Effect of lower irrigation frequency and higher water volumes on vine water deficit
It is commonly admitted that irrigation strategy has an impact on root size and distribution in the soil. Recently it was shown that irrigation strategy also affects vine vascular structure and that delaying the first irrigation contribute to increasing vine hydraulic resistance (here). More irrigation early season makes the vine more sensitive to drought during the later phase of the season.
For those reasons, when changing irrigation volume and frequency, it may be important to do it gradually and over multiple seasons. Adopting changes in irrigation strategies progressively promote vine acclimation to moderate water stress and allows the time needed for the vine to modify its root structure and its vascular structure accordingly.
How does vine water stress respond to increased intervals between irrigation?
To address this question, the following case study is discussed.
In California, the goal of a 3 years experiment was to stimulate root density at greater depths. To do so, the irrigation frequency gradually switched from every 3 days to every 7 to 10 days. During this transitional phase, winegrower wanted to ensure the vine would stay healthy and receive enough water. Therefore, sap flow sensors were installed in the vineyard to monitor variations in water stress level from reference vines in real time. Based on the live water stress readings, the level of vine water stress was maintained within moderate values while the volume of each irrigation was increased to compensate for the now larger time interval between irrigations. A high value for water deficit index (WDI) means vine water use is high and vine experiences little or no stress as a result. Inversely, a low value for WDI means vine water use is too low which induces water stress.
Figure 1, shows that irrigation during year 2 was triggered when WDI reaches 40%. With this monitoring technique, each irrigation volume during year 2 went up (from 5mm to 8mm) which resulted in a reduction of the number of irrigation (from 15 to 9 events).
Figure 1 shows that higher values of WDI are observed along the season compared to year 1. Thus applying irrigation less frequently reduced seasonal amount of vine water stress year 2. Figure 1 also shows that the initial drop in water deficit index is observed earlier in year 2 (around May 10) compared to year 1. This means that water deficit was experienced earlier year 2. Results suggest that even if year 2 seasonal context is more stressful for the vine, applying irrigation less often lead to a lower level of vine water stress resulting in higher values for the seasonal WDI profile.
Figure 1: Comparison of water deficit index dynamics in response to decreasing irrigation frequency and increasing irrigation volume between year 1 (5mm per irrigation) and year 2 (8mm per irrigation) – (Adapted from Scholasch, 2018)
Figure 2 shows that the seasonal WDI profile is significantly higher in year 3. Figure 2 shows that irrigation during year 3 was triggered when WDI reaches 40% and then 50%, as a way to further reduce the seasonal amount of water deficit. Each irrigation volume was comprised between 8 to 12 mm (year 3) and the number of irrigation was reduced to 7.
Figure 2 also shows that the initial drop in water deficit index is observed at the same date in year 3 and year 1 (end of May). This means that water deficit was experienced on the same date, but year 3 the vine is better able to regulate and maintain its water status.
Figure 2: Comparison of water deficit index dynamics in response to increasing irrigation volume between year 1 (5mm per irrigation) and year 3 (8 to 12 mm per irrigation) – (Adapted from Scholasch, 2018)
Practical Take home:
We speculate that applying larger irrigation volumes for 2 consecutive years has increased the depth of root absorption sites as well as vine hydraulic resistance leading to water stress acclimation. Those physiological changes may have helped vine water use regulation leading to a less severe water deficit and contributed to more efficient use of water.
Year 2 and year 3, berry volume and sugar accumulation during ripening showed the usual pattern expected in absence of water supply limitation as described in literature. (data not shown).
Improving irrigation strategies while continuously monitoring vineyard water stress is one of the applications of the sap flow sensor. These sensors provide the most accurate way to track directly plant water status variations. Doing so, the sap flow sensor also opens a lot of opportunities to work on berry quality due to the effects of water stress onto fruit composition.
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