ASSIMILATE FLUX DETERMINES KERNEL SET AT LOW WATER POTENTIAL IN MAIZE

Plant water deficits during flowering came maize (Zea mays L.) kernels to abort. Lack of current and reserve photosynthate account for much of the kernel toss, but partitioning to ovaries at low ovary water pot ential (psi(w)) may also be limited by lack of assimilate demand. To t est this possibility, we measured the water status, carbohydrate conte nt, and growth elf ovaries on plants grown in the field in pots contai ning 22 kg of soil under one of three light environments [Control, 75- cm rows, 43 055 plants ha(-1); Shade, same as Control except under sha de cloth (55% light interception) from the sixth leaf stage until phys iological maturity; or Isolated, 122-cm rows, 6727 plants ha(-1)] and exposed to a water deficit during pollination. Water was withheld at s ilk emergence and plants were hand-pollinated 4 d later when silk psi( w) was approximate to - 1.1 MPa, leaf psi(w) was approximate to - 1.8 MPa, and photosynthesis was completely inhibited. The water deficit de creased kernel set, which was correlated with the inhibition of ovary dry matter accumulation. The concentration of sucrose and glucose incr eased in ovaries of water-deficient plants, and ovary turgor remained at or above control levels. Thus, inhibition of ovary growth at low ps i(w) was not related to a loss of turgor, nor was it caused by a deple tion of ovary sugars. Sugar accumulation at low psi(w) suggested that metabolism may have been impaired. Coupled with a low level of reserve s, failure to utilize available sugars at low psi(w) would severely in hibit assimilate flux to the ear and render kernel set highIy vulnerab le to water deficits during pollination.