Changes in Cell Number and Cell Division Activity during Endosperm Development in Allohexaploid Wheat, Triticum aestivum L

Nuclear or cell number, and the mitotic index, were recorded in endosperms of Triticum aestivum cv. Mardler to test if a particular stage of endosperm development was critical in determining the final grain weight. The basal four florets of emasculated spikelets (controls), and the third and fourth florets of spikelets where the two basal ovaries were removed (ovary-removed), were sampled at various times up to 360 h after hand-pollination. The coenocytic phase in endosperms ended about 84 h after pollination regardless of both grain position and the treatment. The onset of the cellular stage was characterized by the final large fluctuations in the mitotic index reflecting the culmination of the synchronous nuclear division of the coenocytic stage. Thereafter, the mitotic index fluctuated with smaller amplitudes and, by 216 h after pollination, was < 1%. Neither floret position in the spikelet nor the treatment affected the pattern of alteration to the mitotic index. However, ovary removal from first and second florets resulted in significantly heavier grains and higher endosperm cell number in the 3rd and 4th florets compared with the controls. In all florets, mean endosperm cell number peaked at 280 h but decreased by 360 h after pollination. At this time, the mean cell numbers in endosperms of the 3rd and 4th florets of ovary-removed spikelets were significantly higher than in the corresponding endosperms in the controls. Thus, a key contributory factor in determining the final endosperm cell number may be the number of cells which are lost during the late period of the cellular stage of endosperm development.