Moisture stress during pollination of maize (Zea mays L.) can greatly reduce kernel set, yet little quantitative information is available on the effects of plant water status on male and female floral development. The purpose of this study was to establish different drought stress regimes during pollination and to measure synchronization of male and female floral development, pollen viability, and diurnal silk elongation rates. Single cross hybrids were field‐grown in large pots and exposed to different soil moisture treatments at the time of tassel emergence. Compared to well‐watered control plants, mild (no visible wilting) and severe (visible wilting) drought treatments increased the interval from initial silking to initial pollen shed by an average of 3 and 4 days, respectively. Increasing moisture deficits caused no change in in vitro pollen germination even though the severest drought treatment caused visible symptoms of midday wilting and of lower leaf senescence. Diurnal silk elongation measurements indicated that on clear days the majority of silk elongation occurs at night when ear leaf water potentials are highest. At similar morning leaf water potentials, stressed plants maintained a lower silk elongation rate than well#x2014;watered plants. Positive silk elongation ceased at ear leaf water potentials of about #x2014; 9 bars in droughted plants and at #x2014; 14 bars in well—watered plants, suggesting that factors other than water potential may also regulate rate of silk growth. It is concluded that drought beginning at anthesis has a greater effect on female than male floral development.

High temperatures during maize (Zea mays L.) pollinatiou are known to result in poor kernel set, but little is known of the direct effects of temperature on pollen germination. The purpose of this research was to determine how in vitru pollen germination of different maize genotypes is affected by high temperature stress during anthesis. Tassels from field‐grown plants were excised at beginning anthesis, placed in water and transferred to growth chambers maintained at daytime temperatures of 27, 32, and 38 C. Nighttime temperatures were maintained 6 C cooler. In vitro germination was measured after 24 and 48 hours in the growth chamber as well as on pollen collected directly in the field. Genotypes differed in their response to temperature. In some genotypes pollen germination steadily decreased as temperature increased. Others either germinated equally well at 27 and 32 C or germinated better at 32 than at 27 C. All genotypes had a lower germination at 38 C than at 32 or 27 C, and several genotypes exhibited no germination after 48 hours at 38 C. After 24 hours in the 38 C chamber, six inbreds widely used in the 1970's germinated significantly better as a group than inbreds widely used in the 1950's and 1930's. Growth environment affected the absolute in vitro germination percentage, but in general genotypes retained similar relative responses to increasing temperature. Results from this study suggest that prolonged exposure to temperatures above 32 C can reduce pollen germination of many genotypes to levels near zero.

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