No abstract

The traditional bulk elastic modulus approach to plant cell pressure-volume relations is inconsistent with its definition. The relationship between the bulk modulus and Young's modulus that forms the basis of their usual application to cell pressure-volume properties is demonstrated to be physically meaningless. The bulk modulus describes stress/strain relations of solid, homogeneous bodies undergoing small deformations, whereas the plant cell is best described as a thin-shelled, fluid-filled structure with a polymer base. Because cell walls possess a polymer structure, an alternative method of mechanical analysis is presented using polymer elasticity principles. This initial study presents the groundwork of polymer mechanics as would be applied to cell walls and discusses how the matrix and microfibrillar network induce nonlinear stress/strain relationships in the cell wall in response to turgor pressure. In subsequent studies, these concepts will be expanded to include anisotropic expansion as regulated by the microfibrillar network.

Previous mathematical analyses of stomatal mechanics have demonstrated, and experimental measur-ements have confirmed, that the turgor-gcner-ated force of the epidermal cells dominates that of the guard cells in determining apertute, DeMichele & Sharpe (1973) tettned the phenotnenon the mechanical advantage of the epidermal cells, while Cooke et al. (1976) expressed it as an antagonism ratio. Both of these formulations, however, have theoretical or practical limitations.This study presents a biophysical analysis demonstrating that the effective forces in the stomatal system may be studied in ter-ms of sitnple stotnatal geometry. From this analysis, the mechanical advantage can be redefined and interpreted based upon simple geometric relationships calculated from measurable anatomical dimensions, Kev-wotds: epidermal cell, guard cell, mechanical advantage, stomata, turgor pressure

Vicia faba plants grown under water deficit were found to have guard cells considetably smaller than those of plants grown under well-watered conditions. Stomata of plants adapted to drought conditions have been observed in past studies to maintain opening at plant water potentials lower than those of plants not so adapted. By employing the geometric interpretation of the mechanical advantage (Wu, Sharpe & Spence, 1985), an anatotnical/mechanical basis was found that helps explain how sueh opening in drought conditions can occur. The geometry and resulting mechanical properties of small stomata, in contrast to larger stomata, give them the capability of opening or maintaining open pores with lower guard cell turgor pressures, relative to the turgor of the surrounding epidermal cells.

Vicia faba plants grown under water deficit were found to have guard cells considetably smaller than those of plants grown under well-watered conditions. Stomata of plants adapted to drought conditions have been observed in past studies to maintain opening at plant water potentials lower than those of plants not so adapted. By employing the geometric interpretation of the mechanical advantage (Wu, Sharpe & Spence, 1985), an anatotnical/mechanical basis was found that helps explain how sueh opening in drought conditions can occur. The geometry and resulting mechanical properties of small stomata, in contrast to larger stomata, give them the capability of opening or maintaining open pores with lower guard cell turgor pressures, relative to the turgor of the surrounding epidermal cells.