Dynamics of water and carbon dioxide gas exchange on the adaxial and abaxial surfaces of leaves

Abstract

Leaf gas exchange occurs through the stomata and cuticle, with water vapour and CO2 passing through the boundary layer. Assuming moisture saturation inside the leaf enables us to know the H2O vapour concentration gradient that drives transpiration, which is used to estimate the H2O gas exchange parameters with some assumptions about the H2O pathway through the leaf surfaces. Common assumptions in gas exchange estimations lead to different levels of errors in the calculation of internal CO2 concentration, affecting the precision and interpretation of photosynthetic processes and CO2 diffusion inside the leaf. Two main assumptions are to neglect the cuticular conductance and the neglect of the impact of mixing adaxial and abaxial gas exchanges in the calculations, i.e, analysing the leaf gas exchange in leaf surfaces basis without separating the abaxial and adaxial exchange of gases. It is virtually impossible to identify stomatal and cuticular conductance separately when the transpiration occurs through both at the same time. Thus, for convenience and because cuticular conductance to water generally is less than 10% of the stomatal conductance when the stomata are fully open, cuticular conductance is generally neglected. Equally, cuticular conductance to CO2 is usually ignored as it is two orders of magnitude lower than stomatal conductance to CO2. These two neglected items force the assumption that the whole leaf surface conductance is solely stomatal conductance, regardless of real stomatal conductance. One of the most important consequences of neglecting cuticular fluxes is that the ratio of cuticular and stomatal conductances to H2O are incorrectly considered to be the same as the ratio for CO2, which leads to significant errors in the calculations of dynamics of H2O and CO2 gas exchange. For example, if water passes through the cuticle as a liquid, then the CO2 flux may be very small. Thus, while the assumption of negligible cuticular conductance to H2O has been a largely practical one, it can lead to significant overestimates of CO2 conductance and subsequent calculated parameters. The widely used equations for gas exchange calculations derived by von Caemmerer and Farquhar (1981) are usually used assuming that they can be applied directly on gas exchange measurements mixing the adaxial and abaxial leaf gas exchange; even though the derivations were based on a single leaf surfaces. The assumption of neglecting the impact of mixing adaxial and abaxial fluxes has been largely unattended and its actual influence in the outcome of the calculations remains mostly unknown. In this thesis, I examine H2O vapour and CO2 pathways and dynamics in leaves during gas exchange, focusing on three main aspects of it: (1) cuticular conductance to H2O and CO2; (2) adaxial and abaxial leaf gas exchange; and (3) leaf internal CO2 concentration. The objective of the work was to determine the impact of these three aspects of the gas exchange on the calculation of gas exchange parameters such as stomatal conductance and [CO2] inside the leaf, generating a mathematical theory to quantify them in more detail. Particular attention is paid to the cuticular conductance to H2O, which is important to gas exchange measurements, but the cuticle is also the final barrier to leaf desiccation, and water loss through it is a key factor determining plant survival in drought. A theoretical and practical comprehensive analysis of the importance of cuticular conductance is presented, together with methods to indirectly estimate its value. This study presents a technical and mathematical solution to calculating gas exchange parameters most precisely, modified structurally from the most widely used equations. Show more