Aspectos fisiológicos e bioquímicos do estresse salino em plantas de milho

Abstract

ln this paper, the physiological and biochemical responses to salt stress ofrnaize plants were studied. Three experiments were performed in a greenhouse, and the plants were grown in nutrient solution with or without 100 mM NaCI added. The aim of fust experiment was to evaluate the saIt stress effects on growth, water relations, gas exchange and solute accumulation of eight maize genotypes. Salt stress reduced plant growth of ali genotypes but the genotypes BR5033 and BR5011 were characterized as the most salt-tolerant and saltsensitive, respectively. Stomatal response of the saIt-tolerant genotype was not affected by salinity. Among the studied parameters, shoot to root dry mass ratio, leaf sodium content and leaf soluble organic solute content showed no relation with salt tolerance. In contrast, sodium and soluble organic solutes accumulation in the roots as a result of salt stress appeared to play an important role in the acclimation to saIt stress of the maize genotypes studied. The second experiment evaluated the time course of salt stress effects on organic solutes and free amino acids accumulation, as well as the lipid peroxidation and activity of antioxidative enzymes in leaves and roots of maize genotypes differing in salt tolerance (BR5033 and BR5011). In leaves and roots of the two genotypes, soluble amino-N and soluble protein contents were unchanged or increased with salt stress. Soluble carbohydrate contents also stayed constant in BR5033, but decreased in both leaves and roots of B R S O 11 plants. Salt stress increased most of free arnino acids contents, in leaves and roots of two genotypes. However proline, threonine, arginine, serine, aspartate, and glycine were the amino acids that more contributed for the osmotic potential reduction. ln leaves of salt-stressed plants, SOD, APX, GPX and GR activities increased with time when compared to the controls. However the increase was more pronounced in the BR5033 than in the BR5011 genotype. Salt stress had no significant effect on CAT activity in leaves ofBR5033, but it was reduced in the BR5011 genotype. APX, GPX and GR activities remained unchanged in salt-stressed roots ofBR5033 genotype, but reduced the activity of alI studied enzymes in roots of the BR5011 genotype. The data showed that CAT and GPX enzyrnes had the greatest H202 scavenger activity. Moreover, CAT,.APX and GPX activities in conjunction with SOD activity seem to play an essential protective role in the H202 scavenging processes. Lipid peroxidation was enhanced only in salt-stressed leaves of the BR5011 genotype. These results indicate that oxidative stress may play an important role in salt-stressed maize plants and that the greater protection of BR5033 leaves and roots 3 from salt-induced oxidative damage results, at least in part, through the maintenance andlor increase of the activity of antioxidant enzymes. The third experiment evaluated the effects of H202 pre-treatment on plant growth, lipid peroxidation and activity of antioxidative enzymes in leaves and roots of a salt-sensitive maize genotype. H202 pre-treabnent induced an increase of salt tolerance during subsequent exposure to salt stress. This observation was confirmed by shoot and root dry masses and leaf are a measurements. In leaves of unacclimated plants, salt stress increased APX, GPX, and GR. However, in acclimated plants, salt stress increased the activities of all antioxidative enzymes. ln roots, the salt stress did not result in striking changes in enzyme activities, except for an increase in CAT activity in acclimated and a decrease in unacclimated plants at the end of experimental period. Salt stress increased lipid peroxidation in leaves, but had almost no effect in roots. The results suggest that differences in the antioxidative enzyme acti ities may, at least in part, explain the increased tolerance of acclimated plants to salt stress, and that H202 metabolism is involved as signal in the processes ofmaize saIt acclimation.