Modelling Stomatal Responses to Environment in Macadamia integrifolia

Abstract

Gas exchange measurements were made of photosynthetic and stomatal responses of Macadamia integrifolia under controlled conditions. Test leaves were subjected to a range of temperatures, humidities and photon irradiances. When stomatal responses to humidity were plotted as a function of vapour mol fraction difference (D) a similar curvilinear response was observed at all temperatures and at photon irradiances of 200 and 1500 μmol quanta m-2 s-1. By contrast, when expressed as a function of relative humidity, different slopes in the humidity response were observed, and at high photon irradiances, stomatal conductances (gs) appeared to have an optimum temperature below 15�C. Simple equations to quantify responses to leaf temperature (TI) and D were developed, the best of which was gs = [1-k1(1-[Tl/Topt)]/k2√D, where Topt is the leaf temperature at which maximal stomatal opening is observed and k1 and k2 are constants fitted by non-linear least squares regression analysis. Calculation of the gain ratio of CO2 assimilation (A) to transpiration (E) (δA/δE) was complicated by effects of D on the relationship between A and leaf intercellular mol fraction of CO2 (CI). Calculation of δA/δE using A/CI relationships derived by varying external CO2 mol fraction at constant D showed the gain ratio to be virtually constant (1.5 mmol mol-1) across a range of leaf temperatures and vapour mol fraction differences but, when calculated directly from the relationship between A and gs, a decrease in δA/δE with D was observed. Macadamia leaves have heavily sclerified bundle sheath extensions and it is considered that this dependence was an artefact due to non-uniform stomatal closure in response to increasing D. It is shown that, at any given temperature, a stomatal response of the form gsD-1/2 gives rise to an approximately constant δA/δE.