Heatwave and vine monitoring: the Foliar Water Potential under discussion.

vineyard technology

Content updated January 2021

The Foliar Water Potential under discussion. A heatwave affected California during the week of June 10. As global warming is expected to increase the frequency of heatwave events, it seems important to us to take a closer look at vine response to heatwaves and the influence heatwaves have in terms of selecting decision-making tools for irrigation.

Since its inception,  Fruition Sciences has been closely studying the effect of heatwaves on vine transpiration. Our research in real vineyard conditions and in the laboratory, as well as our founding experience in California, led us to believe that Napa’s climatic conditions would eventually become transposable to France, in the context of climate change.

Based on this experience, along with 10 years of observations and data collection, we can now share our conclusions and raise awareness among professionals. One of our major observations is that leaf water potential measured on petioles during a heatwave reaches some limitation. Thus, it is advisable to be cautious when using it.

Explanations in an interview with our co-founder Thibaut Scholasch,  expert in vine physiology in arid environments.

A brief look back at the situation:

  • An early and intense 3-day heatwave.
  • The heatwave peak was reached on June 11th in Napa Valley, California.

Here are some key facts:

The Vapor Pressure Deficit (VPD) measured in the air represents how “oven-like” are current atmospheric conditions. The combined effects of high temperature and low relative humidity are integrated into the VPD. When VPD is higher, more water is lost by the leaves.

How does the vine respond to heatwaves?

Vine tissue’s resistance to extreme heat varies. Recent studies* show that vine cells, especially those in the berry, are more sensitive to high temperatures BEFORE veraison, than after. Heat burn, causing tissue necrosis, may appear more easily on some organs, such as berries. Thus, tissue degradation in response to early-season heatwaves are not only related to heatwave intensity but also to its precocity with respect to vine seasonal cycle. At the pre-veraison stage, some vine tissues are less adapted to undergo extreme temperatures and are therefore more sensitive.

Plant response: beware of 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 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. Our sap flow sensors precisely measure what results from these processes.

During a heatwave event, because it is more intensely pulled towards opposite directions, the water column inside the plant can reach a breaking point. When the water column breaks, cavitation occurs. In other words, during a heatwave, VPD can become so strong that the “rope” can break. Inside the plant, the water column gets filled with air bubbles resulting in cavitation.

What are the consequences of cavitation?

Cavitation is a non-reversible phenomenon. Irrigation will not refill leaf and petiole tissues with water once they have become air-filled.

Good news: Cavitation can be reversed under positive water pressure (ie. next season before pruning when the vine bleeds).

Bad news: It cannot be fixed. During the whole season, xylem vessels will remain partially full of air.  Leaf cooling effect decreases.

The main consequence of cavitation is a drop in hydraulic conductivity, which slows down or may even stop the process of water transpiration at the leaf level. As transpiration decreases, it no longer cools the tissues, which can cause the leaves to burn more easily. This extreme phenomenon can be spectacular as it has been reported recently in Southern France.

The soil water deficit can exacerbate vine heatwave symptoms (if water is retained more strongly by soil particles, rope pulling tension increases even more). Symptoms can also be exacerbated if a sulfur spray is applied a few days before the heatwave.

Why is leaf water potential unreliable to assess vine water status under these conditions?

During a heatwave, the leaf can act as a fuse and “blow”. As leaf petioles get air-filled, the water stream no longer flows. As a result, the leaf no longer communicates with the rest of the plant! Under these conditions, the leaf water potential is no longer connected with the overall vine water status. Therefore, during high heatwaves (VPD above 3.5 kPa), commonly accepted guidelines to assess the need for irrigation can no longer provide reliable information.

Why does leaf water potential become inadequate to assess vine water status under heatwave or post-heatwave conditions?

As cavitation is irreversible, the leaf will remain partially disconnected from the water status of the plant until it falls at the end of the season (Source).  The loss of “hydraulic conductivity” isolates the petiole from the rest of the plant. This results in leaf water status reflecting only partially the vine water status for the rest of the season.

So how do you estimate the plant’s water status after a heatwave?

Today, the only way to establish a RELIABLE estimate for vine water use and water stress is exclusively via the measurement of the water amount leaving the plant… and that is exactly what the sap flow sensor measures.

In fact, estimation of vine water use can be biased, unless direct measurements are performed. If you do not know what the petiole hydraulic conductivity loss is, it is not possible to reliably assess vine water status from the measurement of water potential measured at the petiole level.

In practice:

Here is an article to show how to keep an eye on important indexes after a heatwave and in anticipation of any new heatwave episode 👉🏼 READ THE ARTICLE

To know everything (or almost everything!) about Cavitation,  watch our webinar about heat waves. 

Heat waves and their effects on vines and growing grapes.

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Cavitation épisode 1: What is cavitation and how was it studied historically?
Cavitation épisode 2: Are vines cavitating all the time?
Cavitation épisode 3: Are we missing something?
Cavitation épisode 4 : Vines rarely cavitate!
Cavitation épisode 5: When do vines really cavitate?

Do not hesitate to contact us. Our team is ready to help you reviewing vineyard indexes and better assess your vineyard needs.

What makes Sap Flow sensors unique?
Start saving water

*Thesis of Julie Gouot, 2019

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