Effects of gibberellin and cytokinin on the resistance to cool weather at the young microspore stage in rice plants

The effects of gibberellin, cytokinin and an inhibitor of gibberellin biosynthesis on spikelet fertility and the number of pollen grains were examined in rice plants which were cooled at the young microspore stage. Spikelet fertility in the cooled plants was decreased by the application of GA₃ or trans-zeatin during the period from the primary rachis branch differentiation stage to the young microspore stage, and increased by the application of prohexadione-calcium, an inhibitor of gibberellin biosynthesis. The number of pollen grains per anther in both the cooled and control plants was reduced by the application of GA₃ or trans-zeatin, and increased by prohexadione-calcium application. Spikelet fertility in the cooled plants was positively correlated with the number of pollen grains not only in the cooled plants, but also in the control plants. It was concluded that both gibberellin and cytokinin weakened the cool-weather resistance through inhibition of pollen formation. Since nitrogen is known to weaken the cool-weather resistance, reduce the number of pollen grain, and increase the endogenous levels of gibberellin and cytokinin in rice plants, it appears that nitrogen weakens the cool-weather resistance by increasing the endogenous levels of gibberellin and cytokinin.