, and ureide concentrations into xylem sapoThis reduction as well as its recovering from water deficit was slower than recovery of leaf nitrate reductase activity and nitrate mobilization in the roots suggesting that noclules are more affected than leaves regarding to protein integrity when the plants were subjected to water deficit followed by rehydration. Noclulated Vita 3 and Vita 7 cultivars, when in presence of 5 mM NO-3, showed the same intensity of growth and dry matter distribution. The water deficit induced in Vita 3 a more conspicuous decrease in leaf dry matter. Vita 7 accumulated free proline mainly in the roots while Vita 3 accumulated free amino acids suggesting that they use different strategies of osmotic adjustment when submitted to water deficit. NoclulatedVita 3 in absence of nitrate procluced higher amount of total dry matter than Vita 7. In this condition, Vita 3 was, apparently, more sensitive to the water shortage since its leaves had morphological alterations characterized by an abnormal enlargement and leaf proline accumulation diminished in comparison with the same cultivar in presence of nitrate. Nodulated Vita 7 in absenceof nitrate showed a decline in proline accumulation, both in leaves and roots, in contrast with stressed Vita 7 plants in presence of nitrate. Both Vita 3 and Vita 7 cultivars showed similar accumulation of ureides in stem under drought conditions. In presence of 5mM nitrate, irrespective if under water stress or not, both cultivars showed similarities among N assimilation (nitrate reductase activity and nitrate accumulation) and in the parameters related to N2 fixation (Ieghemoglobin concentration, noclule glutamine synthetase activity, and ureide concentration in the xylem sap). The specific activity of leaf glutamine synthetase was slightly higher in both cultivars under water stress. Nodulated Vita 3, in absence of nitrate, showed a better N2 fixation rate in relation to the presence of nitrate and also in comparison with nodulated Vita 7 in absence of nitrate. This higher N2 fixation capacity was based on a better noclule growth, higher leghemoglobin and ureide concentrations, and higher flux of ureide into xylem sapo Overall the results showed that the two N2 assimilation processes (assimilatory reduction of NO-3 and N2 fixation and NH3 assimilation) were highly sensitive to the water stress in Vita 3 and Vita 7 and that the nodule recovering was slower than in the leaves probably owing to a higher protein degradation rate. The cowpea plants were able to evolve an osmotic adjustment through proline accumulation, particularly in the roots. Furthermore the presence of nitrate contributed positively for the adjustment of the nodulated plants under water deficit conditions.