GROWTH, PRESSURE, AND WALL STRESS IN EPIDERMAL-CELLS OF BEGONIA ARGENTEO-GUTTATA L LEAVES DURING DEVELOPMENT

Conventional growth-turgor analyses of wall yielding do not take into account developmental changes in tissue and cell geometry that could significantly alter the force on and stress borne by expanding walls. To ascertain the role of wall stress in the decline in growth during leaf maturation, we analyzed the relation between leaf elongation and epidermal turgor (P-e), and the leaf and cell wall cross-sectional areas at three different stages of leaf development. As Begonia leaves elongated from 70 to 140 mm (fully expanded), the relative growth rate declined from ca. 20% d(-1) to zero. Yet, there was an increase in P-e of 0.15 and 0.10 MPa in epidermal cells over the midvein and lamina, respectively. Characterization of cell wall cross-sectional areas, through image analysis of stained leaf sections, showed that the fraction of the leaf cross-sectional area occupied by cell wall was approximately 10% larger for epidermal cells of the midvein than for midvein parenchyma cells or for epidermal cells of the lamina. No significant changes were observed in the relative cross-sectional area occupied by cell wall during leaf development. Outer epidermal wall (OEW) thickness was determined by light and interference microscopy. Both methods showed that OEW thickness was greater over the midvein than over the lamina. During maturation there was a moderate increase in thickness of midvein OEW, but not in the lamina OEW. Cell wall stress, estimated as force per unit cross-sectional wall area, increased during leaf maturation. Determination of the refractive index of OEW through interference microscopy indicated that densities of midvein and lamina OEW were similar and that they did not change during leaf maturation. The results showed that changes in the growth of Begonia leaf during development were not attributable to a decline in stress on the epidermal walls or to an increase in OEW density.