The relative amount of roots (dry weight ratio of root to shoot, or root dry weight per spikelet) or the physiological activity of roots is correlated with the cool-weather resistance in rice plants. The objective of this study is to elucidate the effects of the relative amount or physiological activity of roots on the number of pollen grains (NPG) which are generally considered to be closely related with the cool-weather resistance. NPG in the anthers excised from the spikelets just before flowering, was counted using the same plant materials as described in our previous reports. NPG was lower at the higher levels of nitrogen application in both the control and cooled plants. The fertility of the cooled plants tended to decrease with decreasing NPG. NPG tended to decrease with decreasing relative amount of roots, which had already been shown to decrease with increasing nitrogen level. Moreover, NPG was decreased in both the cooled and control plants by root cutting or treatment of roots with a respiratory inhibitor, and was positively correlated with the fertility. These results strongly suggest that the development and activity of roots affect the cool-weather resistance through pollen formation, and that one of the reasons why the cool-weather resistance is decreased by the higher levels of nitrogen application is the decrease of NPG due to the lowering of the relative amount of roots.
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