Response of maize kernel number to plant density in Argentinean hybrids released between 1965 and 1993

We investigated the response of maize kernel number to plant density in four hybrids released in Argentina between 1965 and 1993. Assuming kernel number is the main yield component, and using as a framework the relationship between kernel number per plant (KNP) and plant growth rate bracketing silking (PGR(s)), we tested the alternative hypotheses that modern hybrids produce more kernels because they have (a) greater PGR(s) or (b) more kernels per unit PGR(s) than their older counterparts. Three experiments were carried out including a range of plant densities from 3-5 to 15-18 plants m(-2). PGR(s) was calculated from shoot dry matter measured 10 days before and 20 days after silking. Shoot dry matter, grain yield and its components were measured at physiological maturity. Grain yield of the oldest hybrid averaged 7.7 t ha(-1), and increased with year of release at a rate of 173 kg ha(-1) per year. The response of grain yield to plant density was curvilinear. Kernel number per square meter accounted for most of the variation in yield with both year of release and plant density. For both sources of variation, there was a trade-off between kernel number and mass. Both PGR(s) and KNP decreased with increasing plant density in all four hybrids. Whereas variation in PGR(s) among hybrids was small, the oldest hybrid set 93 (low density) and 113 (high density) kernels per unit PGR(s) in comparison to the newest that set 167 and 193. We conclude that more kernels per unit PGR(s), rather than greater PGR(s), accounted fur the genetic improvement of yield potential in the hybrids investigated. (C) 2000 Elsevier Science B.V. All rights reserved.