CELL POPULATION KINETICS IN CALLUS TISSUES OF CULTURED PEA ROOT SEGMENTS

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Abstract
Two callus tissues, one composed of diploid and the other of a mixture of diploid and polyploid cells, were derived by culturing 1‐mm pea root segments; the mixed callus tissues were obtained by incorporation of kinetin in the culture medium. The callus tissues were used to determine (a) if cell proliferation was altered with the change in cell constituents of a callus; (b) the rate at which polyploid cells increased after kinetin stimulation; (c) the nature of the mitotic cycle in the diploid and mixed polyploid callus tissues; and (d) if the mitotic cycle changed as the tissue aged. Histological, cytological, radioisotope, and radioautographic analyses were made on callus tissue ranging from 1 to 4 days old. The results indicated that gross morphological changes were associated with the anatomical location of the proliferative cells. They showed that the percentage of polyploid division figures after stimulation by kinetin increased rapidly during the first 6 to 7 days in culture and then continued to increase at a much reduced rate. Cell counts revealed that cell proliferation in the mixed callus tissue was initially delayed when compared with the diploid tissue, but that after the delay was overcome cell number increased in each in similar manner. Analysis of the number of DNA‐synthesizing cells showed that their percentage was highest during the first 2 days of culture and then leveled off at a value of about 10%. Mitotic cycle analysis indicated that it could be accurately measured only in the younger diploid callus tissues and that it increased in variability with increased age.