We compared the relationship between grain yield per plant (Y-p) and shoot biomass per plant (S-p) in three annual crops with contrasting reproductive strategies: sunflower, a determinate species with a single inflorescence; maize, a determinate species with a limited capacity to adjust the number of ears in response to resource availability; and indeterminate soybean, a species with a large capacity to adjust the number of inflorescences. Our working hypotheses were: H-1-the relationship between Y-p and S-p is linear; H-2-the intercept of the model is zero, i.e. there is not a threshold plant mass for reproduction. A wide range of Y-p and S-p was generated by manipulation of plant density; S-p varied between 0.3 and 196 g per plant in soybean, between 6 and 873 g per plant in sunflower and between 23 and 697 g per plant in maize. Within these broad ranges of plant size, both hypotheses were rejected in five out of six experiments, i.e. the relationship between Y-p and S-p departed from linearity and there was a threshold for S-p below which no grain set occurred. The S-p threshold for grain set varied widely among species; it was close to 2 g per plant for soybean, 27 g per plant for sunflower and 43-71 g per plant for maize. Because of this size threshold and non-linearity, harvest index (HI = YpSp-1) was stable for mid-size plants, diminished slightly for large plants, and diminished sharply for smaller plants in all three crops. Harvest index stability was highest in soybean, intermediate in sunflower and lowest in maize. Differential stability of reproductive partitioning partially derived from contrasting patterns of melistem allocation. (C) 2000 Annals of Botany Company.
