Abstract
Cooling treatments at different stages of pollen development lowered the percentage of fertilized spikelets through decreasing the number of engorged pollen grains per anther at anthesis. Cooling during the period from anther differentiation to the tetrad phase decreased the number of engorged pollen grains mainly by decreasing the number of differentiated microspores. Cooling in the period from the early microspore phase to the late microspore phase, however, decreased the number of engorged pollen grains primarily by increasing the number of aborted microspores. Cooling at the young microspore stage, which consists of the two phases of the tetrad and the early microspore phase, caused the largest decrease in the number of engorged pollen grains, resulting in the largest decrease in the percentage of fertilization. Cooling at the tetrad phase caused the largest decrease in the number of differentiated microspores, resulting in the largest decrease in the number of engorged pollen grains. On the other hand, cooling at the early microspore phase caused the largest increase in the number of aborted microspores, resulting in the largest decline in the number of engorged pollen grains. These results indicate that the highest susceptibility to coolness of anthers at the young microspore stage, which has been estimated as the percentage of fertilized spikelets, is caused by the high susceptibility to coolness of the differentiation and development of microspores.
