Control by cytokinins of the cellular behavior in the plate meristem of zucchini cotyledons

The temporal and spatial effects of exogenous cytokinins on both cell expansion and division activity in the plate meristem of cultured zucchini cotyledons were studied. N ⁶-benzylaminopurine (1–100 μM) and N-(2-chloro-4pyridyl)-N′-phenylurea (4PU-30) (0.1–100 μM) greatly stimulated the cell growth and division. They provoked multiple cell cycles, formation of larger clusters of daughter cells and an increase of the final number of cells. Both cytokinins led to earlier achievement of final cotyledon size and shortened the cell doubling time. By contrast to the purine cytokinin, phenylurea cytokinin 4PU-30 enlarged the cotyledon predominantly in length. Zeatin and kinetin were less effective, particularly in stimulating cell expansion. In low concentrations, all cytokinins were more effective in stimulating division activity rather than expansion. The cells in the cotyledon margins displayed a higher division activity, especially when treated with exogenous cytokinins. The final cotyledon and cluster areas were not of the strict proportional dependence upon the number of their cells. These results provide a novel example where stimulated cell division fails to evoke a respective increase in the final organ size.