Abstract
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.
Leaf water potential (LWP) and stem water potential (SWP) are commonly used to estimate the degree of water stress in grapevines (van Zyl 1987, Novello et al. 1992, Williams et al. 1994, Greenspan et al. 1996, Carbonneau et al. 2004, Patakas et al. 2005, Shackel 2007). LWP indicates the water potential of fully sunlit, transpiring leaves that often experience more water stress than shaded leaves in the canopy. SWP is determined on non-transpiring leaves enclosed in darkened, air-tight bags after they equilibrate with the water potential of the corresponding shoot. SWP is believed to better reflect the water status of the whole plant than LWP (McCutchan and Shackel 1992, Choné et al. 2001, Salón et al. 2005). LWP and SWP are typically determined at midday when the net solar radiation is greatest (Choné et al. 2001, Smith and Prichard 2002, Williams and Araujo 2002, Levin 2019). It has been recommend that both LWP and SWP be measured within 30 min of solar noon (Williams and Araujo 2002). This recommendation is based largely on observations from well-watered vines on a sprawl trellis, where vine transpiration was maximal at midday (Williams 2000, Williams et al. 2003). Measuring LWP and SWP at midday is also recommended by others, as the midday values are believed to reflect the daily minimum levels of LWP and SWP, and hence the maximal degree of vine water stress on a given day (Choné et al. 2001, Smith and Prichard 2002, Levin 2019). However, studies in vineyards employing a variety of training systems have shown that the daily minimum level of LWP and SWP occurs after solar noon, between 1400 hr and 1700 hr (local time) (Novello et al. 1992, Williams et al. 1994, Greenspan et al. 1996, Carbonneau et al. 2004, Patakas et al. 2005, Shackel 2007). These findings suggest that midday may not be the appropriate time to measure LWP or SWP, if one wishes to understand the greatest level of water stress experienced by vines on a given day.
Vertical shoot-positioning is the most common training system used in winegrapes. We suspected that north-south (N-S) oriented, vertically shoot-positioned (VSP) canopies might not experience the lowest water potential at midday, because most of the canopy is shaded at this time and whole-vine transpiration shows a midday depression (Petrie et al. 2009, Hunter et al. 2016). In addition, weather conditions and the degree of water stress experienced by vines can affect whole vine water use and alter diurnal patterns of LWP and SWP in vineyards with hanging systems or divided canopies (During and Loveys 1982, van Zyl 1987, Greenspan et al. 1996, Carbonneau et al. 2004, Shackel 2007, Tarara and Perez Peña 2015). The extent that the prevailing weather and the degree of vine water stress affect the diurnal changes of LWP and SWP in N-S oriented, VSP canopies remains unclear. Here, we examined the diurnal responses of LWP and SWP in three Willamette Valley vineyards employing a N-S oriented, VSP system to define when each measure reached the lowest daily value and the time interval during which those values were stable. The goal of this work was to define the appropriate time window to obtain LWP or SWP data using the pressure chamber for N-S oriented, VSP vineyards that reflects the greatest level of water stress in exposed leaves or whole shoots (stems) on a given day. Our purpose was not to test which measure of water potential best reflects instantaneous whole vine water status, but rather to provide viticulturists with practical guidelines of when to measure LWP and SWP in N-S oriented, VSP vineyards.
Materials and Methods
Vineyards
Four Willamette Valley vineyards, including a Chardonnay vineyard on a VSP system (vineyard A), two Pinot noir vineyards on a VSP system (vineyard B and vineyard C), and a Pinot noir vineyard on a Geneva Double Curtain (GDC) system (vineyard D), were used to monitor the daily patterns of LWP and SWP. The characteristics of each vineyard, including scion, rootstock, soil type, and use of irrigation are summarized (Table 1). Vine rows at each vineyard were planted on an N-S axis.
