Indicadores de deficiência hídrica em três variedades de milho (Zea Marys L)

Abstract

A study was made of the effect of water stress on three corn (Zea mays L.) cultivars: Piranao, Centralmex and IPEACS III. Under the effect of a - 15 atm stress (using mannitol as osmoticum), Centralmex showed the highest percent germination while IPEACS III showed the lowest. The germination of Piranão seeds fell in between. There also differences in root and coleoptile growths of the three varieties as compared to that of the controls (germinated in demineralized water). At a more advanced stage of grouth (6-8 leaves, plants in pots kept in a greenhouse), the soil-water stress affected the plants differently. IPEACS III continued to be severely affected but Piranão was now the least affected, with Centralmex falling in between. The effects of water stress upon the plant metabolic patterns resulted in a increase in the leaf content of nitrate-N, free amino-N, ammonia-N, free proline and soluble sugars.181 There was on the other hand a reduction in nitrate reductase activity and protein content. Although the same general metabolic pattern was observed for the three varieties, significant differences could be detected among the varieties. These results indicated that the greenhouse environment was adequate for the study of plant responses to water stress, even enabling the distinction between varietal differences. The content of free amino-N and soluble sugars in the plant leaves showed a higher sensitivity to moderate water stress than the other indicators. A special attention was given to the study of the increase in free proline levels in the leaves. The ellucidation of the factors governing the relative accumulation of this metabolite in corn leaves requires more detailed studies. Only then could proline content be considered a safe indicator of water stress resistance for corn plants. The plants under water stress were also submitted to growth analysis. This test was very efficient for the study of dry weight and leaf expansion variation among varieties. The results of this study shows that three main factors are responsible for the plant resistance to water stress, namely: a. Tolerance to dehydration (differences in R.W.C.) b. resistance to loss in protein (differences in protein-N levels and free-N/protein-N ratios)182 c. increasc in root/shoot ratios. The present study shows the necessity of experiments with field grown plants, to check the sensitivity of the various factors studied and to compare the behavior of different corn varieties in the field with that in greenhouse.