For adapted cultivars under normal crop densities, biological yield is largely determined by the pool of available resources, e.g. water, nutrients and photosynthetically active radiation, while the nature and intensity of intraspecific competition plays an important role in determining the magnitude of harvest index (M). Water deficits can drastically reduce the HI from its genetic potential to zero. This study was conducted to determine the effect of drought-weighted intraspecific competition on the HI and, consequently, the grain yield in spring wheat populations along a natural moisture gradient in northwestern China. Along the natural moisture gradient (annual mean rainfall decreased 328 --> 204 --> 185 mm per year, supplemented with 70 mm of irrigation), culm size inequality (as measured by the Gini coefficient of above-ground biomass per culm) always increased, and Lorenz curves were more concave. HI decreased significantly in 1999 (0.364 --> 0.345 --> 0.307) and 2000 (0.341 --> 0.303 --> 0.251). There was a significant negative correlation between the Gini coefficient and the 111 of spring wheat along the moisture gradient (R-2 = 0.92, P < 0.01). These results suggested that size hierarchies in spring wheat populations are closely correlated with the water regime in the field, and that under greater drought stress there are relatively more smaller plants with lower HI (size-dependent reproductive allocation). Size inequality is an index of competitive status in plant populations under stress environments. Agriculturally, greater size inequality may result in a competitive cost for energy and photosynthetic products, in other words, growth redundancy, which is detrimental to reproductive allocation and consequently, grain yield. The results support the view that stand uniformity in field crops is an important mechanism for increasing grain yield. (C) 2002 Elsevier Science B.V. All rights reserved.