Monitoring diurnal water potential in vineyards
The diurnal changes of LWP and SWP were monitored on eight cloudless days between bloom and harvest in 2017 and 2018. To minimize the influence of changing environmental conditions, measurements of LWP and SWP at each time point were accomplished within a period of 40 min. All times reported herein are Pacific Daylight Time, where solar noon in western Oregon occurs at ~1300 hr. Six to eight pairs of leaves (one of each pair for LWP and one for SWP) were selected from different vines spaced throughout the vineyard at each time point. Vines that were atypical in size and leaves that were damaged were excluded from our measurements. Each pair of leaves was located on the same vine when possible. In those cases where a fully sunlit leaf could not be found on the same vine as the leaf chosen for SWP, the closest fully exposed leaf on an adjacent vine was used. Typically, LWP and SWP were determined every two hours beginning at 0900 hr, although measurements were performed every hour in some cases to improve data resolution. The last time point for each measurement day was between 1800 hr and 1845 hr on most days, but was shifted to an earlier time on two occasions late in the growing season when the lower canopy began to be shaded by the adjacent vine row.
In vineyard A, LWP and SWP were monitored on 9 Aug 2017 (three weeks before veraison), 15 July 2018 (five weeks postbloom), and 18 Sept 2018 (one week before harvest). In Vineyard B, LWP and SWP were measured on 1 Sept 2017 (one week postveraison), 3 July 2018 (two weeks postbloom), and 4 Sept 2018 (one week postveraison). In vineyard C, LWP and SWP were determined on 27 July 2018 (three weeks before veraison) in vines that were irrigated (19 L/vine) the previous day (Wet treatment) and in vines that had not been irrigated for 10 days (Dry treatment). The 10 days preceding this were warm and sunny (average daily high temperature was 31.6°C), so vine water status was anticipated to differ between Wet and Dry treatments. In vineyard D, the diurnal changes of LWP and SWP were determined on 1 Aug 2018 (about two weeks before veraison) for vines trained on a GDC system. In this study, vines were considered to be under no or mild water stress when the daily minimum LWP was above or equal to -1.2 Mpa. Vines were defined as moderately stressed when the daily minimum LWP was between -1.2 and -1.6 Mpa. The average daily minimum LWP was greater than -1.6 MPa on all measurement days; thus, vines were not exposed to severe water stress in this study.
Measurements of LWP and SWP
Mature, healthy, fully-exposed leaves on the external canopy were used to measure LWP with a pressure chamber (model 610, PMS Instrument Company). The leaf blade was enclosed in a clear plastic bag immediately before cutting the petiole with a sharp razor blade. The leaf was secured with the base of the petiole protruding through the chamber lid and the plastic bag was removed immediately before placing the leaf in the chamber and pressurization. The time between leaf excision and chamber pressurization was less than 10 sec. The chamber pressure was initially increased at the rate of ~0.05 Mpa/sec; this rate was adjusted to 0.01 Mpa/sec within 0.3 MPa of expected values. LWP was determined when the first drop of sap appeared on the cut petiole surface observed with a magnifying glass. To determine SWP, a mature, healthy leaf blade was first enclosed in a SWP bag made from a foil-laminate material (stem water potential bag for grape, PMS Instrument Company) and allowed to equilibrate for at least 90 min. SWP was then measured with the pressure chamber using the same procedure described for LWP. Different from our practice, leaves are typically kept in the SWP bag during pressurization in the chamber in many studies (MuCutchan and Shackel 1992, Greenspan et al. 1996, Shackel 2007). However, we found that removing the SWP bag after securing the petiole in the chamber lid, so that the leaf was uncovered during pressurization, did not affect SWP values on four occasions on which we also tested the impact of covering leaves on LWP (data not shown).
Effect of covering (bagging) on LWP
We tested how covering the leaf within a clear plastic bag at different steps during measurement of LWP influences the values obtained. Four data sets were collected from two separate N-S oriented, VSP Pinot noir vineyards on two cloudless days in 2020 (Supplemental Figure 1). Four different treatments related to when leaves are covered with a plastic bag were compared in the morning and afternoon at each vineyard, with five to eight replicates at each time point. All leaves selected within each set where the four treatments were applied were located on adjacent vines, fully exposed, and of similar size, age, and position in the canopy. Treatments were: 1) enclose the leaf in a clear plastic bag immediately before cutting the petiole and keep the leaf covered until the completion of the measurement (B + B); 2) cover the leaf with a clear bag prior to cutting the petiole and remove the bag right before placing the leaf in the chamber (B + N); 3) cut the petiole with no bag covering, but bag the leaf just prior to pressurization in the chamber (N + B); and 4) do not cover the leaf at any time during sampling and measurement (N + N). For the first three methods, air was squeezed out of the bag before sealing it.
