Kernel Set at Low Water Potential Does Not Vary with Source/Sink Ration in Maize

Water deficits during pollination increase the frequency of zygotic abortion 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 tested this possibility by altering the vegetative source to reproductive sink ratio in the inbred line A188 prior to imposing a water deficit at pollination. Relative assimilate supply was modified by growing plants 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 chamber. Sink size was modified genetically using a chromosome interchange (reciprocal translocation) mutant (T3-7c) that is near-isogenic to the wild type (A188WT) but sets only 50% of the fertilized ovaries. Six treatment-genotype combinations produced a seven-fold range in photosynthesis 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 photosynthesis was <2 μmol m −2 s −1 and silk water potential (Ψ w ) reached −1.1 MPa. Upon rehydration 2 d after pollination (DAP), leaf Ψ w and photosynthesis rapidly recovered to control values. The water deficit decreased kernel number per ear 40 to 70% compared to the controls. Plants with a high source/sink ratio (e.g., T3-7c, field) were just as sensitive to low Ψ w at pollination as plants with a low source/ sink ratio (e.g., A188WT, growth chamber). These results indicate that assimilate supply per kernel alone does not determine the sensitivity of kernel set to low Ψ w at pollination