Grain yield of wheat as affected by individual and combined heat and drought stress during different growth stages with emphasis on source/sink relations

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In many world regions, higher temperatures and reduced water supply to plants are expected, most likely causing depressions in crop productivity. Both stress conditions occur in the field simultaneously, and incidences of combined stress will increase in the near future. However, relatively little is known about their combined impacts on plants. The grain yield potential of wheat, a rather heat-sensitive crop, is determined by three parameters: number of ear-bearing culms per plant or per area, kernel number per ear, and individual grain weight. These yield determinants are specifically affected at defined growth stages: tillering, stem elongation, flowering, and grain filling. Thus, various phases of wheat development need to be investigated in order to identify the decisive determinants for grain yield performance under stress conditions. So far, most studies focused on the grain-filling period, whereas investigations on individual and particularly combined heat and drought stress effects during vegetative growth and at anthesis are rare. Depressions in wheat grain yield caused by stress conditions can either be due to restricted availability of assimilates (source limitation), or by reduced sink strength. In our previous study, we found that under continuous heat stress wheat plants were able to strongly increase the number of ear-bearing tillers. This potential for yield stabilization could only partly be exploited as kernel setting was strongly reduced, yet the established kernels showed good grain filling. No source limitation was observed, but sink capacity was reduced (fewer and smaller kernels), and probably also sink activity. This merits further studies of the enzymes involved, particularly of acid invertase, plasma membrane H+-ATPase, and starch synthase. In the proposed project, individual or combined heat and drought stress will be applied to two wheat cultivars either during vegetative growth, at anthesis, or during grain-filling. In addition, the effects of drought priming during vegetative growth on grain yield performance of plants stressed during anthesis will be tested, and the ability of the wheat plants to recover from stress is evaluated. Various parameters which determine source or sink strength will be investigated, either with measurements at the growing plants or with analyses of different plant organs derived from harvests during grain filling or at physiological maturity. This study addresses the important question: What is the liming factor for grain yield development when plants are prone to the various stresses during different growth stages. This knowledge will contribute to the identification of traits which determine heat and/or drought resistance, and can be incorporated in breeding programs for higher yield stability under stress. The water- and nutrient-use efficiencies can be improved in order to protect limited resources and favor a more sustainable wheat production.