Pollination at low w
In the grains, losses in kernel number were often ascribed to a lack of fertilization of the egg, resulting in an undeveloped ovule. Pollen was sterile in barley, wheat, and rice when low w occurred during microsporogenesis (Saini and Westgate, 2000). ABA applied to the shoot in various ways caused similar pollen sterility (Saini and Westgate, 2000). Because ABA concentrations increase when plants have low w, high ABA was at first thought to account for pollen sterility (Saini and Westgate, 2000). However, using split roots to change ABA concentrations without lowering w, Dembinska et al. (1992) found that elevated ABA alone did not induce low grain numbers in wheat. Later, Dorion et al. (1996) and Sheoran and Saini (1996) reported decreases in invertase activity and starch accumulation in the developing pollen of wheat and rice when w were low. These investigators ascribed pollen sterility to losses in invertase activity that resulted in an inability of the pollen to metabolize incoming sucrose to hexoses.
In maize, low kernel numbers were also occasionally ascribed to sterile pollen (Lonnquist and Jugenheimer, 1943). However, Westgate and Boyer (1986b) and Schoper et al. (1987) found little evidence for pollen sterility at low w. Instead, Schoper et al. (1987) reported a sensitivity of pollen to high temperatures. High temperatures often accompany low w in the field, and there was genotypic variation for this sensitivity. In the absence of high temperature, pollen that developed normally retained its viability when low w occurred (Westgate and Boyer, 1986b). Silks sometimes emerged too late to be pollinated (Moss and Downey, 1971; Edmeades et al., 1993; Herrero and Johnson, 1981). Bassetti and Westgate (1993) found that silks lost receptivity to pollen as they aged, and receptivity was lost earlier if the plants were subjected to a water limitation. Much emphasis has been placed on minimizing these asynchronous effects by genetic selection. Edmeades et al. (1993) selected for genotypes exhibiting precocious silk growth (i.e. appearing before pollen shed begins), and this early development allowed silks to remain within the pollen-shed interval despite the delaying effect of low w. Precocious silk emergence is evident in many modern maize hybrids and contributes significantly to their improved drought performance.