Appropriate Time to Measure Leaf and Stem Water Potential in North-South Oriented, Vertically Shoot-Positioned Vineyards

The diurnal changes in vine water status and the appropriate time of day to measure leaf water potential (LWP) and stem water potential (SWP) were examined in Willamette Valley vineyards with north-south (N-S) oriented rows employing a single curtain, vertically shoot-positioned (VSP) canopy. Measurements of LWP and SWP were performed on Chardonnay and Pinot noir grapevines on seven cloudless days between bloom and harvest over two years. On warm days, LWP reached the daily minimum value by midday (1300 hr) and remained there for a longer duration when vines experienced moderate water stress (LWP < -1.20 MPa) than minor water stress (LWP > -1.20 MPa). However, on cool days, LWP reached the daily minimum later in the day (1400 hr to 1500 hr) in both stressed and unstressed vines. SWP reached the daily minimum level late in the day (1400 hr to 1600 hr) under all conditions and even increased between late morning and midday on two occasions. Thus, measuring SWP at midday consistently underestimates the greatest level of water stress experienced by vines in N-S oriented, VSP canopies. Results of this study show that LWP can be determined over a four-hour period starting at midday on warm sunny days when vines experience a moderate level of water stress: conditions when it is most critical to assess vine water status to schedule irrigation. SWP should be measured in the two-hour period between 1500 hr and 1700 hr under all conditions tested here in N-S oriented, VSP canopies.