KERNEL SET AT LOW WATER POTENTIAL DOES NOT VARY WITH SOURCE SINK RATIO IN MAIZE/

Water deficits during pollination increase the frequency of zygotic ab ortion in maize (Zea mays L.). Recent work suggests that reproductive failure occurs because assimilate supply in water-deficient plants is not sufficient to maintain growth of all newly formed zygotes. We test ed this possibility by altering the vegetative source to reproductive sink ratio in the inbred line A188 prior to imposing a water deficit a t pollination. Relative assimilate supply was modified by growing plan ts in 20-L pots containing 22 kg of amended soil in three environments : (i) the field, (ii) the field under 55% shade, or (iii) a growth cha mber. Sink size was modified genetically using a chromosome interchang e (reciprocal translocation) mutant (T3-7c) that is near-isogenic to t he wild type (A188WT) but sets only 50% of the fertilized ovaries. Six treatment-genotype combinations produced a seven-fold range in photos ynthesis per kernel and a four-fold range in reserve sugars per kernel at silk emergence. Water was withheld on the first day silks emerged and plants were hand pollinated 4 to 7 d later when leaf photosynthesi s was <2 pmol m(-2) s(-1) and silk water potential (Psi(w)) reached ap proximate to -1.1 MPa. Upon rehydration 2 d after pollination (DAP), l eaf Psi(w) and photosynthesis rapidly recovered to control values. The water deficit decreased kernel number per ear 40 to 70% compared to t he controls. Plants with a high source/sink ratio (e.g., T3-7c, field) were just as sensitive to low Psi(w) at pollination as plants with a low source/sink ratio (e.g., A188WT, growth chamber). These results in dicate that assimilate supply per kernel alone does not determine the sensitivity of kernel set to low Psi(w) at pollination.