The effects of level of water deficit and plant population density on plant barrenness and harvest index were investigated in a strictly controlled field experiment, repeated for four seasons. Differences in soil water supply were obtained by the seasonal variation in rainfall and by supplementing rainfall with irrigation in some of the treatments. In each season the highest level of water represented conditions of adequate soil water supply. The supply of mineral nutrients was abundant in all treatments. At adequate levels of soil water supply plant barrenness occurred only when planting density was raised above 60 000 plants ha#x2212;1(LAI 5). At a planting density of 111 111 plants ha#x2212;1(LAI ofc.8), which was the optimum population for grain when the soil water supply was not limiting, the incidence of plant barrenness ranged from 4 to 10. However, this degree of plant barrenness was not sufficiently high to cause a significant reduction in harvest index. When soil water supply became limiting, plant barrenness occurred at progressively lower planting densities, and within each plant population, the incidence of barrenness increased as the level of water deficit became more severe. There was a close negative correlation between the harvest index and the proportion of barren plants. The results show that increases in the degree of water deficit and increases in intraspecific competition for a limiting amount of available soil water both cause an increase in the incidence of plant barrenness and a reduction in the harvest index of maize.
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