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Histochemical, Physiological, and Ultrastructural Changes in the Maize Embryo during Artificial Drying

Cargill Hybrid Seeds, 2600 West Galena Blv. Aurora, IL 60506
Agronomy Dep., Iowa State Univ., Ames IA, 50011

^* Corresponding author (antonio_perdomo{at}cargill.com).

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 prevent injury due to subsequent high temperature drying. We preconditioned maize seeds under four different environments(35°C / 35% RH, 35°C / 90% RH, 20°C / 35% RH, and 20°C / 90% RH) and determined the level of seed deterioration in terms of reserve stability, enzyme activity, respiration, and ultrastructural changes in the radicle meristem. During PC, respiration rates were significantly higher for the 35°C than the 20°C treatments; however, overall respiration during PC was considerably lower than during germination. After 48 h of PC, amylase activity was present in all of the PC treatments with the highest levels and additional isozymes in the 35°C / 90% RH treatment. This treatment was also the only one in which we found degradation of starch grains in the histochemical analysis. During PC, transmission electron micrographs revealed migration of lipid bodies toward the cell wall and formation of protein bodies within vacuoles. Both of these processes were more likely to appear in those treatments that allowed higher rates of desiccation or in which 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 were preconditioned at moderate (35°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 and morphological processes resulting in high temperature desiccation tolerance, while minimizing the negative impact of the catabolic events such as degradation of starch grains and higher respiration rates.

Received for publication June 2, 1997.

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