LAUSR

Abstract The rapid growth of the giant reed (Arundo donax L.) is sustained by high rates of photosynthesis (PN) and stomatal conductance (Gs). High rates of Gs would render A. donax vulnerable to desiccation during episodes of high evapotranspirative demand and/or low water availability if not accompanied by effective stomatal control. Stomatal control involves the adjustment of stomatal pore aperture to the prevailing environmental conditions and physiological status of the plant to optimise water use efficiency. We assessed stomatal response to environmental signals (light intensity, [CO2] and leaf to air vapour pressure deficit − VPD) and the foliar concentration of abscisic acid ([ABA]) of field grown A. donax under irrigated (control) and rain-fed (drought) conditions. Drought-stressed A. donax showed more rapid reductions in Gs to lower light intensity/darkness, a slower rise in Gs following increased light and enhanced sensitivity to variations in [CO2]. The stomatal response to leaf to air VPD was unaffected by the water status of the plant. The rates of stomatal response to light/dark and [CO2] were strongly correlated with the concentration of free-ABA within the cytosol but not with the relative water content of the leaves. When exposed to drought, stomata became increasingly sensitive to [CO2] in comparison to PAR and leaf to air VPD. This pronounced increase in stomatal sensitivity to [CO2] was replicated by supplying exogenous ABA to cut leaves from a well-watered control plant. The results of this study indicate that the high PN of A. donax is underpinned by highly effective stomatal control. The stomata of A. donax respond rapidly to changes in environmental conditions and their behaviour is sensitive to the concentration of ABA within the leaf. The high potential for gas exchange and stomatal control observed in A. donax makes it a suitable model species for enhanced stomatal control of PN and the optimisation of stomatal behaviour.