SHORT AND LONG TERM EFFECTS OF HEAT AND OSMOTIC STRESS ON SORGHUM1

Abstract

Drought stress frequently occurs accornpenied by high temperatures. Therefore, the combined effects of three temperature levels (30/22, 35/22, and 40/22°C; day/night) and three levels of osmotic stress (-0.5, -4.0 and -7.5 bars) on the development, growth and grain yield of a hybrid sorghum (Sorghum bicolor L. Moench), growing in a hydroponic culture, were studied. These treatments were imposed at the anther differentiation stage for six days. At the end of the treatment period it was found that in this sorghum the panicle, followed by the root, were the most susceptible organs to both heat (HS) and osmotic stress (OS), considering dry matter accumutatíon. Culm growth, however, was arrested only by HS, whereas leaf area and net co₂ exchange rate were disminished only by OS. At harvest, after the recovery périod, the OS effects on the vegetative growth remained at a significant level, while those of HS had vanished; grain yield did not show significant losses because the observed reductions in grain number, mainly caused by OS, were largely compensated by gain in grain size. The HS X OS interaction had no significant effects, on most of the studied traits, either in the short or in the long term; however, the net grain of carbon by the entire plant during the sixday treatment period, was clearly declined by the combined action of OS and HS, although these factors had little effect when imposed alone.