Relationship between hydraulic and stomatal conductance and its regulation by root and leaf aquaporins under progressive water stress and recovery and exogenous application of ABA in Vitis vinifera L. 'Syrah'

Aquaporins (AQPs) are cellular water channel proteins known to play a key role in regulating the hydraulic conductance of roots (L_o) and leaves (K_leaf) in plants. Using potted grapevines (Vitis vinifera L. 'Syrah') on a gravimetric irrigation platform, we examined the effects of progressive water stress and recovery therefrom, and soil application of abscisic acid (ABA) on the relationships between L_o, K_leaf, leaf stomatal conductance (g_s) and gene expression of leaf and root AQPs. We test the hypothesis that water stress (potentially resulting in endogenous ABA biosynthesis) and exogenous ABA application increase L_o due to enhanced root AQP activity but decrease g_s and K_leaf due to stomatal closure and decreased leaf AQP activity. Leaf and stem water potentials, g_s, L_o and K_leaf were measured at different time points along the experiment and leaf and roots samples were taken at the same time to determine AQP expression. Both, water stress and ABA application reduced g_s to 50 mmol m^-2 s^-1 three days after the experiment was started. While K_leaf decreased by 75% in the water-stressed plants compared to the well-watered plants, it was unchanged with exogenous ABA. Upon rewatering, the water-stressed plants reached 66% of the initial g_s and had K_leaf values similar to the well-watered plants. Positive linear relationships were observed between L_o and g_s, and between L_o and transpiration that were related to vine water status and ABA application.