Blum, A., Institute of Field and Garden Crops, The Volcani Center, ARO, PO Box 6, Bet Dagan, Israel
Sinmena, B., Institute of Field and Garden Crops, The Volcani Center, ARO, PO Box 6, Bet Dagan, Israel

Tolerance of wheat (Triticum aestivum) to chronic heat stress and the subsequent effect on growth and yield are largely mediated by the plant carbon balance at high temperature. Seed endosperm utilization for seedling growth in the dark can be a simple model for studying tolerance to chronic heat stress in terms of the carbon balance. Seed endosperm utilization efficiency (EUE) was defined as the ratio of gain in seedling (shoot and radicle) dry mass to loss in seed dry mass during germination This study was designed to evaluate heat tolerance in terms of EUE and its relations to heat tolerance in the autotrophic plant. Endosperm utilization efficiency was measured by periodic sampling of germinating seed in the dark under normal (25°C) and high (35°C) temperatures in 18 wheat cultivars. Heat tolerance of the autotrophic juvenile plant was measured in the same cultivars in terms of plant growth rate (GR) between 17 to 24 days after emergence under normal (15°/25°C) and high (25°/35°C) (night/day) temperatures. Mean final seedling dry mass in the dark was greater at low than at high temperature. Total dry matter loss from the seed did not differ much between temperatures, indicating that the lower seedling dry mass under high temperature was probably due to a relatively greater loss of carbon to respiration. Averaged over 18 cultivars, EUE was reduced by chronic heat stress from 0.64 g g-1 at 25°C to 0.53 g g-1 at 35°C. There were significant differences among cultivars in EUE at both low and high temperatures and there was a very significant (P≤0.01) temperature by cultivar interaction for EUE. Endosperm utilization efficiency at high temperature ranged from 0.30 g g-1 to 0.85 g g-1 among cutlivars and was independent of cultivar seed mass. Percent GR reduction from 15°/25°C to 25°/35°C ranged from null to 53% in the different cultivars. A significant negative correlation (r=-0.81; P≤0.05; df=16) was found between EUE at 35°C and the rate of GR reduction from low to high temperature, indicating an association across cultivars between heat tolerance in carbon utilization during seed germination and heat tolerance in growth of the autotrophic plant. The common mechanism underlying the variation among cultivars in heat tolerance at both stages could be in catabolic processes affecting carbon use efficiency. © 1994.