P. T. ANNIE and P. GOPINATHAN NAIR
Department of Botany, University of Kerala, Karisvattom, Trivandrum, 695 581, India
Cytology of developing endosperm has been studied in many plants by several workers. But no cytological study has so far been reported in this nutritionally vital tissue in rice. Due to the very early accumulation of starch, it is extremely difficult to obtain good cytological preparations of the endosperm cells in cultivated rice. The present report is the result of cytological studies on the endosperm in two diploid wild species, Oryza granulata and O. officinalis and a tetraploid form of O. punctata.
Very young endosperm 4-6 days after selling in O. officinalis and O. punctata and 2-3 days after selfing in O. granulata were collected at about 1.30 PM from plants maintained in the Botanical Garden of the Botany Department, Kerala University and fixed in Carnoy's fluid (3 ethyl alcohol: 1 acetic acid) after removing lemma-palea. After 24 hours the materials were transferred to fresh fixative and kept in the refrigerator for a week. Thereafter the endosperm was dissected out and squashed in 2% acetocarmine solution. A drop of ferric acetate was added to the fixative for better stainability of the chromosomes.
In all the three species frequency of dividing cells was found to be very low (3.6% in O. granulata, 2.05% in O. officinalis and 8.54% in O. punctata) and the few cells at metaphase showed mostly triploid chromosome number of 3n = 36(3X) in O. granulata (Fig. 1A) and O. officinalis (Fig. 1B) and hexaploid number of 3n = 72(6X) in O. punctata. Further, cells with 3n = 72(6X) chromosome number were observed in O. granulata (0.67%) and in O. officinalis (0.67%) and 4.67% cells in O. punctata (Fig. 1C) showed 3n= 144(12X) chromosome number. In the tetraploid species still higher ploidy of 3n=288(24X) was also seen in one cell. Endopolyploidy was found to be mainly due to C-mitosis type of division. Moreover, a few cells in which fusion of two daughter nuclei at prophase and cytomixis involving the fusion of the entire chromosome complements of two neighbouring cells at metaphase (Fig. 1D) were also observed.
The present observations on the endopolyploidy and the mode of endopoly- ploidization i.e. C-mitosis type of division and fusion of nuclei at prophase are in conformity with similar observations on the endosperm cytology in plants like coconut (Abraham 1963; Abraham and Mathew 1963) and Borassus and Datura (Stephen 1974, 1980). However, cytomixis observed in the tetraploid rice is another cause of endopolyploidization which is not yet reported in the endosperm tissue in any other plant. The present study reveals that the frequency and level of endopolyploidy are very limited in the endosperm of wild species of Oryza. Accumulation of mitotic abnormalities was earlier reported to be characteristic of degenerating endosperm (tandon and Kapoor 1962; Abraham et al. 1965;Stephen 1977). But in the present study such mitotic abnormalities were not seen. Another notable feature in the present study is the difference between the diploid and tetraploid species regarding the frequency and levels of endopolyploidy, both being higher in the tetraploid. Varietal difference in the frequency and level of endopolyploidy was earlier reported in coconut (Abraham et al. 1966). The present study indicates that the process of endopolyploidization of the endosperm tissue in the wild species of Oryza might be under genetic control involving also the ploidy status of the species.
We thank Prof. Susan Abraham for providing facilities and Dr. T. T. Chang. International Rice Germplasm Center, IRRI, Philippines for sending the rice materials. P. T. Annie acknowledges the associateship received under the UGC Departmental Special Assistance Programme.
Fig. 1. Endosperm mitoses in three wild species of Oryza. x 1500.
A. A cell showing triploid [3n=36(3X)] number of chromosomes in Oryza granulata.
B. A cell showing triploid [3n=36(3X)] number of chromosomes in O. officinalis.
C. A cell showing triploid 3n = 144(12X)] number of chromosomes in O. punctata.
D. Two adjacent cells showing cytomixis in O. punctata.
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