Weather conditions
The hourly average air temperature, relative humidity, and actual vapor pressure of the air were obtained from the nearest Agrimet weather station for each vineyard site (US Dept. of the Interior - Bureau of Reclamation, https://www.usbr.gov/pn/agrimet/webarcread.html). The Aurora station is ~37 km from vineyard A and B. The Corvallis station is ~35 km from vineyard C and 19 km from vineyard D. The vapor pressure deficit (VPD) of the air was calculated as the difference between actual vapor pressure and saturated vapor pressure (Murray 1967). The hourly temperature as recorded at each vineyard using a bi-metal laboratory thermometer placed in the shade was within 2℃ of the corresponding hourly temperature obtained from the nearest Agrimet weather station on all measurement days. Days when the maximum air temperature was above 30℃ were defined as warm days and other days were considered cool days in this study.
Data analysis
The LWP and SWP data from each measurement day were analyzed separately using analysis of variance (ANOVA), with time of day as the main factor. Normality and homogeneity of variance were tested prior to ANOVA using the Shapiro-Wilk test and Cochran’s test; data from all measurement days met these assumptions. The effect of time of day on LWP and SWP was considered significant at p < 0.05. Means of LWP or SWP were compared using Fisher’s protected least significant difference (LSD) test at 95% confidence. Replicate was included in the model for the data sets obtained from vineyards A and B in 2017, because those data were collected from an ongoing field experiment that was blocked over a large area. Replicate was not included in the model for all other data sets, since the sampling locations at each time point were confined to 10 adjacent rows with similar vigor. Data from vineyard C were analyzed separately for the Wet and Dry treatments. Additionally, the test of bagging leaves or not at different stages of measuring LWP were compared using ANOVA with bag treatment as the sole factor, after confirming that data met the assumptions of normality and homogeneity of variance. Means of LWP were compared using Fisher’s protected LSD test at 95% confidence, if the ANOVA had p < 0.05. Raw data of LWP and SWP obtained at 1300 hr and 1500 hr from all diurnal data sets from the VSP vineyards examined here were pooled and analyzed using linear regression. This analysis was conducted to compare the relationship of LWP and SWP between midday and 1500 hr (when both measures reached their daily minimum on most measurement days). Prior to the regression analysis, the assumptions of linearity and homoscedasticity was evaluated using the plot of residuals versus fitted values, and the assumption of normality was examined by Kolmogorov-Smirnov and Shapiro-Wilk tests. LWP was included in the model as a response variable and SWP, time of the day, and their interaction were used as predictor variables. The p value of the interaction term was <0.05, indicating that the slopes differed between 1300 hr and 1500 hr. Simple linear regression was then conducted for data obtained at each time point and the model fit was evaluated by the coefficient of determination (R2). Data analysis was performed using R (R version 3.3.3, The R Foundation for Statistical Computing).
Results
In vineyard A, the diurnal changes of LWP and SWP were monitored on two warm days between bloom and veraison (9 Aug 2017 and 15 July 2018) and on one cool day close to harvest (18 Sept 2018) (Figure 1). The temperature and VPD of the air reached the daily maximum between 1700 hr and 1800 hr on all three days (Figure 1A, 1C, and 1E). Vines experienced moderate water stress on 9 Aug 2017 (Figure 1B) and mild water stress on 15 July 2018 (Figure 1D). Despite the difference in the degree of vine water stress, the diurnal patterns of each LWP and SWP were similar on these two days, although they diverged from each other. LWP reached the daily minimum at 1100 hr and stayed at this level until 1700 hr, while SWP reached the daily minimum level at 1500 hr and remained until 1700 hr. On 15 July 2018, SWP at 1100 hr did not differ from the daily minimum value that occurred late in the day, but SWP at midday was greater than the daily minimum. Both LWP and SWP increased between 1700 hr and 1830 hr on 9 Aug 2017 and 15 July 2018. Vines were not stressed on 18 Sept 2018, and both LWP and SWP reached their daily lowest values at 1400 hr and remained at this level until 1600 hr, before recovering by 1700 hr (Figure 1F). Interestingly, SWP increased between 1100 hr and 1200 hr before reaching the daily minimum level at 1400 hr, but LWP did not.
