Regulation of Grain Weight by Supply of Assimilates and Starch Granule Development in Three Winter Wheat Varieties

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Abstract
On removing the top half of the ear (halving) on several dates after anthesis, dry weight per grain increased in three winter wheat (Triticum aestivum L.) varieties, in two pot experiments; the increase was greater with early than with late halving. The variety Splendeur had a lower dry weight and water percentage in grains than either Hobbit or Maris Huntsman. The ratio of the green area integrated over the post-anthesis period to the number of grains per ear (green area per grain) was highest in Splendeur and lowest in Hobbit in the first experiment; in the second, Splendeur gave a lower ratio than the other two varieties, which showed similar values. The green area per grain was greater the earlier the ear was halved. The number of A-type starch granules per endosperm, but not the volume per A granule (modal volume) and the modal volume of B starch granules, but not their number, increased to a greater extent with early than with late halving. In Splendeur the grains had fewer A starch granules, although these were of greater modal volume than in Hobbit and Maris Huntsman and a number of B starch granules similar to the other two varieties, but of smaller modal volume. Maris Huntsman had more A granules than Hobbit, but with smaller modal volume. Dry weight per grain increased linearly with the number of A starch granules per endosperm, which in turn increased asymptotically with green area per grain. The regressions for the three varieties differed significantly. The influence of the supply of assimilates and the capacity for starch granule formation in the regulation of grain weight is discussed.