MODELING STOMATAL 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 mol fraction difference (D) a similar curvilinear response was observed at all temperatures and at photon irradiances of 200 and 1500-mu-mol 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 15-degrees-C. Simple equations to quantify responses to leaf temperature (T(l)) and D were developed, the best of which was g(s) = [1 - k1(1 - \T(l)/T(opt)\)]/k2 square-root 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) (partial derivative A/partial derivative E) was complicated by effects of D on the relationship between A and leaf intercellular mol fraction of CO2 (C(i)). Calculation of partial derivative A/partial derivative E 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 partial derivative A/partial derivative E 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(s) is-proportional-to D-1/2 gives rise to an approximately constant partial derivative A/partial derivative E.