Effects of Gibberellin on the Morphogenesis in Rice Plants : 9. Relation between abnormal spikelets induced by gibberellin and their fertility

Abstract

Macro-and microscopic observations were carried out to clear the causes for lowering of fertilization due to the application of gibberellin in rice plants, using a slightly late variety Sachikaze. Gibberellin was applied at the stage of young panicle differentiation mixed with the culture solution at 100 ppm. Though remarkable increase, about 40 percent, of spikelets in number was brought about by the treatment, simultaneously about 62 percent of the total spikelets has resulted in sterility in the mature panicles. As shown in fig. 1, tables 2 and 3, the sterile spikelets due to the so-called white hulls took place at the basal portion of the lower primary branches and the other sterile, particularly the perfectly sterile ones, spikelets were observed on the middle and upper primary branches. With the immature panicles of the gibberellin plot, collected about two weeks after the heading date when the gibberellin induced morphogenetic abnormalities in spikelets were clearly discernible, receiving no disturbance due to the withering, and yet the conditions of fertilization could be judged by the degree of ovary development, the morphogenetically abnormal spikelets and the white hulls mounted up to about 60 percent of the total number of spikelets, being almost the same value as the percentage of sterile spikelets in the mature panicles mentioned above. Distributive pattern of the abnormal spikelets in the immature panicles was also very similar to that of sterile ones in the mature panicles. Not only the white hulls but also nearly all spikelets which had any morphogenetic abnormality in their floral organs, such as, glumes, stamens and pistils had resulted in the complete sterility or parthenocarpy as shown in figs. 2-11. From the above results, a major cause for the lowering of fertilization due to the gibberellin treatment was considered to be the morphogenetie abnormality which had been induced by gibberellin in the process of spikelet formation, though the immediate relationship between them was not clear in this study. The histological observations were shown in figs. 12-15. As seen in them, the development of the inner tissues of carpel, such as, epidermis, crushed parenchyma, about two layers of cross cells and a layer of tube cells, was almost normal, but that of ovule, especially nucellus was abnormal. No alleuron layer nor starch tissue as well as embryo have generated in the embryo sac. On the parthenocarpic caryopses seen in the gibberellin plot, it was indicated that they resembled closely to those which were reported to be caused by auxins with castrated rice flowers and were usually found in rice plants grown under an unfavourable low temperature, both in morphological and histological characteristics. The relation between auxins and gibberellin on the revelation of parthenocarpic caryopses, and that between these hormonic substances and the physiological background of spikelet morphogenesis under an unfavourable low temperature were discussed to some extent.