Growth and productivity of irrigated Sorghum bicolor (L. Moench) in Northern Australia. II. Low solar altitude as a possible constraint to productivity in the tropical dry season

Abstract

A growth study was carried out during the dry season on irrigated grain sorghum cultivar NK 300F at latitude 16¦S. in northern Australia. The apparent efficiency of the canopy in the photosynthetic conversion of solar radiation increased progressively in high density stands between June and September, while low density stands showed no change. An hypothesis is advanced that the rise in canopy efficiency was due to increasing solar altitude combining with a suitable canopy structure at high density to give increased light penetration into the canopy. A parameter named weighted mean solar altitude (WMSA) is used in conjunction with noon solar altitude (NSA) to assist in the interpretation of published models of light penetration. This solar altitude effect, if verified by further work, would lower the expectations, based on mean daily solar radiation, for dry season yield of irrigated sorghum and possibly other cereals in the semi-arid tropics.