Whole-canopy and single-leaf gas-exchange responses to partial rootzone drying in potted 'Cabernet Sauvignon' grapevines

Single-leaf and whole-canopy gas exchange were monitored for two weeks on 6-year-old 'Cabernet Sauvignon' grapevines (~ 6 m2 leaf area) grown with their root system split between two 40 L pots. Beginning July 28, water was withheld from one pot while the second was kept well watered (referred to hereafter as the PRD treatment). The control vines had both pots well watered. The PRD vines were rewatered on 11 August. One half of each vine's canopy was enclosed in a flowthrough, whole-canopy chamber system for continuous monitoring of CO2 and H2O differentials. The other half of the canopy was used for concurrent single-leaf measurements of net CO2 assimilation (A), stomatal conductance (gs), transpiration (E) and midday water potential (Ψ1) taken 3 days before July 28 and 2, 5, 9 and 12 days thereafter and at re-watering. Volumetric soil water content was monitored at the 30 cm depth by Time Domain Reflectometry. Pre-dawn leaf water potential (ΨPD) was measured on day 11 of stress. Soil moisture approached the wilting point in the dry pot 2 days after water was withheld. The ΨPD of the PRD treatment (-0.25 MPa) was not significantly different from that of the control (-0.17 MPa) at day 11. Canopy net CO2 exchange rate (NCER) was reduced by 24.4% in the PRD treatment for the two week period and was similar to the reduction (-27.2%) in a single-leaf basis. Leaf g s and E were reduced for the PRD treatment during the two weeks by 23 and 25%, respectively, as compared to the control. Water use evaluated on a canopy basis for the PRD treatment was reduced by only 15% compared to the control. Leaf water potential was unaffected by treatments. Water use efficiency calculated from both single-leaf and whole-canopy readings over the 2-week period showed no differences between treatments.