Abstract

Genotypic variation for phenology is important when considering the adaptation of grain sorghum (Sorghum bicolor (L.) Moench) to adverse environments, but little is known about its role under environmental conditions that result in low soil nitrogen (N) availability. We examined the role of phenology in relation to other traits considered to contribute to the adaptation of sorghum to low soil N conditions.

Four hybrids with contrasting maturity date were examined (2 early and 2 late) under conditions of full irrigation supply. The late-maturing hybrids had higher yield than one of the early hybrids only in optimum N conditions (960 v. 815 g/m²). The high yield of the late-maturing hybrids was a result of greater biomass production due to a longer period of radiation interception, rather than a greater fraction of radiation interception at any time. Longer growth duration had no positive effect on N capture, resulting in a lower grain N concentration at maturity relative to the early-maturing hybrid (1·42% v. 1·67%). The other early-maturing hybrids achieved a comparable amount of biomass production and grain yield (997 g/m²) to the late-maturing hybrids, and higher grain N concentration (1·55%). This was attributed to their higher plant N uptake by maturity, which contributed to higher grain N and maintained higher radiation use efficiency (RUE) relative to the other hybrids.

Under N-limiting conditions, the advantage of the late-maturing hybrids was small in terms of radiation interception, and there was no advantage in terms of total plant N content. One of the early-maturing hybrids continued to absorb more N and accumulated larger amounts of N to grain for a longer period after anthesis than the other hybrids, resulting in higher grain N concentration (1·10% v. 0·92%). Genotypic variation for RUE, N utilisation, and harvest index was observed, but was confounded with the other components, resulting in a small difference in yield (392–454 g/m2).