School of Agriculture, Food and Ecosystem Sciences - Theses
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ItemEffects of post-anthesis drought on wheat( 1985)The experiments presented in this thesis investigate, firstly, the effects of drought on the cellular processes occuring in the wheat grain and, secondly, some of the possible causes of the reduction in grain growth. The main conclusions are: 1. Drought occurring during the early period of grain growth leads to a reduction in number and size of endosperm cells and a reduction in the number of starch granules initiated per cell. The reductions in numbers of endosperm cells and starch granules are greater in grains of top spikelets (distal grains) than in basal grains of middle spikelets (basal grains). 2. Final grain weight is closely correlated with the number of endosperm cells in basal grains of the variety Warigal. However, in basal grains of Condor, a variety more drought-sensitive than Warigal, grain weight is more reduced than cell number. This is also the case in distal grains of both varieties. The greater reduction in final grain weight relative to cell number is at least partly due to to a reduction in number of small starch granules per cell. 3. Although the supply of sucrose to the endosperm cells is reduced under drought, this is unlikely to cause the reductions in numbers of cells and starch granules per cell. Rather, it is concluded that the reduction in sucrose supply to the endosperm observed under drought is a response to, and not a cause of, the reduction in numbers of endosperm cells and starch granules per cell. This conclusion is based on two results: (i) despite a constant level of sucrose per cell, the number of small starch granules per cell is reduced under drought, and (ii) a reduction in photosynthesis of ca. 40% is accompanied by a reduction in grain growth and an accumulation of stem reserves in droughted plants of Warigal. Stem reserves would not be expected to accumulate to the extent occurring in control if assimilates were in short supply for grain growth. 4. Among the photosynthetic organs supplying assimilates to the grains, the glumes show the greatest drought-tolerance. Glumes have a better osmotic adjustment and a greater integrity of the cell membranes under drought than leaves. These two characters are probably at least partly due to anatomical characteristics of the glumes, in particular, small cell size and thick cell walls. 5. Carbon and nitrogen budgets of droughted plants indicate that the roots and the stem play an important role in the transfer of assimilates to the grains. Larger osmotic adjustment of the roots and greater accumulation of stem reserves are two aspects of the better drought-tolerance of Warigal relative to Condor. 6. The loss of turgor and the accumulation of abscisic acid in the grains are the most likely causes of the reduction in numbers of endosperm cells and starch granules. Distal grains lose turgor more rapidly and accumulate more ABA per endosperm cell than basal grains. The implications of these results for drought tolerance of wheat are discussed.