HISTOCHEMICAL, PHYSIOLOGICAL, AND ULTRASTRUCTURAL-CHANGES IN THE MAIZE EMBRYO DURING ARTIFICIAL DRYING

When drying high moisture (450-500 g H2O kg(-1) fw) Zea mays L. seeds, a preconditioning period of drying at low temperatures (PC) may preve nt injury due to subsequent high temperature drying. We preconditioned mite seeds under four different environments (35 degrees C I 35% RH, 35 degrees C / 90% RH, 20 degrees C / 35% RW, and 20 degrees C / 90% R H) and determined the level of seed deterioration in terms of reserve stability, enzyme activity, respiration, and ultrastructural changes i n the radicle meristem, During PC, respiration rates were significantl y higher for the 35 degrees C than the 20 degrees C treatments; howeve r, overall respiration during PC was considerably lower than during ge rmination, After 48 h of PC, amylase activity was present in all of th e PC treatments with the highest levels and additional isozymes in the 35 degrees C / 90% RH treatment. This treatment was also the only one in which we found degradation of starch grains in the histochemic al analysis, During PC, transmission electron micrographs revealed migrat ion of lipid bodies toward the cell wall and formation of protein bodi es within vacuoles. Both of these processes were more likely to appear in those treatments that allowed higher rates of desiccation or in wh ich the seeds were more mature (Harvest 2). Mitochondrial cristae were well differentiated with no evident impairment of their integrity in any of the PC treatments, Better seed quality was obtained when seed w ere preconditioned at moderate (35 degrees C) temperature and low (35% ) relative humidity. We conclude from these results that the positive effects of preconditioning may occur as a balance between metabolic an d morphological processes resulting in high temperature desiccation to lerance, while minimizing the negative impact of the catabolic events such as degradation of starch grains and higher respiration rates.