Hydraulics and gas exchange recover more rapidly from severe drought stress in small pot-grown grapevines than in field-grown plants.

Modifications of plant hydraulics and shoot resistances (R) induced by water withholding followed by rewatering, and their relationships with plant water status, leaf gas exchange and water use efficiency at the leaf level, were investigated in pot-grown and field-grown, own-rooted Syrah grapevines in an arid climate. Water stress induced anisohydric behavior, gradually reducing stomatal conductance (g) and leaf photosynthesis (A) in response to decreasing midday stem water potential (Ψ). Water stress also rapidly increased intrinsic water-use efficiency (A/g); this effect persisted for many days after rewatering. Whole-plant (K), canopy (K), shoot (K) and leaf (K) hydraulic conductances decreased during water stress, in tune with the gradual decrease in Ψ, leaf gas exchange and whole plant water use. Water-stressed vines also had a lower Ψ gradient between stem and leaf (ΔΨ), which was correlated with lower leaf transpiration rate (E). E and ΔΨ increased with increasing vapour pressure deficit (VPD) in non-stressed control vines but not in stressed vines. Perfusion of xylem-mobile dye showed that water flow to petioles and leaves was substantially reduced or even stopped under moderate and severe drought stress. Leaf blade hydraulic resistance accounted for most of the total shoot resistance. However, hydraulic conductance of the whole root system (K) was not significantly reduced until water stress became very severe in pot-grown vines. Significant correlations between K, K and Ψ, K and leaf gas exchange, K and Ψ, and K and A support a link between water supply, leaf water status and gas exchange. Upon re-watering, Ψ recovered faster than gas exchange and leaf-shoot hydraulics. A gradual recovery of hydraulic functionality of plant organs was also observed, the leaves being the last to recover after rewatering. In pot-grown vines, K recovered rather quickly following restoration of Ψ, although gas exchange recovery did not directly depend on recovery of K. In field-grown vines, recovery of water status, gas exchange and hydraulic functionality was slower than in pot-grown plants, and low g after rewatering was related to sustained decreased K, K and K and lower water transport to leaves. These results suggest that caution should be exercised when scaling up conclusions from experiments with small pot-grown plants to field conditions.