The Physiology of Heterosis in Sorghum with Respect to Environmental Stress

Abstract

The hypothesis that heterosis in biomass production of sorghum (Sorghum bicolor L. Moench) may be ascribed to stability in carbon exchange rate (CER) over a wide range of environmental conditions was evaluated. This hypothesis was based on previous results from detached leaves that hybrids sustained gTeater CER over a wider temperture range than their parents. Two grain sorghum hybrids (ATx378/RTx430 and ATx378/RTx434) and their parental lines were grown in the greenhouse in a gradient of ambient temperatures under two water regimes (well-irrigated and drought up to heading). Plant water-use (estimated by weighing pots), leaf area, leaf gas exchange, grain yield, and above-ground biomass were determined. Significant heterosis was found for biomass, grain yield per plant, and grain number per panicle. No heterosis occurred for harvest index, indicating that heterosis in grain yield was due to heterosis in biomass. Neither growth duration nor leaf area could explain heterosis in biomass. CER and stomatal conductance for hybrid ATx378/RTx43O in the controls were greater than for both its parents at leaf temperatures above 38 °C. This, however, was not observed in the other hybrid which was less heterotic for biomass and grain yield in the controls. When CER data were subjected to a stability analysis by joint linear regression, the two hybrids had greater CER than their respective parents especially under conditions favouring high CER. When extreme stress conditions developed, the hybrid's performance depended on its genetic background more than on heterosis.