Introduction Stomata, epidermal pores that control the uptake of CO2 into, and water vapour loss out of, leaves (Cowan, 1978), consist of two guard cells. Changes in guard cell turgor adjust pore apertures. The maximum level of stomatal conductance (gs) is dependent on the size of the stomatal pore and the density of stomata (Franks et al., 2009), and steady state values of stomatal conductance and carbon assimilation (A) are often tightly correlated (Wong et al., 1979; Farquhar & Sharkey, 1982).