|TIAN, TIAN - Oregon State University|
|Schreiner, R Paul|
Submitted to: American Journal of Enology and Viticulture
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 9/14/2020
Publication Date: 1/1/2021
Citation: Tian, T., Schreiner, R.P. 2021. Appropriate time of day to measure leaf and stem water potential in vineyards using vertical shoot positioning. American Journal of Enology and Viticulture. 72(1):64-72. https://doi.org/10.5344/ajev.2020.20020.
Interpretive Summary: Grape growers rely most heavily on two measures that show how much water stress vines are experiencing in order to manage water resources in vineyards. These measures are known as leaf water potential and stem water potential. It has been recommended that both measures should be determined within a one hour period at midday (solar noon), but we suspected that this time period may not be correct for grapevines trained vertically as is common in wine grapes. We measured leaf water potential and stem water potential throughout the day on a numerous occasions in vineyards with vertically trained canopies. Our findings show that both measures of water stress do not follow the same pattern over the day and that each measure was stable over different time intervals. Measures of stem water potential taken at midday also underestimated the level of water stress that vines experienced later in day. New recommendations for the time of day that leaf water potential or stem water potential should be measured in vertically trained vineyards are given.
Technical Abstract: The diurnal changes of vine water status and the appropriate time of the day to measure leaf water potential (LWP) and stem water potential (SWP) were examined in Willamette Valley vineyards 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 fruit set and harvest over two years. On warm days, LWP reached the daily minimum value by midday (1300 hr) and remained at this level for a longer duration when vines experienced moderate water stress (LWP < -1.20 MPa) as compared to minor water stress (LWP > -1.20 MPa). However, on cool days LWP reached the daily minimum later in the day (1400 hr-1500 hr) in both stressed and non-stressed vines. Stem water potential reached the daily minimum level late in the day (1400 hr-1600 hr) in all cases and even increased between late morning and midday on two occasions. Thus, measuring SWP at midday consistently underestimates the level of water stress experienced by vines in VSP canopies. Results of this study show that LWP can be determined over a four-hour period starting at midday on warm days when vines experience moderate levels of water stress; conditions when it is most critical to assess vine water status and schedule irrigation. Stem water potential should be measured in the two-hour period between 1500 hr and 1700 hr under all conditions tested here in VSP canopies.