Determination of grain number in sorghum

Grain number is an important component of grain yield in sorghum. Research in wheat and maize has indicated a dependency of grain number on the crop or panicle growth rate around anthesis (CGRa and PGRa respectively), but little quantitative information is available for sorghum. The aim of this paper was firstly to quantify the effect of CGRa and PGRa on grain number and secondly, to identify other parameters that could be used as substitutes for PGRa. Analyses included data from a number of experiments, covering a range in nitrogen and drought treatments and including both tall (single dwarf) and short (triple dwarf) hybrids. CGRa and SGRa (stem growth rate) were calculated from the derivative of a curvilinear function fitted to experimental data, and PGRa was obtained by subtraction of SGRa from CGRa. Results indicated a linear relationship between grain number and CGRa, but the slope differed for tall and short hybrids. This was due to a difference in the proportion of dry matter allocated to the reproductive organs around anthesis (Pr), as PGRa was closely related to grain number, irrespective of crop height. Since panicle dry mass at maturity (excluding grain) was closely correlated with reproductive biomass shortly after anthesis, this indicator represents an integration of panicle growth during the critical period for yield determination in sorghum (i.e. flag leaf until start of grain filling). Panicle biomass at maturity (excluding grain) was thus also linearly related to grain number, and the relationship was independent of crop height and of the timing, severity, or type of stress. Our results indicate that panicle mass at maturity could provide an alternative to PGRa for the estimation of grain number.