In vineyard B, LWP and SWP were examined on three days, including a warm day just after veraison (1 Sept 2017), a cool day between bloom and veraison (3 July 2018), and a cool day postveraison (4 Sept 2018) (Figure 2). The temperature and VPD of the air reached the daily maximum values between 1700 hr and 1800 hr on all three days (Figure 2A, 2C, and 2E). On 1 Sept 2017, vines were moderately stressed (Figure 2B). LWP reached the daily minimum level by midday (1300 hr) and remained at this level through 1700 hr before increasing at 1800 hr. SWP reached the daily lowest value two hours after LWP and remained stable until 1800 hr on this day. On 3 July 2018, when vines were clearly not water-stressed, both measures of water potential followed a similar diurnal pattern (Figure 2D). However, SWP rose significantly between 1100 hr and midday before declining to the daily lowest value at 1500 hr, but the slight increase of LWP that occurred between 1100 hr and midday was not significant. Stable minimum values for both LWP and SWP occurred from 1500 hr to 1800 hr on this day, with the caveat that values of both measures at 1100 hr did not differ from the daily minimum, but their midday values were greater than the daily minimum. On 4 Sept 2018, vines experienced moderate water stress even though it was a cool day with a low VPD (Figure 2F). The diurnal patterns for LWP and SWP were similar, as both measures showed a gradual decline throughout most of the day, with the only difference being that LWP reached the daily minimum at 1500 hr, one hour before SWP. Both LWP and SWP remained at the daily minimum levels until 1800 hr.
The Wet versus Dry treatment comparison in vineyard C occurred on a warm day, when the temperature and VPD of the air reached their daily maximum values at 1700 hr (Figure 3). Vines in the Wet treatment experienced no water stress on this day (Figure 3B), but vines in the Dry treatment were moderately stressed (Figure 3C). LWP and SWP both declined steadily throughout the day, reaching the daily minimum at 1500 hr in the Wet vines (Figure 3B). Thereafter, LWP increased by 1700 hr while SWP remained at its daily minimum level before recovering later, at 1845 hr. The diurnal pattern of LWP and SWP in the Dry vines was similar to moderately stressed vines in vineyards A and B on warm days. LWP declined to the daily minimum by midday (1300 hr) and remained there until the last measurement of the day at 1845 hr, but SWP did not reach the daily minimum until 1500 hr, where it remained stable until 1845 hr (Figure 3C). Although the last measurement at this site was later in the day than the other data sets in this study, neither measure of water potential recovered to be greater than the daily lowest level by the end of the day in the Dry vines. In contrast, both LWP and SWP recovered to higher values after 1700 hr in the Wet vines.
In the GDC vineyard (vineyard D), LWP and SWP were monitored on a warm day before veraison, where the temperature and VPD of the air peaked at 1700 hr (Figure 4). Even though vines at this site were not stressed on this day, LWP reached the daily minimum level by midday and remained stable until 1700 hr, before recovering at 1800 hr. SWP reached the daily minimum level two hours later (1500 hr) and remained at this level for one hour before recovering to a higher value by 1700 hr (Figure 4B).
The relationship between LWP and SWP at midday (1300 hr) and at 1500 hr were assessed in VSP vineyards using linear regression (Figure 5). LWP and SWP correlated strongly at each time point, but the slopes of the regression differed between 1300 hr and 1500 hr. When LWP was greater than -1.0 MPa, the values of SWP were similar at the two time points. However, when LWP ranged between -1.2 and -1.6 MPa, the value of SWP was 0.05 to 0.15 MPa greater at 1300 hr than at 1500 hr.
Covering the leaf with a clear plastic bag at different stages of measurement had no influence on LWP when vines experienced no water stress on 25 June 2020 (Supplemental Figure 1A and 1B). On 21 July 2020, the mean values of LWP were less negative when leaves were bagged throughout the entire measurement (B + B) than when the bag was not present at the time the petiole was cut (N + B and N + N) (Supplemental Figure 1C and 1D). The difference in mean LWP for B + B versus the N + B and N + N treatments was 0.13 MPa in the morning and 0.07 MPa in the afternoon. However, the B + N treatment, where the leaf was bagged before cutting the petiole but removed before pressurization in the chamber (the method we used to monitor diurnal changes of LWP in this study) never differed from the B + B treatment, where leaves were covered for the entire measurement.
