Much of California has now come out of extreme drought. However, even if we are in a low water deficit year, there are always sites suffering from drought conditions. How much should we worry when vine experiences water deficit? What are some insights from latest research into vine resistance to droughts?
Sylvain Delzon (INRA) who presented at this year’s Napa Vintage Report conference provided an overview of past research efforts to enhance our understanding of cavitation. Cavitation refers to the presence of air bubbles inside the plant. Severe water deficit can induce cavitation, making vine vessels no longer functional. How much water stress can vines sustain? This is the key question underlying Delzon’s research into the movement of water within a vine’s xylem tissue.
Let’s look at a simple analogy: for humans, embolism is a heart condition when the blood cannot circulate due to the formation of blood clots. For plants, embolism is discussed within the context of plant mortality. Every time we irrigate, it is still unclear whether we’re helping with the process of embolism.
The cavitation controversy – Chapter 1: What is cavitation and how was it studied historically?
In viticulture, both cavitation and embolism are often trated as the same thing. Cavitation is the phenomenon of gas-filled cavities in liquids. Cavitation occurs in the xylem of vascular plants when the tension of water within the xylem becomes so high (ie. when water potential becomes so negative) that dissolved air within water expands and fill the vessels. The blocking of a xylem vessel by an air bubble or cavity is called embolism.
Many people believe plants cavitate daily. However, there is still a lack of consensus in our understanding of plant cavitations. Is cavitation really happening? Are the plants full of air at some point during the day? How do they refill their vessels?
What’s unique about vine vessels is they are very long, about 80 to 100 cm in length. This makes the study of xylem vessels very challenging and sensitive to measurements. Every time one looks at a shoot section, one is only looking at a subsection of the vessel.
The study of cavitation with vine sections has historically been a challenge because of the artifacts created when the stem is physically sectioned.
- Robert Hooke (1635-1703) is one of the first scientists to make observations with a microscope. He came up with the very first view of how xylem vessels work. He found that wood contains pores that carry the ascending sap.
- That was the consensus until Nehemiah Grew (1648 – 1712) claimed that this was not the way plant tissues are used. Grew looked at the stem section of the plant around mid-day and saw that the plant was full of air. This was because when he sectioned the plant, he was actually inducing cavitation.
- In 1914, Henry Dixon (1869-1953) proposed a revolutionary idea called the tension-cohesion theory. In his views, water moves under negative pressure, which was confirmed later by Scholander in the 1960s. Attractive forces between water molecules induce cohesion along the water column. Thanks to the cohesion, water is able to rise. For some time, this became the dominant point of view on how the sap moves within the plant.
- In 1997, Canny published a study that challenged the tension-cohesion theory. To demonstrate his point, he performed a stem section of a living tree in the early morning. Immediately after, he froze the stem section in Nitrogen and observed the section under a microscope. He observed the presence of frozen water and the absence of air in the vascular tissues. At noon, he repeated the same measurements and observed the presence of air in the vessels. Canny concluded from these observations “water columns are weak and are already much broken early in the day and full of air”. He used these result to contradict the assumptions of the water tension-cohesion theory.
Figure 1: Presence of frozen water in the vessels in the morning, presence of air in the vessels at noon. Observation used by Canny to challenge the “tension cohesion theory” (slide from Sylvain Delzon’s presentation, Vintage report Napa, 2016)
- It is only in 2000, that Cochard and his team demonstrated that Canny’s results were in fact a measurement artifact. Authors explained that in fact, the mere action of sectioning the stem can induce the cavitation.
As a take-home, the scientific understanding of cavitation and vessels refilling is still under development with direct and important implications for understanding vineyard sustainability and response under drought.
Today, scientists are using x-ray micro-CT scans and synchrotron technology to provide real-time, non-invasive views of vine vessels and cavitation. In future blog posts, we’ll dive deeper into newest findings of Delzon’s team as well as others. Stay tuned!
Fruition Sciences offers a full suite of products addressing a variety of vine health monitoring needs.
If you want to monitor embolism in your vineyard, our product Sap Flow helps to address that question while saving significant amounts of water. Additionally, since a moderate level of stress has been demonstrated to benefit fruit and wine quality, it is in the best interest of winemakers and vineyard managers to track plant susceptibility to embolism.
Typically, when after a drought episode, irrigation can no longer help vine sap flow reaching its maximal level of water use, chances are that an embolism has occurred.
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