Early germination physiology in hybrid maize (Zea mays L.) embryos damaged by high-temperature desiccation
Artificial drying of hybrid maize (Zea mays L.) seed using air temperatures in excess of 40°C can reduce seed vigor without affecting viability. Seed that was dried using high-temperature (45°C) air exhibited lower early seedling dry weight accumulation when compared to low-temperature (35°C) treatments. In embryo culture experiments, an extended germination period was observed for excised embryos from high-temperature (HT) treatments compared to low-temperature (LT). This observation coincided with reduced respiration rates for imbibing axis tissue from HT samples. In addition, uptake rates continued to increase for LT samples through the first 6 h of imbibition while rates for HT axes were constant. Mitochondria isolated from HT treated axis tissue that had been imbibed for 4 h exhibited no respiratory control. However, ADP/O values of 0.79 and 0.77 were recorded for NADH and succinate assays, respectively, for mitochondria from LT treated axes. Electron micrographs of cells from the radicle meristem region showed that mitochondrial development during the initial 24 h of imbibition was impaired in HT treated samples compared to LT tissue. A visible disruption of the matrix structure in HT tissue after 0 and 6 h of imbibition resulted in the apparent degeneration of some mitochondria at 24 h while others appear to have achieved repair. Analysis of nucleotide extracts of imbibed axis tissue by high performance liquid chromatography showed lower triphosphate nucleoside levels in HT samples. After 2 h of imbibition in air-saturated water, ATP levels in LT samples had increased dramatically and were 45% larger than HT treatments. Though ATP levels increased in HT treated tissue through 6 h of imbibition, the pool size did not attain the level measured for LT samples at 2 h. A reduced capacity for de novo synthesis of adenine nucleotides was also evident in the HT samples with total adenylates increasing only slightly in the first 6 h of imbibition. Adenylate energy charge values for LT samples after 4 h of 0.87 were consistent with metabolically active tissue while HT samples did not attain the same energy status. These results indicate that mitochondrial function, during the initial stages of embryonic tissue hydration, is a key element in determining the rate of germination (whole seed) and subsequent seedling growth.