Before and after the heatwave, how to use 360Viti?
California experienced an intense and early heatwave the week of June 10th. The heatwave peak was reached on June 11th in Napa. During heatwave events, competition for water occurs between the soil and the atmosphere. Vine water supply can be low due to low soil water content and vine water demand can be high due to strong air vaporization, resulting in the risk of cavitation.
The following are recommendations and cues to understand how to closely monitor vineyard performances and take “informed” action.
Here are the climate data (anonymized) that were collected on the 360viti app during the June heatwave:
The VPD corresponds to the air Vapor Pressure Deficit and it represents the “oven-like effect” of the atmosphere. The combined effects of high temperature and low relative humidity are integrated into the VPD. VPD values greater than 3.5 kPa are considered high. In our example the critical threshold has been largely exceeded for all the regions considered.
What are the vine’s physiological responses to early-season heatwaves?
The risk of burns and necrosis of vine tissue
Recent studies* show that vine cells, especially berry cells, are more sensitive to high temperatures BEFORE veraison than after veraison. When a heatwave occurs during the early stage of the vine seasonal cycle, tissue necrosis resulting from high heat exposure can appear more easily on some organs, such as berries, which are more susceptible to extreme heat.
The phenomenon of vine cavitation
Water flows through the vine from the soil to the atmosphere. The water absorbed by the roots is retained by the soil particles, more or less strongly, depending on soil water deficit. The water transpired by the leaves is vaporized in the air, more or less quickly, depending on the air vapor pressure deficit. Thus, as it goes up, the liquid water column inside the plant stretches from the roots to the leaves. It is put under tension because it experiences two opposite forces, like in a rope pulling game.
To illustrate the phenomenon, here is some VPD data collected in the vineyard: the peak is well defined signifying a day at risk for cavitation since the demand from the atmosphere is high.
What does the analysis of transpiration by Sap Flow Sensors during the heatwave tell us?
The measurement of transpiration by sap flow is the direct measurement of vine water consumption. During heatwave events, sap flow measurements can indicate whether cavitation risk is avoided or minimized thanks to an increased transpiration rate.
During the heatwave:
- If we observe an increase in transpiration: this is good news. It means that the plant has compensated the rise in climatic demand by “”spending”” more water: the water column works and circulates through the plant correctly.
- If we observe a decline or the maintaining of transpiration, -particularly early season-: the phenomenon of cavitation is likely to occur. The transpiration patterns from these plants should be closely monitored over the next few days and for the rest of the season. Indeed, at the end of the season, ripening disorders may be observed if vine hydraulic integrity has been degraded. This can be the case when cavitation has occurred early season.
To prevent the water column from breaking (see diagram above), the plant can speed up its water flow to the atmosphere, i.e. its transpiration. This implies that the amount of water circulating in the plant can increase when higher light or higher VPD levels contribute to drawing more water into the atmosphere. In situations where increased transpiration is observed, plants have better preserved their hydraulic integrity. Thus, differences in plant responses observed under these conditions, make it possible to assess their ability to resist a heatwave.
Vine transpiration increase is measured in a vineyard experiencing a heat wave (June 24 to July 3rd)
On the graph above, from 360viti, we can see that the vine increased transpiration DURING the heatwave. This is a good transpiration response: the vine compensates for the increase in climatic demand with more transpiration. The quantities of water transpired have increased compared during the heat wave: from an average of 1.8 to 2 mm/d to 2.6 to 2.8 mm/d. Transpiration decreases after heat wave (July 3rd), which is normal because the climatic demand has become “normal” again. However, transpiration should not decrease too much, otherwise, it means that the vine has closed its stomata too much and is experiencing water stress.
What are the consequences of cavitation and what indexes should be monitored in this case?
After cavitation, the “Leaf water Potential” loses its relevance as a support to irrigation management. When cavitation occurs, it causes a disconnect in the water flow between the leaf and the rest of the plant.
The leaf acts as a “hydraulic” fuse to keep the rest of the vine under good hydraulic functioning. However, in extreme case, the water disconnect between the leaf and the rest of the plant can lead to leaf burn symptoms.
The consequence of leaf hydraulic disconnect resulting from cavitation is that leaf water potential can no longer be the reference measurement to assess vine water need for irrigation > See our blog post dedicated to leaf water potential here.
VPD and WSI are reliable indexes to support irrigation decisions
On your 360viti dashboard, you can analyze the plant’s reactions during a heatwave event using transpiration data from sap flow sensors.
To characterize vine water stress profile, we express actual vine transpiration as a percentage of satisfaction of maximum transpiration. It expresses how actual vine transpiration is reduced compared to what it could potentially be considering climatic conditions.
- When root reservoir water is not limited, each vine can reach maximum transpiration and WSI = 100%.
- As the root reservoir water declines, vine can no longer reach maximum transpiration and WSI declines proportionally.
Before a heatwave, you can closely monitor the Water Satisfaction Index (WSI). According to WSI variations, you may support decisions such as irrigation (if WSI is low or declining steeply) or the reduction of a fraction of the transpiring leaf area (e. g. hedging or lateral removal).
In the following days after a heatwave, we must be very attentive to the WSI. In fact, heatwaves can speed up the depletion of water available in the root reservoir and increase the severity of soil water deficit. Thus, maintaining the level of vine water satisfaction can rapidly become more difficult immediately after the heatwave is over. In practice, it is necessary to be very attentive to WSI’s decline. The slope of the decline will indicate the rate at which water stress develops and how it is perceived by the plant.
Practically speaking: what to do BEFORE the next heatwave?
In view of the expected benefits when the plant is able to maintain its hydraulic integrity, it is advisable to enter the heatwave with a high WSI. To maintain a high WSI in anticipation of a heatwave, several actions are possible.
Reducing water losses during a heatwave can be achieved through several means such as cover crop removal, tillage, transpiring leaf area reduction…. It is also possible to reduce water demand from the atmosphere. VPD can be reduced by increasing air humidity via different methods (misters, etc.).
These solutions should be discussed with your technical team and advisors along with other crop protection programs. In particular sulfur sprays before a heatwave should be avoided as they increase the risk of leaf burn.
Our team will be happy to help you set up the monitoring and control systems for your vineyard. To learn more, please call us : +1 (888)295-5187
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👁🗨 To know more about this subject, please explore the video: Heat waves and their effects on the vine.
Grapegrower & Winemaker Magazine talking about sap flows Notre blog and our website
Thibaut holds a Ph.D. in viticulture from the French National Institute of Agronomy at Montpellier, France. His research focused on vine water status variations under dry climates and their consequences on berry ripening. Thibaut also serves as a scientific consultant for various high end vineyards in Napa Valley. Prior to his Ph.D., Thibaut worked as a winemaker for various companies throughout the world (Chile, California, France and Australia). In 2001, he was hired by Robert Mondavi winery as a research viticulturist: his projects focused predominantly on mapping the vineyard variability, analyzing vineyard practices and vine water deficit impact on fruit composition. Thibaut earned a Masters degree in Viticulture and Enology in 1997 and a Masters degree in Winemaking in 1998 from SUPAGRO, one of the top agronomy school in France.
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