INTRODUCTION

The leaves of most crop plants have evolved with a large surface area per unit leaf mass that lends efficiency in the interception of light and provides an extended surface and a short pathway for the diffusive exchange of atmospheric gases. The pathways for CO2 and water vapor differ in length because evaporation of water takes place mainly from the capillary water in the exposed walls of the mesophyll cells while CO2 exchange involves additional transport across the plasmalemma and into the cytoplasm of the cell. The stomatal pores in the epidermis can exert considerable control over diffusion of both CO2 and water vapor.

The water vapor pressure in the protected interior spaces of the leaves is always near to the saturation value (e*) determined by leaf temperature (see Fig. 6.11, Eq. 6.14). Even when leaf water potential (Ψ1) falls to – 2 to – 3 MPa, the depression of water vapor pressure below saturation is small. Ambient air is generally much drier, therefore CO2 uptake through open stomates during photosynthesis is inevitably coupled with a substantial loss of water in transpiration along the strong vapor pressure gradient between leaf and air.

Water is a primary reactant in photosynthesis but the proportion of water required by plants that is chemically incorporated in their structure, or is used to maintain their water content as they grow, is very small. Transpirational losses are several hundred times greater.