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Grain yield and the stability of harvest index are greater in newer than in older Argentinean maize (Zea muys L.) hybrids. The objective of this study was to elucidate mechanisms underlying the superior yield and harvest index stability of newer Argentinean maize hybrids using the relationship between kernel number per plant (K[N.sub.P]) and plant growth rate during the period bracketing silking (PG[R.sub.S]). Three experiments were performed at Balcarce, Argentina, during two growing seasons (1998-2000). Maize was grown under a wide range of plant densities (from 2 up to 30 plants [m.sup.-2]) to generate contrasting availability of resources per plant. Growth of individual plants during the period bracketing silking was estimated through a nondestructive method on the basis of relationships between actual shoot dry matter and morphometric variables, including stem and ear diameters and ear length. Detasseling and silk pollination synchronization treatments were imposed in one experiment to also modify available resources per kernel and kernel sink strength. Newer hybrids set more kernels per unit PG[R.sub.S] than older hybrids as is indicated by (i) the lower threshold PG[R.sub.S] for kernel set and (ii) greater potential kernel number at high availability of resources per plant, for newer than for older hybrids. At low and intermediate PG[R.sub.S], the greater kernel set per unit PG[R.sub.S] in newer vs. older hybrids was attributable to greater partitioning of dry matter to the topmost ear during the period bracketing silking, whereas number of kernels set per unit of ear growth rate did not differ. In contrast, kernel set per unit of ear growth rate was greater in newer than in older hybrids when PG[R.sub.S] was high. Results of this study indicate that genetic yield improvement in maize is attributable, in part, to increased partitioning of dry matter to the ear during the critical period bracketing silking., YIELD OF NEWER ARGENTINEAN MAIZE hybrids is greater than that of older ones (Echarte et al., 2000). Genetic yield improvement in North American maize hybrids has been associated with increased [...]

Final kernel number in maize (Zea mays L.) is closely associated with the physiological condition of the crop during the critical period bracketing silking. The objective of this study was to determine whether there is a common underlying relationship between kernel number per plant (KNP) and plant growth rate (PGR) during that critical period when plant growth varies because of different abiotic stresses. A relationship between KNP and PGR obtained from a previous study of variation in plant density and incident radiation was used as reference. KNP and PGR were measured in experiments in which incident radiation per plant, nitrogen (N), and water availabilities were the experimental sources of variation. The equation fitted to the data obtained at different radiation levels explained 72% of the variation in the data obtained at different levels of N or water availability. Moreover, different models for each set of data did not provide a significantly better fit than a single model for the two sets of data combined. A common relationship between KNP and PGR was also obtained when N supply and water availability were variable. The relationship between KNP and PGR obtained for treatments in which PGR was varied through plant density and shading also could predict KNP for conditions in which PGR was affected by water and/ or N deficiencies. The PGR during the critical period of kernel set is a good predictor of the capacity of the maize plant to set kernels under a wide range of environmental and management practices., MAIZE GRAIN YIELD is closely associated with kernel number at harvest and this yield component is a function of the physiological condition of the crop during the period bracketing flowering [...]