Histochemical, physiological and ultrastructural changes in the maize embryo during drying

Abstract

<p>A critical decision that hybrid maize seed producers confront each year is how to properly dry large volumes of seed and still maintain a high quality product suitable for planting. In many production areas, harvest may begin quite early when the seed is at high moisture content (45-50%) in an effort to avoid damage by early frost or to escape the action of insects and diseases. In this scenario, varying rates of moisture removal and/or high drying temperatures (>45°C) may jeopardize seed quality. Previous studies (Herter and Burris, 1989) have shown that initial drying at lower temperatures (20°-35°C) before subsequent high temperature drying resulted in improved seed quality. However, during this period seed moisture content may remain high thus increasing the risk of seed deterioration. Here evidence is reported of increased amylolytic activity and degradation of starch grains in seeds that were preconditioned at 35°C and high humidity (90% RH). Higher respiration rates were observed in those seeds held at 35°C, compared to 20°C. Although overall respiration during preconditioning was low, as compared to a germinating seed, it may still represent a source of deterioration. Seeds preconditioned at a lower temperature (20°C) and lower relative humidity (35%) did not show signs of such deterioration. Cells in the radicle meristem exhibited a distinct migration of the lipid bodies toward the cell walls in response to desiccation. In addition, a relationship between lipid alignment and leakage during imbibition is described. Protein body formation was also evident in these cells and this accumulation began early during preconditioning. Only in seeds that experienced the highest level of desiccation was this process nearly complete. No apparent deterioration was observed in the mitochondria. They appeared abundant, swollen, well differentiated, and remained intact throughout the entire period. The highest seed quality was obtained in seeds preconditioned at 35°C/35% RH. These conditions have a positive effect on the metabolic and/or morphologic changes that confer high temperature desiccation tolerance, while minimizing the negative impact of the catabolic events described.</p>