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 (g_s) appeared to have an optimum temperature below 15ºC. Simple equations to quantify responses to leaf temperature (T_I) and D were developed, the best of which was

g_s = [1-k₁(1-[T_l/T_opt)]/k₂√D,

where T_opt 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 (C_I). Calculation of δA/δE using A/C_I 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 g_s, 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 g_sD^-1/2 gives rise to an approximately constant δA/δE.