Aplicação foliar de prolina como meio de minorar os efeitos do estresse salino em plantas de milho

Abstract

The present study aimed to evaluate the influence of foliar application of proline in growth and development of maize plants under saline conditions, analyzing physiological and biochemical alterations involved. Seeds of maize (Zea mays) genotype BR 205, being carried two experiments in greenhouse. At first experiment, we evaluated the physiological and biochemical responses in maize plants under saline conditions and subjected to foliar application of proline. For this, the maize plants with 10 days old growing in individual buckets with 10 L, containing nutrient solution of Clark. Four treatments were conducted: 1. plants growing in nutrient solution and leaves sprayed with distilled water, 2. plants growing in nutrient solution and leaves sprayed with 30 mM proline 3.plants growing in nutrient solution containing 80 mM of NaCl and leaves sprayed with distilled water, and 4. plants growing in nutrient solution containing 80 mM NaCl and leaves sprayed with 30 mM proline. The experimental design was completely randomized, with two levels of proline (0 or 30 mM), two levels of salinity (0 or 80 mM NaCl) and two haverst times (plants 7 and 14 days after treatment application. ) with six replications. In this experiment, plants were collected at 7 and 14 days after the imposition of the treatments, the plants were separated into leaves, stems and sheath and roots. Were analyzed: dried weight of shoots, roots and whole plant, leaf area, concentrations of inorganic solutes (Na+, K+ and Cl-) and organic (soluble carbohydrates, soluble proteins, N-aminosolubles and proline). As result, it was found that the foliar application of proline itself had no effect on the growth of maize plants. However, in saline conditions, such treatment was effective in alleviating the reduction in growth by salinity. This was due, in part, by lower accumulation of toxic ions (Na+ and Cl-), mainly in shoot, combined with a smaller reduction in K+/Na+ plants treated with exogenous proline. Treatment with proline reversed the reduction in soluble protein content by salinity in shoots, roots while these levels were increased to levels higher than those of the controls themselves. The soluble carbohydrates and proline have been changed in different ways by treatment with proline, depending on the organ examined. The second experiment was conducted under similar conditions to the first and following the same treatments and experimental design. The objective was to evaluate the effect of foliar application of proline 30 mM in gas exchange, the levels of malondialdehyde and H2O2, and the activities of protection enzyme system and oxidative metabolism of proline in leaves and roots of maize plants under saline conditions. Salinity reduced gas exchange and exogenous proline caused no changes in these parameters, except for a small increase in the rate of transpiration. In leaves and roots, salinity increased the levels of H2O2 and malondialdehyde, both effects being partially reduced by foliar application of proline in maize plants. This treatment increased the activities of the enzymes superoxide dismutase and catalase, this last especially in leaves, having reversed its reduction in activity by salinity and possibly this may have contributed to the reduction of the levels of H2O2 and malondialdehyde, an indicator reliable membrane damage. Moreover, the study on the enzymes of the metabolism of proline was observed in leaves, which activity Δ1-pyrroline-5-carboxylate synthase (P5CS), enzyme of proline anabolic pathway, had increased his activity by salinity, but the treatment with exogenous proline strongly reduced its activity after 14 days of treatment. Already proline dehydrogenase (PDH), the enzyme proline catabolic pathway, was strongly stimulated by exogenous proline, under control or salt stress, which suggests that this enzyme is induced by excess proline in tissues. The role of these enzymes in tissue levels of proline could not be clearly established, since the response pattern in their activities by treatment with exogenous proline was not consistent between the two haverst times for maize.