Discussion
Our goal was to understand when LWP and SWP stabilized at their lowest respective levels during the day to define the appropriate time to measure these indicators of vine water status in N-S oriented, VSP canopies. LWP consistently reached its lowest daily level by midday and remained stable until 1700 hr (four hours), when vines experienced some degree of water stress on warm days. Indeed, on two occasions LWP reached the daily minimum by 1100 hr and was stable for six hours (Figure 1B, 1D). Thus, the time interval to measure LWP in VSP vineyards under these circumstances can be extended considerably as compared to previous recommendations that midday LWP be determined within a one-hour period (30 min before and after solar noon) to ensure data reproducibility (Williams and Araujo 2002). Such a short time frame to measure midday LWP limits its application, leading to a suggestion that the window be extended by one hour for irrigation scheduling in commercial vineyards (Smith and Prichard 2002). Our results on warm days when vines experienced some water stress indicate that viticulturists can allocate more time per day to collect LWP data in N-S oriented, VSP vineyards. This will allow viticulturists to increase the number of vineyard blocks they assess on a given day, or alternatively, to improve accuracy by collecting more data per block. However, this longer time window to collect LWP does not apply to cool days. We only obtained one data set when vines experienced moderate water stress on a cool day and LWP was stable at the daily minimum level later in the day from 1500 hr to 1800 hr (Figure 2F). The midday value for LWP on this day was 0.2 MPa greater than the lowest value that occurred at 1600 hr. Thus, LWP taken at midday underestimates the degree of water stress that vines may experience later in the day on cool days. In addition, when vines did not experience significant water stress, LWP also reached its lowest daily level after midday (between 1400 hr and 1500 hr) on both warm and cool days (Figures 1F, 2D, and 3B). Underestimating the level of water stress under these conditions by taking measurements at midday is not a significant concern for viticulturists, since vines did not become stressed later in the afternoon.
Measurements of SWP should not be conducted at midday in N-S oriented, VSP canopies. Under all conditions encountered in this study, midday SWP was always greater than the daily lowest value. The maximum level of vine water stress on a given day was underestimated to a greater extent in stressed vines than in non-stressed vines, based on midday SWP (Figure 5). Overall, the time period in which SWP was stable at its daily lowest level was more consistent across our data sets than was LWP. SWP was stable at the daily minimum level for at least two hours between 1500 hr and 1700 hr in six of the eight data sets from N-S oriented, VSP vineyards. This occurred on all warm days, whether vines experienced water stress or not (Figure 1B, 1D, 2B, and 3B), and on one cool day when vines were not stressed (Figure 2D). The two exceptions when SWP was not at the daily minimum from 1500 hr to 1700 hr occurred on cool days. In one case, late in the season when vines were not stressed, SWP had recovered from the daily lowest level earlier (by 1700 hr), likely because shading from adjacent rows began at this time (Figure 1F). The other exception occurred on the only cool day in this study when vines were moderately stressed. This shifted the time period at which SWP was at its minimum level to even later in the day (1600 to 1800 hr), similar to what we observed for LWP on this day. Thus, measurements of SWP at 1500 hr can also underestimate the level of vine water stress on some cool days. Our observations for SWP generally support previous work in VSP vineyards, in that SWP reached the daily minimum level after solar noon and most often between 1400 hr and 1700 hr (local time) (Novello et al. 1992, Carbonneau et al. 2004, Patakas et al. 2005). However, we also found that SWP can increase between late morning and midday before declining later in the day, which was not reported previously.
SWP increased between late morning (1100 hr) and midday (1300 hr) on two days in this study (Figures 1F and 2D). We attribute this rise in SWP at midday to reduced canopy sunlight interception and whole-vine transpiration that is known to occur at midday in N-S oriented, VSP systems (Poni et al. 2003, Petrie et al. 2009, Hunter et al. 2016). This increased SWP at midday did not occur at other times in our study, suggesting that it may only occur when it is cool and vines are not stressed. Indeed, vines were under greater water stress during prior diurnal studies of SWP in VSP systems than on the two days here when this midday increase was observed (Novello et al. 1992, Carbonneau et al. 2004, Patakas et al. 2005). The decline in SWP after solar noon can be explained by the increasing solar interception of N-S oriented, VSP canopies, the increasing temperature, VPD, and expected drawdown of soil water (Petrie et al. 2009, Hunter et al. 2016).
