STEM RESERVE MOBILIZATION SUPPORTS WHEAT-GRAIN FILLING UNDER HEAT-STRESS

The grain filling of wheat (Triticum aestivum L.) is seriously impaire d by heat stress due to reductions in current leaf and ear photosynthe sis at high temperatures. An alternative source of carbon for grain fi lling is stored stem reserves. Two spring wheat cultivars (V5 and V218 3) of very similar phenology and plant stature, which had previously b een found to differ in grain shrivelling under drought and heat stress conditions in the field, were used to evaluate the hypothesis that th e mobilisation of stored stem reserves into the growing grain is an im portant source of carbon for supporting grain filling under heat stres s. In two experiments in Israel (1990 and 1991), the rates of stem dry matter (DM) and stem total non-structural carbohydrates (TNC) loss, g rain growth and leaf senescence were monitored under optimal (control) and high (stressed) temperatures in the glasshouse (1990) and the gro wth chamber (1991). Cultivar V5 always sustained a smaller reduction i n grain dry weight under heat stress, than V2183. Irrespective of temp erature, V5 had a higher stem DM and TNC content at the onset of grain filling, greater depletion of stem dry matter (or TNC) during grain f ining, and longer duration of grain filling, than V2183. During grain filling V5 generally exported about two to three times more DM from th e stems than V2183, under both non-stressed and stressed conditions. O n the other hand, V5 was more heat-susceptible than V2183 in terms of leaf longevity, in vivo chlorophyll stability and grain abortion under heat stress. In a third experiment (1992) five cultivars (including V 5 and V2183) were subjected to chemical desiccation (0.3% potassium io dide) of the canopy in the field in order to destroy the photosyntheti c source of the plant after anthesis. The same cultivars were subjecte d to heat stress (35/25 degrees C) or non-stressed (25/15 degrees C) c onditions after anthesis in the growth chamber. It was found that grai n dry weight reduction by chemical desiccation was highly correlated w ith grain dry weight reduction by heat stress (r(2) = 0.89). Therefore , the superior capacity of V5 for grain filling from mobilised stem re serves is a constitutive trait which supports grain filling under heat stress which can be tested for by chemical desiccation of plants unde r non-stressed conditions.