Rafael M. Aragão

Nitrate uptake, xylem NO3- flux, and nitrate assimilation in cowpea exposed to salinity.

This work was carried out to evaluate what is the nitrate acquisition stage (nitrate uptake, xylem nitrate flux or assimilatory reduction) most influenced by the presence of NaCl in cowpea. Twelve day-old seedlings were treated with 50 mM of NaCl in nutrient solution during four days and measurements carried out under two contrasting environmental conditions: typical day (full sun) and completely cloudy day (cloudiness). The salinity affected more intensely the xylem sap flux and nitrate flux than transpiration. Plants treated with NaCl showed a strong decrease in both nitrate uptake rate and leaf nitrate reductase activity as in the full sun as in cloudy day. Transpiration was reduced by the cloudiness while xylem sap flux and nitrate flux remained unchanged, in both salt-treated and control. Moreover, nitrate uptake and nitrate reductase activity were less affected by cloudiness than the transpiration. In addition, NaCl negatively affected nitrate accumulation in roots, stems and leaves while the cloudiness affected only the leaf nitrate accumulation, both in control and stressed plants. Salinity affects more negatively the nitrate xylem flux, as compared with the nitrate uptake and nitrate assimilatory reduction in cowpea leaves.

Salinity affects indirectly nitrate acquisition associated with glutamine accumulation in cowpea roots

The aim of this study was to test the hypothesis that salinity can affect indirectly the nitrate acquisition by a negative modulation triggered by glutamine accumulation. Cowpea plants were exposed to a mild NaCl concentration (50 mM) in order to restrict growth and N-demand. After 21 d, pretreated plants and control plants were supplied with 0, 5 and 10 mM of Ca(NO3)2 for 3 d in absence of NaCl. Salt pretreated plants showed a great limitation in acquisition of NO3−, indicated by decline in the nitrate uptake rate, NO3− accumulation, nitrate reductase activity and protein content. The restriction of NO3− utilization was positively associated with increased glutamine synthetase activity and glutamine accumulation, especially in roots.

Temperaturas elevadas afetam a distribuição de íons em plantas de feijão caupi pré-tratadas com NaCl1

The purpose of this study was to characterize the effect of temperature on the distribution of Na+, Cl- and K+ in roots, stems and leaves of cowpea pre-treated with NaCl. After the acclimation period, plants were divided into two groups: 0 NaCl and 100 mM NaCl for 2 days. Subsequently, the plants were divided into 4 sub-groups and subjected to increasing temperatures of 27, 32, 37 and 42 oC, separately, for a photoperiod of 12 hours at each temperature. The treatment without NaCl at 27 oC was used as reference. The concentrations of Na+ and Cl- in different organs were increased by pretreatment with NaCl and this accumulation was intensified by exposure to high temperatures. In roots, for example, at temperature of 42 oC concentrations of Na were four times higher than in leaves. While Cl- accumulated mainly in leaves, with a 20-fold increase in the concentration of this ion in relation to reference plants. Inversely, K+ concentrations were reduced in all organs analyzed. Thus, it is conclude that high temperatures affect the distribution of ions in different organs directly affecting ion homeostasis in plants.

Salinidade modula negativamente a absorção e assimilação de NO3- em plantas de feijão de corda

+ ), foram submetidas a KNO 3 e NaCl 100 mM por dez dias. Avaliou-se a absorcao e o conteudo de NO 3 - , atividade da Redutase do Nitrato (RN), transpiracao, proteinas soluveis e aminoacidos livres totais (ALT). A aplicacao do NaCl resultou na reducao da absorcao liquida e no conteudo de NO 3 - , principalmente, em raizes. A reducao da atividade da RN foi positivamente correlacionada com a reducao da transpiracao, mas nenhuma relacao foi estabelecida com a reducao nos niveis e proteinas soluveis, que foi mais eminente em raizes. Os niveis de ALT foram superiores em raizes de plantas controle e tratadas com NaCl. Os resultados indicam um possivel mecanismo de modulacao devido a menores niveis de proteinas e maiores niveis de ALT em raizes sob condicoes de salinidade. Em resumo, a salinidade modula negativamente a absorcao e assimilacao de NO 3 - em pl antas de feijao de corda, a principio pela reducao nos niveis de proteinas soluveis e pelo acumulo de aminoacidos em raizes, e a inibicao do tipo nao-competitiva pelo NaCl indica que a competicao entre NO 3 - e Cl - pelos sitios de absorcao nao e o fator mais limitante net uptake and content, nitrate reductase activity (RN), transpiration, soluble proteins and total free amino acids (ALT). Application of NaCl resulted in reduced net uptake and the content of NO 3 - , mainly in roots. The reduction of NR activity was positively correlated with the reduction of transpiration, but no relation was established with the reduction in the soluble proteins levels, which was more conspicuous on roots. ALT levels were higher in roots of control plants and treated with NaCl. The results indicate a possible mechanism of modulation due to lower protein levels and higher ALT levels in roots under saline conditions. In summary, salinity modulates the uptake and assimilation of NO 3 -

Salt-induced NO3 - uptake inhibition in cowpea roots is dependent on the ionic composition of the salt and its osmotic effect

Salinity remarkably inhibits NO3- uptake but the mechanisms are not well understood. This study was addressed to elucidate the role of ionic and osmotic components of salinity on NO3- influx and efflux employing classic kinetics involving a low affinity transport system (LATS) and a high affinity transport system (HATS). In the presence of KCl, NaCl, and Na2SO4 at 100 mM concentrations, in both LATS and HATS, Michaelis constant (Km) was similar for the three salts and maximum rate (Vmax) decreased as follows: KCl > NaCl > Na2SO4, compared to control indicating a non-competitive interaction with NO3-. Unexpectedly, iso-osmotic solutions (osmotic potential Ψπ = -0.450) of polyethylene glycol (PEG, 17.84 %, v/v) and mannitol (100 mM) remarkably increased Km in both the LATS and the HATS, but Vmax did not change indicating a competitive inhibition. Under the PEG and mannitol treatments, Km and Vmax were higher than under the salt treatments. The salts increased slightly NO3- efflux in the following order KCl > NaCl > Na2SO4. In contrast, mannitol strongly stimulated and the PEG inhibited NO3- efflux. The obtained data reveal that salinity effects were not dependent on the anion type (Cl- versus SO42-) indicating a non-competitive inhibition mechanism between Cl- and NO3-. In contrast, the cation types (K+versus Na+) had a pronounced effect. The osmotic component is important to net NO3- uptake affecting remarkably the influx in both LATS and HATS components of cowpea roots.