Ovary abortion at low w
Despite the improvements in synchronization of pollen shed and silk emergence, kernel numbers often continue to be low when w is low around the time of pollination (Westgate and Boyer, 1985). Westgate and Boyer (1986b) focused on this question by shortening the exposure to low w. Rather than subjecting the plants to low w from pollination until maturity, they shortened the exposure by rewatering the plants soon after pollination. As few as 3 d of low w (water withheld for 6 d) caused the same decrease in kernel numbers as did the prolonged exposure. The failure of kernels to develop was irreversible and indicated that abortion had occurred.
Reciprocal cross-pollinations between plants at high and low w revealed that pollen having w as low as –15 MPa remained viable and able to fertilize the egg (Westgate and Boyer, 1986b). The female florets were not viable at such low w and were unable to form kernels at w below about –1.2 MPa. Therefore, the abortion was controlled by the female inflorescence.
In general, the silks of female florets were wetter than pollen. The silks hydrated the pollen and promoted pollen tube growth. Fertilization was typically successful even at the lowest floret w, and each unformed kernel contained an undeveloped embryo and remnants of an endosperm (Westgate and Boyer, 1986b). Reproduction was thus successful except for the last phase, when the embryo and endosperm were developing. The ears had low kernel numbers at maturity (cf. Fig. 1A, C).
Westgate and Boyer (1986b
varied the time around pollination when low w
was present. As the time increased after pollination, exposure to low w
resulted in less abortion. By 10–14 d after pollination, exposure to low w
rarely caused abortion (Jurgens et al.
; Westgate, 1994
). Conversely, low w
before pollination but followed by pollination at high w
resulted in nearly total abortion (Westgate and Boyer, 1986b
). Evidently, the decision to abort occurred at low w
and events were set in motion that terminated embryo development a few days later when w
were high. Because no embryo or endosperm was present when abortion was initiated, the ovary itself was the target for the abortion signal. It appeared that the abortion involved the entire ovary unit, regardless of whether an embryo and endosperm were present.