Our findings show that LWP and SWP did not have similar diurnal patterns in vineyards with N-S oriented, VSP canopies and should not be measured at the same time of day. Under conditions where vines experience some water stress and days are warm, LWP should be measured from 1300 hr to 1700 hr, while SWP should be determined from 1500 hr to 1700 hr. This result differs from prior recommendations that both LWP and SWP should be measured close to solar noon (Choné et al. 2001, Smith and Prichard 2002, Williams and Araujo 2002, Schultz 2003, Levin 2019). One reason for this discrepancy could be due to different training systems used in various studies that alter diurnal patterns of LWP and SWP. However, divergent patterns between LWP and SWP were found here in a vineyard on a GDC trellis (Figure 4B) and also in reports from vineyards employing hanging systems or divided canopies (Novello et al. 1992, Schultz 2003, Carbonneau et al. 2004, Shackel 2007). In addition, many previous studies of diurnal patterns for LWP and SWP were limited to only a few days (During and Loveys 1982, van Zyl 1987, Greenspan et al. 1996, Choné et al. 2001). In this regard, our study highlights the importance of collecting data over many days. It should be noted that this study was conducted in a cool region where vertical shoot-positioning on N-S oriented vine rows is the most widely used training system. Further work is needed to better define the time period when LWP and SWP are stable for warm grapegrowing regions in vineyards using N-S oriented, VSP systems.
In addition to measuring LWP at the appropriate time, using the proper method is critical to ensure the accuracy of the values obtained. Bagging leaves before and during measurement of LWP is generally recommended, since significant water loss may occur and artificially reduce the values obtained for LWP (Turner and Long 1980, Shackel 2007). However, it is unclear how much water loss prior to placing the leaf in the chamber, rather than desiccation of the leaf in the chamber during pressurization with a dry gas, contributes to the lower LWP in uncovered leaves. Our tests indicate that covering the leaf before cutting the petiole protects against water loss and a lower LWP more than covering the leaf in the chamber itself. Only those leaves that were not bagged before cutting the petiole had lower LWP than leaves covered by a bag throughout the entire procedure, whether the bag was added during pressurization in the chamber or not. This occurred only on the second day that the tests were conducted, when it was warmer and less humid, but the impact on LWP observed here was much smaller than reported previously (Turner and Long 1980). More importantly, leaves that were bagged before the petiole was cut, but then uncovered inside the pressure chamber, never differed from leaves that were bagged throughout the whole procedure.
Conclusions
Our findings show that viticulturists can accurately determine LWP of N-S oriented vineyards using a VSP system over a four-hour period beginning at midday (1300 hr to 1700 hr), when the daily maximum temperature is above 30℃ and vines experience some degree of water stress. On cool days, however, the effective measurement time for LWP should be reduced to a two-hour period (1500 hr to 1700 hr). Extending the recommended time frame to measure LWP in N-S oriented, VSP canopies offers practical convenience, allowing viticulturists to increase the number of vineyard blocks that can be assessed on a given day and improve their capacity to manage water resources. Measurements of SWP should not be conducted at midday, since this underestimates the level of water stress experienced by grapevines on N-S oriented, VSP systems. Rather, SWP should be measured later in the afternoon between 1500 hr and 1700 hr in these VSP canopies.
Acknowledgments:
The authors kindly thank Ryan McAdams, Morgan Garay, Alex Cabrera, and Elizabeth Clark for providing access and assistance in the vineyards. The authors also thank Matthew Scott for technical support and gratefully acknowledge support for Tian Tian from the Michael Vail scholarship from the American Society for Enology and Viticulture. Mention of trade names or commercial products in this publication is solely for the purpose of providing specific information and does not imply recommendation or endorsement by the U.S. Department of Agriculture.
Footnotes
↵Supplemental data is freely available with the online version of this article at www.ajevonline.org.
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- Received April 2020.
- Revision received July 2020.
- Accepted September 2020.
- Published online January 2021
- Copyright © 2021 by the American Society for Enology and Viticulture. All rights reserved.