Stomatal Plasticity Maintains Water Potential Homeostasis in <i>Pinus radiata</i> Needles

Vapour pressure deficit (VPD) is a primary determinant of stomatal behaviour and water balance in plants. With increasing global temperature, the accompanying rise in VPD is likely to have a significant impact on the performance of plant species in the future. However, the plasticity of stomatal response to VPD remains largely unexplored. This study examines the plasticity of whole plant stomatal conductance (g_c) response to VPD in Pinus radiata plants grown under two temperatures and a water-deficient treatment over a period of 3 months. The soil-stem water potential gradient (ΔΨ), g_c and soil-stem hydraulic conductance (K_s-s) were evaluated. The different treatment groups showed significant differences in maximum g_c relating to differences in K_s-s, however, g_c dynamic response to VPD was very similar in all treatments such that ΔΨ was conserved once VPD increased above an average threshold of 0.64 kPa. The ability to robustly quantify water potential regulation in Pinus presents opportunities to explore variation in this globally important tree genus as well as providing a new approach to characterize the regulation of gas exchange in response to VPD.