Transient Heat and Mass Transfer from a Leaf Undergoing Stomatal Closure in a Low Pressure Environment

Abstract

Stomatal control of water loss was studied in a low pressure environment by using detached leaves of Crassula argentea Thunb., Peperomia obtusifolia (L.) A. Dietr., and Setcreasea pallida Rose cv. ' Hg) the molecular mean free path of water vapor, X, is large compared with the significant flow dimension, the stomatal pore width, 2b large Knudsen number (defined as X/2b) characterizes free molecule flow, wherein intermolecular collisions are rare, and the flow is determined solely by molecular collisions with the wall. For the species used, the ratio of the guard cell thickness to the stomatal pore width is of the order 10, designated hereafter as 0 (10) for the mass flow rate through a unit area of leaf surface. In the above expression , is the ratio of the vapor pressure outside to that inside the stomatal cavity; L is the length of the stomatal tube; a and b are the semimajor and semiminor axes of the elliptical cross-section of the stoma, respectively; R is the specific gas constant; Hfg is the difference in the specific enthalpies of liquid water and vapor; D is a constant with units of pressure, and T is leaf temperature.In Equation 1 the total pressure inside the leaf has been taken to be the saturation vapor pressure over a pure, free water surface at leaf temperature. It has been tacitly assumed that flow is quasisteady and that the ratio of the pore diameter to the substomatal intercellular area is so small that it, and not the vaporization process at the liquid-gas interface, controls the net rate of molecules leaving the leaf surface. According to the arguments of Slatyer (17)