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Fertilization and Development of Embryo on Satsuma Orange (Citrus Unshiu MARC.) and Natsudaidai (C

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Fertilization and Development of Embryo on Satsuma Orange (Citrus Unshiu MARC.) and Natsudaidai (C Fertilization and development of embryo on Satsuma orange (Citrus unshiu MARC.) and Natsudaidai (C. natsudaidai HAYATA) HSU-JENYANG Collegeof Agriculture,University of OsakaPrefecture , Sakai,Osaka Summary The mechanism of the Citrus pollen-tube growth 3. The fertilized eggs and nucellar cells began into style and ovary, fertilization and development to divide 40 to 50 days after pollination both in of embryo were studied. The results obtained were selfed Natsudaidai and crossed Satsuma orange with summarized as follows Natsudaidai. 1. The pollen tubes of Natsudaidai grew faster 4. In Natsudaidai, embryo development was very than Satsuma orange in pistil of Natsudaidai or slow and reached the globular stage 58 days, heart- Satsuma orange. shape embryo stage 72 days, and the complete 2. The fertilization took place about 8 days in embryo stage 120 days after self-pollination. selfed Natsudaidai, 12 days in the crossing of 5. The nucellar cells of one to three layers near Satsuma orange with Natsudaidai, and 14 days in the micropylar of embryo sac and the region of self-pollination of Satsuma orange after pollination, the micropylar end to one-third of embryo sac have respectively. the ability to form nucellar embryos. Introduction The polyembryony has been found extensively in Citrus, Poncirus and Fortunella plants. This phenomenon was first recognized in orange seed by LEEVWENHOEKin the beginning of the eighteenth century. According to the works of OSAWA(19) ToxopEUs (21) FROST5, FURUSATO(7) and MAHESHWARIand SACHAR~I5~,the extra embryo originate from the nucellar tissue in ovules. It has been considered that pollination is positively necessary for the occurrence of the nucellar embryos (IKEDA(11~,NAGAI and TANIKAWA{17~,FROST~4~, WONG(23)), however, WRIHGT~24~,reported that nucellar embryos occur even without pollination. The fertilization and its mechanism of the initiation in the development of the nucellar embryos have not been studied in detail except for the studies of C. tamurana (MIwA~IS~)and Poncirus trifoliata (OSAWA(19)). In order to obtain a clear understanding of this problem, the author carried out this experiment to study the mechanism of Citrus pollen tube growth into the style and ovary, its fertilization and development of nucellar embryos. Materials and Methods Materials used in this experiment were 25-year-old Satsuma orange (Citrus unshiu MARL.) and 8-year-old Natsudaidai (C. natsudaidai HAYATA). All these plants are grown in the Citrus Garden of Osaka Agriculture Research Center, Osaka Prefecture. The experiments were conducted during the years from 1964 to 1966. Mature and uniform flower buds for artificial crosses were selected and emasculated . The emasculated flowers were covered with paraffin paper bags. Pollen for desired crosses was Received for publication November 20, 1967 8 HSU-JEN YANG : Fertilization and development of embryo on Citrus 103 gathered from newly opened flowers which had been previously covered with paraffin paper bags. Pollination was carried out from 10: 30 a.m. to 4: 00 p.m. The pistils were cut off and fixed in chloroform-acetic-alcohol (3: 2 : 2) at the various intervals after pollination. The materials for microtome were processed in ethanol-xylene dehydration series and embeded in paraffin. Sections were cut out 10 to 15 /2 thickness and stained by Haidenhain's iron-alum- haematoxylin procedure. Results 1. Pollen tube growth The germination of Natsudaidai and Satsuma orange pollens were observed on its own and reciprocal stigmas two hours after pollination (Fig. 1). Pollen grains of Natsudaidai germinated earlier on its own stigmas as well as the stigma of Satsuma orange in comparison to those of Satsuma orange pollen grains. Regarding the growth of pollen tube, the fastest one was Natsudaidai pollen tube in its own style, the second was in the style of Satsuma orange, the third was Satsuma orange pollen tube in its own style, and the slowest was noted in the style of Natsudaidai (Figs. 2 and 3). Natsudaidai pollen tubes reached to its own ovary 4 days after pollination, at that time, Satsuma orange pollen tubes penetrated just only one-half of its own style and one-fourth of Natsudaidai style. All of the Natsudaidai fruits crossed with Satsuma orange dropped off 8 days after pollination (Table 1). 