Abstract
Leaf gas exchange measurements can be used to monitor plant water status and identify the onset of water stress. The main aim of this study was to assess thresholds of soil and midday stem water potential that limit grapevine transpiration and stomatal conductance. The transpiration of 7 adult vines ('Schiava' grafted on SO4) grown in pots was measured by means of compression load cells, able to continuously detect small weight changes. Total leaf area ranged from 1 to 3 m 2 vine-1. Plants were exposed to 3 cycles of water stress (no water supply for 6 days, followed by water application). Soil evaporation from the pots was prevented. Predawn and midday stem water potential as well as net leaf photosynthesis and stomatal conductance (gs) were measured. The daily leaf transpiration rate per unit of leaf area largely varied across the experiment, increasing after each water supply to reach maximum values above 0.5 kg m-2 day-1 and then decreased to values often below 0.1 kg m-2 day-1 before the next watering. Regardless the canopy size of the different vines, daily transpiration rate of non-stressed plants followed the vapor pressure deficit (VPD) pattern and showed a peak of transpiration rate of about 0.17 kg h-1 m-2 of leaf area. Stressed vines had lower maximal daily transpiration rates (0.10 kg h-1 m-2) and transpiration rates were partially uncoupled from the VPD daily pattern. Our data suggest that the onset of a water stress effect on photosynthetic rate and stomatal conductance occurs in the range-0.60 to-0.80 MPa ᴪSTEM. When soil water potential reaches values lower than-0.26 MPa, then the stomatal conductance is also severely reduced. These indications could improve the management of irrigation supplies, especially when deficit irrigation strategies have to be implemented.