Aspectos fisiológicos e bioquímicos de plantas de dois cultivares de feijão de corda inoculadas com Bradyrhizobium sp. e submetidas à salinidade

Abstract

The use of inoculants containing nitrogen-fixing bacteria, increases annually in cowpea crop in North, Northeast and Midwest regions of Brazil. This work was carried out to evaluate the effects of symbiosis with Bradyrhizobium sp. in plant physiology under salt stress conditions. For this purpose, an experiment was conducted in greenhouse, with two cultivars: Pitiúba (tolerant to salinity) and TVu 2331 (sensitive to salinity), both inoculated with the strain of Bradyrhizobium SEMIA 6461. The experimental design was completely randomized with the following factorial arrangement: 2 (cultivars) x 2 (inoculated and non-inoculated plants) x 2 (stress levels – 0 and 75 mM NaCl). For treatment with rhizobium, seeds were sown in the presence of 1 mL of the culture broth containing the inoculum, the seedlings being maintained for 12 days in this situation. After, inoculated and non-inoculated plants were subjected to salt treatment for 15 days. The results revealed that the bacteria strain reduced the efficiency in nitrogen fixation in cowpea subjected to salt treatment when compared to control treatment. The growth of c plants cowpea was inhibited by salt stress and inoculation with rhizobium was not able to minimize the deleterious effects of salinity, especially in cultivating TVu, which had its biomass most affected by this stress. The levels of toxic ions (Na+ and Cl-) in plants inoculated in relation to non-inoculated showed no significant differences, but the levels of potassium, nitrogen and phosphorus had increases in inoculated plants subjected to salinity. Regarding the levels of organic solutes in saline group plants, we can highlight that: inoculation with bacteria increased carbohydrate levels only in shoots of Pitiúba and roots of TVu; inoculation increased N-amino levels in all parts of both cultivars, except for the roots of Pitiúba; proline levels have increased in the whole plant, except the roots of TVu; protein content invariably increased in all parts of the plants. The net photosynthesis of Pitiúba were not altered in any treatment, but TVu inoculated treatments caused increases in this parameter compared to their respective controls. Excess salt inhibited stomatal conductance in both cultivars, but the maximum quantum yield of PSII did not change in saline treatments, proving that the photosynthetic apparatus of cowpea is tolerant to salinity. Total chlorophyll contents increased in response to salinity, both in inoculated plants as in non-inoculated plants. Oxidative stress induced by salinity led to an increase in the activity of peroxidase and ascorbate peroxidase guaiacol in leaves and roots of both cultivars of nodulated plants. Furthermore, salinity increased activity of the enzyme superoxide dismutase in the roots, despite inoculation have higher enzyme activity in roots of Pitiúba. The increased activity of these enzymes in saline conditions may be related to the inoculation, whereas there was a reduction in lipid peroxidation in plants of both cultivars inoculated and stressed. The salinity negatively affected biomass production of cowpea due to effects caused by the accumulation of toxic ions. Although the nodulation has raised osmoprotectors levels, increased activity of antioxidant enzymes in roots and reduced oxidative damage, there was no significant improvement in growth of the plants of both cultivars studied when subjected to salinity.