Utilização de nitrato e ajustamento osmótico em plantas de feijão-de-corda [ Vigna unguiculata (L.) Walp ] submetidas a diferentes níveis de estresse salino

Abstract

Salinity is largely the main environmental factor accountable for reduced agricultural productivity, mainly in arid and semi- arid regions. Nitrate ion is the most available form of nitrogen in the soil, often limiting plant growth. In addition, N-N03- seems related to plant osmotic adjustment and salinity tolerance. In this work, we have evaluated the salinity (NaCl) effect on some steps of the N03- utilization, from its uptake, N03- assimilatory reduction and ammonium assimilation in relation to the adaptive mechanisms and osmotic adjustment in cowpea [Vigna unguiculata (L.) Walp] plants, cv vita 7 - a moderately salinity tolerant glicofitic species. The plants were subjected to different salinity intensity (O, 50, 100 and 200 NaCImM) and N03- levels (O, 5 and 10 mM), under the experimental condition of summer season in Fortaleza, Ceará, Brazil, under high solar radiation and temperature rates from 26 to 36°C during the day and from 22 to 26°C during the night and relative humidity of 39 (day) to 80% (night). When cultured from germination with NaCI 50mM the plants presented N03- uptake rates and NR activity values at a levei compatible with the requirement of amino acids to support protein synthesis and plant growth. In spite of a low leaf GS activity, it seems likely that NH4+ assimilation from exogenous N03- was not a limiting step for the plant amino acids supply. Thus, under moderate stress, the intensity of the N03- uptake appeared to be adjusted by the metabolic neecl for the plant growth. At that condition, the cowpea plants did show a sharply adaptive response to salinity through an efficient mechanism of the osmotic adjustment. The salt effect was more pronounced at the initial stage of the plant growth. Already from the onset of the exponential growing there was a clear recovery in the rates of the plant growth. This salt adaptive response was related to a compartmentalization and accumulating of the Na+ And CI- ions at the root and stem tissues preventing its accumulation on the leaves, mainly chloride. The plants performed a efficient control on the cr uptake that, in turn, was proportional to the plant growth rate. On the other hand, the plants were not able to use the available K for osmotic adjustment. At a high NaCl levei (100 mM), the salinity led to a decrease on the N03- uptake, water transpiration and NR activity rates. On that salt condition, the plant was able to develop an efficient mechanism of osmotic adjustment. The NaCl effect on the NR activity seemed to be indirect most likely through a decrease on the N03- flux coming from leaf and fali in the photosynthesis rate. When the NaCl levei was raised to 200 mM, both NR activity and transpiration rates, were drastically reduced. It could be taken in account the salt direct effect such as lonlc toxicity and still due to some other indirect effect from plant metabolic disturbance induced by the Na+ and CI- ions. Despite a strongly reduction on the growth, the plants did not exhibite turgescency 1055 or chlorophyll degradation. However, salt treated plants were not able to accumulate free amino adds, mainly proline in the leaves, enough to contribute with the osmotic adjustment. The plant processes such as N03- uptake, nitrate reduction and ammonium assimilation pathway were not limiting for the adaptation and growth of cowpea plants subjected to a moderate NaCl leveI. However, under high NaCI levei, N03 uptake and nitrate reduction might llrnlt plant adaptation and growth. Despite Vigna unguiculata to show a moderate tolerance to salinity, it seems to be important to introduce genetic improvement related to salinity tolerance by the introduction of physiological traits more efficient to accumulate inorganic ions and compatible organic solute in response to environrnental stress such as soil salinity, water deficit and high temperature.