WATER STATUS, HIGH TEMPERATURE, AND GENOTYPE EFFECTS ON SOME PROCESSES WHICH INFLUENCE MAIZE SEED SET (ZEA MAYS L., LOW WATER POTENTIAL, POLLEN VIABILITY, IN VIVO ASSAY, STRESS)

Drought and high temperatures during pollination and fertilization greatly reduce kernel number in maize (Zea mays L.). Studies were conducted to determine the response of ear receptivity (the ability of the ear to set seed if the silks are pollinated), pollen viability, and pollen production to heat and water stress. These processes are involved in determining kernel number.;Silk turgor was maintained at a higher level in the heat-sensitive hybrid than in the other hybrids when the plants were uniformly water stressed as measured by ear leaf water potential. There were no differences in silk osmotic potential among the hybrids. Therefore, the heat-sensitive hybrid maintained higher silk turgor due to some characteristic other than superior silk osmoregulatory capability. These data also indicate that the variations in silk turgor did not affect pollen germination and fertilization because seed set was decreased the most in the heat-sensitive hybrid. Water potentials of the pollen were unaffected by varying whole plant water levels and were considerably lower than those of the ear leaf. Pollen from a heat-stressed tassel was irregular in shape and/or shriveled.;Analysis of data collected from a diallel set of crosses for in vitro pollen viability, anther emergence, and tassel death or "blast" indicated both pollen production and viability must be considered in tassel heat tolerance. The tassel of the inbred Mo17 was most tolerant to high temperatures and was the only inbred with a Lancaster background. Some of the inbreds (e.g., B73) would be useful as testers for the traits measured since they were poor in the traits measured but well adapted generally.;Ear receptivity, measured by seed set, of a prolific, a commercial, and a high temperature sensitive hybrid decreased by 9, 15, and 31%, respectively, when the whole plant was water stressed. The water deficit was imposed from the day before pollination unit two days after pollination. Pollen viability was unaffected by a water deficit imposed one day before pollen collection. Kernel numbers were reduced 53, 72, and 87% for the commercial, prolific, and heat sensitive hybrids, respectively, when the tassel was heat stressed. The percent reduction in kernel number was higher for the heat-stressed pollen than for the well-watered pollen when the female plant was water stressed. However, the decreases in kernel number were not very different.