The Diffusive Conductivity of the Stomata of Wheat Leaves

Abstract

A leaf chamber (described in detail) was used alternately with a resistance porometer to measure resistance to viscous flow of air through the leaf, and with a diffusion porometer to measure the differential diffusive flow of hydrogen and air (V_H−V_A) through the leaf and the component of hydrogen flow (V'_H) moving straight across the leaf. The resistance of the mesophyll is needed for interpretation: estimates by three different methods for viscous flow did not agree very well, but two different methods for diffusive flow gave good agreement. For wheat leaves, only very large errors are important. Formal analysis is in three appendixes: I. Interpretation of viscous and diffusive flow in small pores involves some problems in molecular physics, complicated by the particular geometry of the wheat stoma. With some uncertainty, formal expressions are derived for the viscous resistance of a single stoma, rv, and for the resistances to diffusion of hydrogen and air, and of water vapour and carbon dioxide, all expressed as r_s per square centimetre of leaf surface. The analysis for hydrogen/air is the most uncertain; that for water vapour and carbon dioxide is more reliable. II. An indication is given of the flow characteristics of the leaf-chamber system, from which rv can be derived, and of the basis for estimating mesophyll resistance. III. The method of converting estimates of r_s into estimates of V_H−V_A and V'_H is given. The results presented are expressed as nearly as possible in terms of the quantities which were measured. For five leaves the dependence of V_H−V_A on V'_H agrees well with theoretical predictions; the dependence of V_H−V_A (and V'_H) on rv, on average, agrees well with prediction, but involves the assumption that the stomata get shorter as they close. The agreement is good enough to suggest that the formal expressions for r_s in terms of stomatal dimensions and molecular gas constants are reliable enough to be carried forward into future transpiration and assimilation studies. The minimum value of ra for water vapour (c. 3 sec cm⁺¹) is close to values found elsewhere by different techniques. At very small stomatal openings there was a large deviation from predicted behaviour, such as would occur if the imposed excess air pressure further closed the stomata during viscous flow experiments.