Abstract
Comparative effects of gibberellin and auxins-naphthaleneacetic acid (NAA) and 2, 4-dichlor-ophenoxyacetic acid (2, 4-D)-on the induction of parthenocarpic caryopses were studied using three rice varieties, "Fujisaka No. 5", "Sachikaze" and "Tokaiasahi". The chemicals were sprayed on young flowers which were emasculated just before anthesis and the flowers cut two weeks after treatment were used for observations. NAA and 2, 4-D, especially the latter, produce the parthenocarpic caryopses efficiently but unexpectedly at any concentration of 0.1, 1.0, 10.0 and 50.0 ppm of gibberellin failed to promote the growth of the ovary wall parthenocarpically. Above results indicate that in rice plants the effective growth substances to induce the parthenocarpy may be auxins and not gibberellins when they are applied at the stage of anthesis. Several workers have reported on the effectiveness of gibberellins in stimulating parthenocarpy in various plants. The reason why gibberellin does not display the same effect on the rice plants may be an interesting problem to be resolved in the near future. Furthermore, it was found that even in the control plots (emasculated and sprayed with distilled water) the parthenocarpic caryopses were observed frequently in two varieties used, especially in "Tokaiasahi", the latest maturing variety of all. This indicates that in rice plants there is possibility to induce the parthenocarpy without the pollination process and that in some varieties there seems to be enough endogenous auxin in the ovary at the stage of anthesis to promote the growth of the ovary wall by itself. The result previously reported by the writer that frequent parthenocarpic caryopses can be induced by gibberellin treated at the stage of young panicle primordium differentiation should be considered to be resulted not with the direct but the auxin-mediated effect of gibberellin during the pistillate organ development with or without pollination. Also, on the prevailing phenomenon of parthenocarpy in rice plants caused by an unfavourable low temperature, the endogenous auxin should be considered to be an important factor stimulating the ovary wall growth with or without pollination process.
