In 2016, Charrier and his colleagues reported results to quantify vulnerability to cavitation as water potential gets more negative. Using a synchrotron, authors let the vines dry and scanned stem sections several days apart as water deficit develops. Focusing on Cabernet Sauvignon plants at different water potentials, they observed that:
- At -10 bars, less than 10% cavitation occurred in stem vessels (figure 1 – picture B).
- At -17 bars, 50% of stem vessels cavitated (figure 1- picture C).
Figure 1: Vine cavitation in stem section at different water potential levels from Charrier et al. 2016 (Source: Sylvain Delzon’s presentation at Vintage Report Napa 2016). Functional (gray) and air-filled (black) xylem vessels are represented in blue and red in the insets. The theoretical loss of hydraulic conductivity for each image is indicated as percentage loss of conductivity (PLC (%)).
Figure 1 results show that vines are more resistant to drought than previously thought. Cavitation doesn’t happen on a daily basis. Moreover, authors compared vine susceptibility to cavitation under drought for various organs. They found out that leaf petiole vessels are cavitating more easily than the stem. Figure 2 shows that:
- At -12 bars, almost 50% of petiole vessels cavitate.
- At -17 bars, almost 100% of petiole vessels cavitate.
Figure 2: Vine cavitation in petiole section at different water potential levels from Charrier et al. 2016. (Source: Sylvain Delzon’s presentation at Vintage Report Napa 2016).
(same color coding as described in figure 1 for inset)
Thus, comparing Figure 1 and Figure 2, we observe that leaf petiole gets easily disconnected or “segmented” from the rest of vine hydraulic system under drought conditions. Such “hydraulic segmentation” – disconnecting petiole from vine stem – questions the validity of measuring leaf water potential with a bagged leaf as it is routinely done in California to decide when to irrigate.
How is cavitation repaired?
To repair cavitation, vessels must be refilled with water. Water refilling can only happen when water pressure gets positive, ie. after a large rain event or after a large water volume has fully refilled the root reservoir. In 2015, a California team lead by Knipfer showed that cavitation repair is always associated with positive root pressure. Charrier and his colleagues confirmed those results.
Conclusions and take-home
It is thanks to their recent works that scientists have demonstrated that some hydraulic techniques are prone to artifacts. These artifacts are particularly relevant in species with long vessels, like vine. Such artifacts have fuelled the cavitation controversy at different periods throughout history:
- In the 18th century: measurement artifact misled Nehemiah Grew into thinking plant vessels were full of air (chapter 1).
- In 1997: measurement artifacts misled Canny into thinking xylem vessels were full of air at noon (chapter 1).
- In 2012: measurement artifact misled Jacobsen and Pratt’s into thinking xylem vessels were full of air under low level of water deficit (chapter 2).
Today, thanks to the use of synchrotron, research has confirmed that the method of sectioning long stem vessels originates the cavitation. Finally after 4 centuries of debate, we can close – for now- the cavitation controversy and be confident that vineyards can sustain greater levels of water deficit!
By profiling hydraulic response to drought for different rootstocks, varietals (Chardonnay, Syrah, Cabernet Sauvignon,….) and organs, research makes possible the evaluation of new thresholds to better manage vine resistance to drought. To improve irrigation practices and water savings, it is essential to incorporate such knowledge.
Thanks to the cavitation controversy, we learn that:
- Winemakers and vineyard managers can impose moderate levels of vine water deficit without threatening vine health or inducing severe cavitation.
- Applying small irrigation volumes prevents optimal vessels refilling after cavitation. Because xylem vessel refilling does not happen under negative pressure, a large irrigation is more effective to bring water potential in the root reservoir to value higher than 0. Bringing water potential to value above 0 is the only way to effectively refill the vessels.
- Vine hydraulic system is able to resist drought and is rarely at risk. Consequently, between two large irrigations, vineyard water deficit management aims essentially at shaping fruit composition and yield.
- Relying on water potential measured with a bagged leaf to assess vine water needs may have limited value, particularly during drought if the petiole gets hydraulically disconnected from the stem.