2. Fertilization and development of endosperm Pollen tubes of Natsudaidai reached to its own embryo sac and fertilized 8 days after self- pollination (Fig. 5), and to the Satsuma orange embryo sac 12 days after crossing of Satsuma orange with Natsudaidai. However, Satsuma orange pollen tube penetrated into its own embryo sac 14 days after self-pollination. Then, several endosperm nuclei were observed in the embryo sac 10 days after self-pollination of Natsudaidai (Fig. 6), 14 days after crossing of Satsuma orange with Natsudaidai, and 16 days after self-pollination of Satsuma orange. The endosperm nucleus which had divided succes- sively formed many scattered free nuclei in a thin layer of cytoplasm during 32 days after crossing of Satsuma orange with Natsudaidai (Fig. 7). After then, these free nuclei divided more and more and cell wall was formed surrounding each nuclei and filled the embryo sac up completely. Finally, the endosperm gradually disappeared with the formation of complete embryo (Figs. 15, 16 and 17). 3. Development of embryo Fig. 4A. Diagram depicting longitudinal section From the observation of this experiment, the of an ovule of Citrus unshiu, showing the oospore remained dormant relatively for a long position of pollen tubes. A. B. C. D and E indicate the position of pollen tubes. a, time after the egg cell has fertilized. The antipodal cell, eg, egg cell. f, f uniculus. 1, oospore did not divide within 40 days after locule. o, ovary, ov, ovule. p, polar nuclei. s, synergid. st, style. vb, vascular bundle. pollination (Fig. 8). Four-celled proembryo was 1.04 Jour. Japan. Soc. Hort. Sci. 37 (2) Table 1. Length of pollen tubes and the number of endosperm nuclei of C. natsudaidai and C. unshiu, in each of self- and crossing(s) observed 44 days after self-pollination of Natsudaidai (Fig. 10), and six- and ten-celled pro- embryo was found 52 and 54 days after crossing of Satsuma orange with Natsudaidai (Figs. 11 and 12). So it may be said in general, the oospore remained dormant for about four to five weeks after fertilization. In Natsudaidai, embryo development was very slow and reached the globular stage 58 days (Fig. 13), heart-shape embryo stage 72 days (Fig. 14), and the complete embryo stage 120 days after self-pollination (Figs. 15 and 16). While, in Satsuma orange the formation of globular embryo was observed 63 days and the embryo which was being differen- tiated and formed the various parts of complete embryo was found 108 days after the crossing (Fig. 17). It was confirmed that when oospore began to divide a few of large nuclei which could be stained with hematoxylin were found in the nucellar tissue of one to three layers near the micropylar of the embryo sac (Fig. 9). Some of these cells began to divide and formed masses of cells and protruded into the embryo sac 84 days after crossing of Satsuma orange with Natsudaidai (Figs. 20 and 21). It was also found that globular nucellar embryo was observed from the region of the micropylar end to one-third of embryo-sac (Figs. 18 and 19). From the above mentioned results, it may be said that not only the nucellar cells near the micropylar end, but also the cells from the micropylar end to one-third of embryo sac have the ability to form nucellar embryos. Discussion Fertilization is the fusion of a sperm nucleus with an egg nucleus. The first step of the fertilization is begun with the germination of pollen grains and growth of pollen tube in the stigma tissue. MiwA~16~reported that C. tamurana has not a behavior of self-fruitfulness, owing Hsu-JEN YANG : Fertilization and development of embryo on Citrus 105 to their self-incompatibility. He further reported regarding the growth rate of pollen tube in style that the pollen tube of C. limon grew more quickly when C. tamurana was crossed with C. limon than of self ed C. tamurana. TON and KREZDORNX20) observed that pollen tubes of cross-pollination among Clementin, Minneola and Orlando, with the exception of cross-incom- patibile Orlando x Minneola, and Minneola x Orlando, grew faster than those of the self s. The similar result was obtained from Satsuma orange by the author, i.e., the growth of pollen tube in crossing with Natsudaidai was faster than selfing. However, Natsudaidai showed completely reverse results ; the growth of pollen tube in selfing was more rapidly than the crossing with Satsuma orange. It is also found that many pollen tubes of Natsudaidai were observed in the ovary of self ed Natsudaidai and crossed Satsuma orange with Natsudaidai, but'few pollen tubes of Satsuma orange detected in its self ed ovary, and could not be ascertained any of them in the base of style and ovary of crossed Natsudaidai with Satsuma orange. While, the pollen tube of Satsuma orange grew only one-half of the style of Natsudaidai. Moreover, all of the fruits of Natsudaidai crossed with Satsuma orange dropped off 8 days after pollination. From these results it can be assumed that some kind of inhibitors are formed in the style or ovary of Natsudaidai and these inhibitors suppress the growth of pollen tube of Satsuma orange. It may also be said the cross-incompatibility would be exist when Natsudaidai was crossed with Satsuma orange. As soon as the pollen tube reached the embryo sac by way of the micropyle, a sperm nucleus passed into the embryo sac, and fused with the egg nucleus, thus fertilization took place.
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