Turgor and Cell Wall Yielding in Dicot Leaf Growth in Response to Changes in Relative Humidity

Epidermal cell turgor ( P ) and leaf growth in Begonia argenteo-guttata L. were monitored simultaneously following changes in air humidity in order to evaluate P –growth relations. A decrease in air humidity from 70 to 5% caused a decrease in P of 0.05 MPa. This small decrease in P resulted in cessation of growth. Subsequently, growth recovered partially at constant P , indicating an increase in wall yielding to P . Notwithstanding this increase in wall yielding, the steady growth rates showed a marked dependence on P . Decreases in P of 0.05 MPa caused a 30–40% reduction in the steady rate of elongation. These results were reversible. Upon a step increase in air humidity from 5 to 70%, P and growth rate rapidly increased. Subsequently, growth declined without a corresponding decrease in P , although the rate of growth remained higher than at low humidity. The partial self-stabilization of growth following P changes and the positive relationship between steady growth rate and P are consistent with the notion that wall yielding is controlled by interactions between P and metabolism. Results are discussed in relation to biophysical factors that control growth and to present theories that accommodate variable wall yielding