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Two experiments were conducted on stress-tolerant Suneca and stress-susceptible Batavia wheat varieties which were grown in pots in a temperature-controlled naturally illuminated glasshouse (March–May) under 25/18 °C day/night cycle. Plants were divided into two groups at 8 days after anthesis (DAA). Plants of one group were subjected to four water deficit cycles, while the others were irrigated daily as before. Both groups were sub-divided: one was exposed to a higher temperature (30/25 °C day/night cycle) for 3 days toward the end of the first drought cycle, and the second was maintained at previous conditions. Plants of the first experiment (excluding high temperature exposed Batavia plant) were arranged in three groups: defoliated, decapitated and intact. Dry weights of leaves, stems, ears and kernels were determined at 6–7 days intervals. A mechanistic model was used to analyze the daily rates of dry matter transport from vegetative organs to kernels, and its contribution to kernel weight. Water deficit did not affect kernel number, while high temperature reduced it significantly. The rate of dry matter accumulation by kernels was considerably decreased by water deficit in both cultivars. High temperatures also reduced the rate of dry matter accumulation in Suneca kernels, but to a lesser extend than water deficit. Dry weight of the intact plants’ vegetative organs (stems plus leaves) decreased during grain filling, probably due to export of non-structural carbohydrates to the developing kernels. By contrast, in decapitated plants, dry weight of the vegetative organs increased during the same period. Rates of transport (probably of non-structural carbohydrates) from vegetative organs to kernels were much higher in Suneca than in Batavia during drought. High temperature reduced this rate in Suneca. Initially, the relative contribution of stem and leaf dry matter to kernels was only 40% of their total dry weight increase in unstressed Suneca plants, indicating that the ear was active. This increased gradually up to 1.00 at 26–27 DAA. The relative contribution of Batavia vegetative organs increased from 0.30 to 0.60. In general, water deficit and high temperature increased the relative contribution of transported dry matter to kernels. The second experiments showed that thousand-kernel weight (TKW) and weight of kernels per ear were more severely decreased by water deficit than by heat in both varieties, and more by water stress in Batavia than in Suneca. Plant exposure to high temperatures reduced the response of TKW to water deficit in both varieties. |
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