The grain filling of wheat (Triticum aestivum L.) is seriously impaired by heat stress due to reductions in current leaf and ear photosynthesis at high temperatures. An alternative source of carbon for grain filling is stored stem reserves. Two spring wheat cultivars (V5 and V2183) of very similar phenology and plant stature, which had previously been found to differ in grain shrivelling under drought and heat stress conditions in the field, were used to evaluate the hypothesis that the mobilisation of stored stem reserves into the growing grain is an important source of carbon for supporting grain filling under heat stress. In two experiments in Israel (1990 and 1991), the rates of stem dry matter (DM) and stem total non-structural carbohydrates (TNC) loss, grain growth and leaf senescence were monitored under optimal (control) and high (stressed) temperatures in the glasshouse (1990) and the growth chamber (1991). Cultivar V5 always sustained a smaller reduction in grain dry weight under heat stress, than V2183. Irrespective of temperature, 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 filling, and longer duration of grain filling, than V2183. During grain filling V5 generally exported about two to three times more DM from the stems than V2183, under both non-stressed and stressed conditions. On 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 V5 and V2183) were subjected to chemical desiccation (0.3% potassium iodide) of the canopy in the field in order to destroy the photosynthetic source ofthe plant after anthesis. The same cultivars were subjected to heat stress (35/25�C) or non-stressed (25/15�C) conditions after anthesis in the growth chamber. It was found that grain dry weight reduction by chemical desiccation was highly correlated with grain dry weight reduction by heat stress (r2 = 0.89). Therefore, the superior capacity of V5 for grain filling from mobilised stem reserves is a consti- tutive trait which supports grain filling under heat stress which can be tested for by chemical desiccation of plants under non-stressed conditions.
Diverse landraces of wheat, collected from the semi-arid (150 to 250 mm of total annual rainfall) Northern Negev desert in Israel were considered as a potential genetic resource of drought resistance for wheat breeding . These materials were therefore evaluated for their reponses to drought stress in agronomical and physiological terms . Up to 68 landraces, comprising of Triticum durum, T aestivum, and T compactum were tested in two field drought environments, in one favourable field environment, under post-anthesis chemical plant desiccation which revealed the capacity for grain filling from mobilized stem reserves, under a controlled drought stress in a rainout shelter and in the growth chamber under polyethylene glycol (PEG)-induced water stress . Biomass, grain yield and its components, harvest index, plant phenology, canopy temperatures, kernel weight loss by chemical plant desiccation, growth reduction by PEG-induced drought stress and osmotic adjustment were evaluated in the various experiments .Landraces varied significantly for all parameters of drought response as measured in the different experiments, which was in accordance to their documented large morphological diversity . Variation in grain yield among landraces under an increasing drought stress after tillering was largely affected by spike number per unit area . Kernel weight contributed very little to yield variation among landraces under stress, probably because these tall (average of 131 cm) landraces generally excelled in their capacity to support kernel growth by stem reserve mobilization under stress . Yield under stress was reduced with a longer growth duration of landraces only under early planting but not under late planting . Landraces were generally late flowering but they were still considered well adapted phenologically to their native region where they were always planted late .Landraces differed significantly in canopy temperature under drought stress . Canopy temperature under stress in the rainout shelter was negatively correlated across landraces with grain yield (r = 0 .67**) and biomass (r = 0 .64* *) under stress . Canopy temperature under stress in the rainout shelter was also positively correlated across landraces (r = 0 .50**) with canopy temperature in one stress field environment . Osmotic adjustment in PEG-stressed plants was negatively correlated (r = -0 .60**) with percent growth reduction by PEG-induced water stress . It was not correlated with yield under stress in any of the experiments .In terms of yield under stress, canopy temperatures and stem reserve utilization for grain filling, the most drought resistant landrace was the 'Juljuli' population of T durum . 88
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