GROWTH, WATER-USE, AND KERNEL ABORTION OF MAIZE SUBJECTED TO DROUGHT AT SILKING

Maize (Zea mays L.) grain yield is particularly sensitive to water deficits that coincide with the tasseling-silking period, causing marked reductions in grain number. More knowledge about crop responses to water supply is required, however, to explain the causes of kernel number reductions under the mild stresses characteristic of humid regions. The objectives of this study were to: (i) quantify crop evapotranspiration, E(c), and its relationship with shoot biomass production, grain yield, and kernel number; and (ii) determine the impact on final kernel number of supplying fresh pollen to silks whose appearance is delayed by water deficits at silking. Field experiments were conducted at Balcare (37-degrees 45'S, 130m) during 1988/89 and 1989/90 with two sowing dates (6 weeks apart) to provide differences in evaporative demand. Plastic covers were placed on the ground of water-deficit plots to generate a 40-day period of lowered water supply bracketing silking. Control plots received rain plus additional furrow irrigation in order to keep the ratio between crop (E(c)) and potential (E(p)) Penman evapotranspiration greater than 0.9. Plant water status indicators revealed differences between treatments, but failed to reflect soil water status. Water deficit reduced plant height, maximum leaf area index, and shoot biomass. Shoot biomass accumulation was correlated with E(c), but higher water-use efficiencies (WUE) were found for the water-stress treatments. Grain yield was correlated to kernels m-2 (r = 0.88; 6 d.f.), and both grain yield and kernels m-2 were related to E(c) during the treatment period, resulting in reductions of 4.7 grains m-2 and 17.7 kg ha-1 for each mm reduction in E(c). The number of kernels per ear did not improve when fresh pollen was applied to late appearing silks, suggesting that ovaries which failed to expose their silks synchronously with pollen shedding were deleteriously affected by water stress.