In order to obtain a more definite information on the ploidy level of nodule cells, an attempt has been made to correlate the data obtained from measurements of nuclear and nucleolar volume in the cells of the nodule meristem and bacteroid region of nodules of
Lathyrus sativus as compared against those of normal uninfected root meristem. By this method it has been possible to locate each cell, with their ploidy level, in the different tissues studied.
The number of nucleoli per nucleus varied from one to four in the root meristem, one to five in the nodule meristem and one to three in the bacteroid region of nodules. The absence of the expected number of nucleoli in the 4n cells was due to the early fusion of these nucleoli during their origin in telophase.
Disomatic cells were not observed in the root meristem either prior to or after the initiation of nodule formation. This was confirmed from measurements of nuclear and nucleolar volumes of cells. Only 2n number was counted in the root meristem but in the cells of the nodule meristem both 2n and 4n number were evident.
A 2:1 relationship existed between the nuclear as well as nucleolar volume of cells of the bacteroid region and those of the root meristem, irrespective of the nucleolar number. In the nodule meristem, the mean nuclear and nucleolar volumes of cells were more than those of the root meristem but less than the bacteroid region.
Infection by
Rhizobium is correlated to initiation of nodule meristem. Such increased physiological activity of the cells, accelerated chromosome duplication without the corresponding cell plate formation. Hence most of the immediate cell progenies were rendered polyploid. Continued division in these cells was interrupted due to change in the relative viscosity of the cytoplasm produced by the multiplication of the invading rhizobial cells in them. For this reason the relative frequency of divisional figures with polyploid number of chromosomes in the nodules varied from plant to plant (cf. Tatuno and Kodama 1965, Kodama 1967). The nature of invasion of the rhizobia and the consequent pattern of distribution of meristem determined the relative amount of monosomatic and disomatic cells in the nodules.
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