Figure 6.

A model summarizing the observed cellular changes related to the formation of the vacuolized and embryogenic cell types, respectively. Exogenous auxin is required for cell activation characterized by an increase in pHc, but the morphology and following development of the cells can be modulated by parallel mild stress treatments. The time required for endogenous auxin synthesis and the activation of cell division is shortened and the vacuolar pH is raised under high 2,4-D concentrations or mild stress conditions. Buffering of the medium pH by MES delays endogenous auxin synthesis and consequently delays cell activation and blocks the developmental pathway toward the formation of the small, cytoplasmically rich embryogenic cell type.