Dr. Patty Skinkis, Viticulture Extension Specialist & Associate Professor
Excessive plant water stress can cause damage to grapevines, particularly young vines. The recent hot, dry seasons of 2014 and 2015 and the west coast drought drew awareness to water stress and its potential detrimental impacts. However, water stress can occur in any season, and an understanding of how to monitor and manage it is important to the production of healthy vineyards in any region.
This article briefly describes ways to determine whether vines are under water stress. This is the first step used in determining when to begin irrigation in response to stress. For sites that are not irrigated, the methods may be used to determine whether management tactics should be employed to conserve soil moisture or reduce soil water competition. The three most common methods to determine water stress include 1) visual symptoms, 2) leaf or stem water potential and 3) stomatal conductance.
Visual symptoms may be used to determine whether a plant is under stress. It requires training and understanding of the vine’s lifecycle, as symptoms may also be caused by other factors. When visually assessing canopies, it is important to look at shoot tips, tendrils, leaves, clusters, and overall growth. When vines are under water stress, tendrils become limp, shoot tips begin to flop downward, leaves begin to bend (petiole juncture at leaf blade begins to form a shepherds hook) and berries may begin to shrivel. Under prolonged water stress shoot tips stop growing and abscise, tendrils dry up and fall off, and basal leaves may turn yellow and abscise. It is important to consider that tendrils and shoot tips may dry up and fall off as a result of the natural cessation of growth late season, in preparation for dormancy and may not indicate water stress. Visual symptoms are important to document and are strengthened by quantitative measures. Keep in mind that water stress can lead to nutrient deficiencies which may lead to other visual symptoms.
Leaf water potential is the most common measure conducted by commercial vineyards to determine whether vines are under water stress. A pressure chamber is used to measure the amount of pressure required to push water out of the cut end of the petiole and reflects the amount of water potential (or tension) of the water column in the leaf. It is a measure of negative pressure (- bars), although the gauge on most pressure chambers does not indicate a negative number. Typically, irrigation is initiated when leaf water potential readings reach -12 bars, which is considered moderate stress. Stem water potential, also uses a leaf to measure plant water stress, but it requires additional steps in the process, including covering a leaf with a special reflective bag for at least one hour prior to measurement. Covering the leaf limits transpiration, equalizing the water potential in the leaf close to what is experienced in the shoot (or stem). Stem water potential is usually 1-2 bars less negative as shoots are under less tension than leaves. For example, if leaf water potential readings are at -12 bars, the stem water potential on the same plant may be at -10 bars, and the leaf reading would suggest that you need to start irrigating while the stem reading does not. More information about monitoring plant water stress can be found here.
Above: The pressure chamber. The leaf is placed inside a sealed chamber and pressurized gas is added to the chamber slowly. As the pressure increases water will be forced out of the xylem and will be visible at the cut end of the stem. Note the magnifying glass for easier viewing.
Above: Stem water potential, also uses a leaf to measure plant water stress, requires additional steps in the process, including covering a leaf with a special reflective bag for at least one hour prior to measurement. Covering the leaf limits transpiration, equalizing the water potential in the leaf close to what is experienced in the shoot (or stem).
Above: When measuring water or leaf potential, make sure to cut end of the petiole for an accurate measurement.
Above: A leaf attached to a petiole is placed inside a sealed chamber and pressurised gas is added to the chamber slowly.
Some find the pressure chamber to be cumbersome due to its bulky size and have been interested in using a portable leaf porometer. A leaf porometer measures the rate of water that moves out of the stomata, known as stomatal conductance. When a vine is under water stress, stomata close and stomatal conductance of water is reduced. A vine that is not under any water stress can have stomatal conductance of >250 mmol H20/m2/s while a vine under moderate stress has <150 mmol H20/m2/s. There is a porometer on the market that is affordable for commercial vineyard use, and the cost is comparable to a pressure chamber. It may be small, but it requires training, attention to detail in collecting the data, and requires a calibration step before using the meter. This calibration needs to be done each day, before sampling begins, and again hen environmental conditions change (approximately every hour).
Both leaf water potential and stomatal conductance measures require the right equipment and training. Both tools should be used on clear, cloudless days within 1 hour before and after solar noon and not under extreme heat events, to give the most accurate readings. We have tested both pressure chambers and porometers in various trials under western Oregon conditions to monitor plant water stress (Skinkis and Schreiner Labs), and both can be good tools to determine when vines are under stress. If you are interested in those findings, contact Patty Skinkis or Paul Schreiner for more information. Whenever using quantitative measure of plant water stress, it is important to make note of visual symptoms, as they may help interpret conditions of the vines.
NOTE: The reference to porometers and pressure chambers are for local companies that have developed these devices. There may be other companies that provide similar products, and the mention of these two companies does not imply recommendation or endorsement of those products over any other similar product.