Studies on the behavior of incompatible pollen in Brassica. IV

Abstract

Quite different opinions about which will be discussed later have been proposed by three wor-kers, with regard to the behavior of incompatible pollen in Brassica oleracea (SASAOKA 1928, KAKIZAKI 1930, and SEARS 1937). The situation makes this plant of particular interest for pollen tube studies. In order to elucidate this problem the writer has attempted some experiments with cabbges and cauliflowers. In these investigations the following varieties were used as material : Cabbage; Nozaki-Wase, Nakano-Wase, and Succession. Cauliflower; Gokuwase, and Gokuwase-Taikyu. Both self- and cross-pollinations were made simultaneously on the same plant. In these ex-periments the pistils were collected at intervals, killed in acetic-alcohol, slit with a razor blade lengthwise into three sections, and stained with 0.5 percent lactic blue for about 2 hours. After being differentiated in lactic acid for about 24 hours, mounts were made in a drop of glycerine. Sufficient pressure was applied to the cover glass to squeeze the style into a thin layer only two or three cells in thickness. The results obtained are summarized as follows: I. Brassica oleracea L. var capitata L. In self-pollinated flowers, a few of the pollen grains _ were found on the stigmas. It is thought that most of the pollen grains which failed to germinate had been washed off in the treatment of fixing and staining. The germination of pollen was decreased. Pollen tubes usually ceased to develop when they were about as long as the diameter of a pollen grain, and failed to penetrate into the stigma. Some tubes coiled around the papillae. Emptied pollen grains were scarcely found which had emptied their contents into their tubes. Nozaki-Wase, No. 7 was an only exception, which may probably be a self-compatible plant (Table 1). Consequently, the behavior of incompatible pollen was found to be similar to that of the broccoli by SEARS (1937). It may be concluded that the self-incompatibility of the cabbage is attributed to decreased germination of incompatible pollen and to no penetration of tubes into the stigma. On the behavior of pollen after cross-pollination, there was a clear distinction between incompatible and compatible matings. 1) In cross-incompatible combinations, the behavior was found to be quite similar to that of incompatible pollen after self- pollination. 2) In cross-compatible matings, nu-merous pollen grains were found on the stigmas, the percentage of pollen germinated being much higher than that of self-pollination. A high proportion of emptied pollen grains was obtained. The tissue of the style wsa penetrated freely by pollen tubes. The longest tube reached the ovary in about 8 hours after pollination. Numerous pollen tubes could be found in the style and ovary in about 24 hours after pollination (Table 2). When a long time had passed after self-pol-lination, the percentage of pollen germinated was increased, and only a few emptied grains were also found, even in incompatible flowers (Table 3). This phenomenon might be explained more reasonably by assuming of the rendering com-patible of incompatible pollen in long time inter-vals after self-pollination, than by assuming that pollen germination and tube growth were very slow after self-pollination. This idea might be supported by the fact that the pollen viability of the cabbage was maintained satisfactorily for 7 days at 11°C (PEARSON 1933), and by the fact that cabbage flowers selfed 2 or 3 days after open-ing gave much better seed production than those selfed on the same day when flowers opened, namely, the secretion of inhibiting substance that prevents self-fertilization might be decreased in the old pistils (KAKIZAKI 1930, MOHAMMAD 1935, and SIIINOHARA 1942)