Cytokinin Production in Relation to the Growth of Pea-root Callus Tissue

The natural occurrence of free cytokinins was examined in relation to the growth of serially propagated pea-root callus tissue cultures. The relatively slow-growing pea-root callus was harvested at regular intervals throughout a 12-week period and fresh weight, dry weight, cell number, and cytokinin activity were determined. At the end of the culture period the fresh weight had increased about 34 times, the cell number about 100-fold, and the dry weight approximately 19 times over that found on inoculation. Purified ethanol extracts from pea-root callus were tested for cytokinin activity by the soybean callus bioassay. Cytokinin activity was detected in extracts made at all stages of growth tested. There was a sharp rise in cytokinin content during the early period of culture. The peak in cytokinin activity established at the beginning of the phase of growth was associated with a high frequency in mitoses. As growth proceeded there was a decline in both cytokinin content and in the mitotic index.