Modelling Stornatal Responses to Environment in Macadamia integrifolia

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 rnol fraction difference (D) a similar curvilinear response was observed at all temperatures and at photon irradiances of 200 and 1500 amol quanta m-2 s-'. 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,) 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 where To,, is the leaf temperature at which maximal stomatal opening is observed and kl and k2 are constants fitted by non-linear least squares regression analysis. Calculation of the gain ratio of C02 assimilation (A) to transpiration (E) (BADE) was complicated by effects of D on the relationship between A and leaf intercellular mol fraction of C02 (Ci). Calculation of aA/BE using A/Ci relationships derived by varying external COz mol fraction at constant D showed the gain ratio to be virtually constant (1 $5 mmol mol-') across a range of leaf temperatures and vapour mol fraction differences but, when calculated directly from the relationship between A and g,, a decrease in BA/BE 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,=~-'" gives rise to an approximately constant BA/BE.