What will happen to our wine? A study reveals how grapevines survive the dry heat of Israeli summers. Will they tolerate significantly worsened conditions due to climate change?
As winter shifted into spring and colder temperatures gave way to warmer ones, the feeling of thirst came back to mind. It is easy for humans to find and drink a glass of water to quench our thirst. For plants, this can be much more complicated, as they depend on resources in their immediate environment. Millions of years of evolution, however, have given plants a few tricks.
A new Israeli study examined grapevines (Vitis vinifera), a particularly popular local crop, and found that the plants can alter their hydraulic traits and internal structure over the course of a growing season to accommodate the hot, dry Israeli summer.
At the center of the new research is the plant’s ability to utilize water.
“Plants are able to carry water to all their parts without any investment of energy, sometimes even to a height of 100 meters,” said Dr. Uri Hochberg of the Institute of Soil, Water and Environment at the Volcani Center for Agricultural Research in Rishon Lezion.
“This is very different from the processes that take place in our body. For example, in order for nutrients and fluids to be transported from the heart to the rest of the body, we are required to burn energy, which is measured in calories.”
A plant, however, transports water, nutrients and minerals upward from the roots to the stem and leaves – moving against gravity – without any investment of energy. The water moves in a fairly long and vertical path in a system of tubes called the xylem.
How exactly does this happen? It all starts when water evaporates from the leaves of the plant through the stomata – the microscopic openings on the leaves through which air from the environment enters the plant. Water molecules within the plant have tension between them that attracts them to each other. So, when some of the molecules evaporate through the stomata, a tension is created in these tubes, pulling the water upward.
“The weak point of the process is that when the plant is in severe drought conditions, the tension in the transport tubes increases to the point where the water molecules may even completely separate from each other,” explained Hochberg. “When this happens, the liquid in the transport tubes actually turns into gas in a process called cavitation, and when there is gas in the heart of the transport system, it damages the flow of water in the plant.”
The study was conducted in Hochberg’s laboratory by doctoral student Yonatan Sorek with the assistance of Dr. Yishai Netzer from Eastern Regional R&D and Ariel University. The researchers examined vines of the well-known Cabernet Sauvignon variety using an innovative method to examine the cavitation processes
They photographed the vine leaves and analyzed the subtle changes occurring due to the amount of light passing through the leaves, which indicated the presence of gas bubbles within the plant’s transport system. As the water turns from a liquid to a gas, a smaller amount of light reaches the camera, revealing the areas where there are gas bubbles, which appear darker.
The researchers found that the “secret” vines use to deal with dryness is deformation. The leaves of the vines change their xylem structure in a way that prevents cavitation. Therefore, the water flow within the vines is not damaged or altered in any way, even in extreme conditions.
THE RESEARCHERS found that the changes in the internal structure of the vine gradually develop as the growing season progresses. This is because of the vines’ life cycle. Every year as winter approaches, the vines sink into a deep dormancy until they regrow vigorously in early spring with the blossoming of new young leaves, and grapes afterward.
In early spring, the young leaves use the water that has accumulated in the soil during the rainy winter months. As a result, they do not suffer from a significant lack of fluids. However, toward the end of the growing season, when the leaves are already mature, the reserves left for the plant are already thinner, forcing the plant to act in ways that will allow it to survive in drought conditions.
The researchers found that in addition to changing the xylem structure, the vines used another mechanism to adapt itself to the dry season: the accumulation of dissolved solids. Within the vines’ leaves are dissolved solids such as salts, sugars, organic acids, amino acids and other minerals, which are present in high concentrations. Because the natural tendency of water is to flow from a place where the concentration of solutes is low to a place where their concentration is high (osmosis), raising the concentration of solutes in the leaf makes it easier for the plant to attract water to it in dry periods.
According to Hochberg, vines are not the only crop in which relatively little cavitation is generated when there is a water shortage.
“In the past year, in addition to vines, we also examined other deciduous plants, such as pear, pomegranate and eucalyptus, and we also saw that they also adapt to dry conditions in this way,” he reported.
Despite the grapevine’s impressive weather-coping strategy, it is unclear whether the plant will be able to properly adapt to harsher drought conditions projected for the future. The climate crisis has already led to a rise of almost one degree Celsius in the world average temperature, with the minimum expectation being a warming of 1.5°-2°C (2.7°-3.6°F) on average. In addition, extreme weather events, such as droughts and heat waves, are expected to continue becoming more frequent and severe.
“In Israel, many heat waves occur in the spring, the period when vine leaves are still relatively young and therefore more sensitive to dry conditions,” explained Hochberg. “Last year, for example, we experienced a 10-day heat wave in May, in which temperatures reached more than 40°C [104°F]. Therefore, if the heat waves become more frequent, there is a reasonable chance that the plant will have more difficulty coping with extreme temperatures,” he warns.
Heat accelerates the ripening of sugar, which grapes turn into alcohol during the fermentation process. Phenols (the substances that give wine its unique properties like color, feel and smell), however, need time to ripen. Therefore, by the time the phenols mature under high heat conditions, the squeezed grape juice might already have too much sugar, resulting in an unbalanced wine with too much alcohol for the palates of most consumers. As Hochberg put it, “Grapevine crops and the farmers who grow them are greatly affected by rising global temperatures.”
In order to better handle the situation, options are currently being considered in Israel, such as replacing the cultivated varieties (mainly French varieties, such as Cabernet Sauvignon, Merlot and Chardonnay) with varieties that better suit the new conditions. It is very possible that it will soon be much more difficult to grow wine grapes in warmer areas where wine grapes are grown today, such as Gedera or Zichron Ya’acov.