SORGHUM HYBRID DIFFERENCES IN GRAIN-YIELD AND NITROGEN CONCENTRATION UNDER LOW SOIL-NITROGEN AVAILABILITY - II - HYBRIDS WITH CONTRASTING PHENOLOGY

Genotypic variation for phenology is important when considering the ad aptation of grain sorghum (Sorghum bicolor (L.) Moench) to adverse env ironments, but little is known about its role under environmental cond itions that result in low soil nitrogen (N) availability. We examined the role of phenology in relation to other traits considered to contri bute to the adaptation of sorghum to low soil N conditions. Four hybri ds with contrasting maturity date were examined (2 early and 2 late) u nder conditions of full irrigation supply. The late-maturing hybrids h ad higher yield than one of the early hybrids only in optimum N condit ions (960 v. 815 g/m(2)). The high yield of the late-maturing hybrids was a result of greater biomass production due to a longer period of r adiation interception, rather than a greater fraction of radiation int erception at any time. Longer growth duration had no positive effect o n N capture, resulting in a lower grain N concentration at maturity re lative to the early-maturing hybrid (1.42% v. 1.67%). The other early- maturing hybrids achieved a comparable amount of biomass production an d grain yield (997 g/m(2)) to the late-maturing hybrids, and higher gr ain N concentration (1.55%). This was attributed to their higher plant N uptake by maturity, which contributed to higher grain N and maintai ned higher radiation use efficiency (RUE) relative to the other hybrid s. Under N-limiting conditions, the advantage of the late-maturing hyb rids was small in terms of radiation interception, and there was no ad vantage in terms of total plant N content. One of the early-maturing h ybrids continued to absorb more N and accumulated larger amounts of N to grain for a longer period after anthesis than the other hybrids, re sulting in higher grain N concentration (1.10% v. 0.92%). Genotypic va riation for RUE, N utilisation, and harvest index was observed, but wa s confounded with the other components, resulting in a small differenc e in yield (392-454 g/m(2)).