Handbook for Irrigation of Wine Grapes in South Africa

Chapter 8

8.4.2 PLANT BASED MEASUREMENTS 8.4.2.1 GRAPEVINE WATER POTENTIAL

Measuring grapevine water potential does not tell a grower how much to irrigate, but is a useful aid for making decisions concerning the timing of irrigations to achieve a specific wine quality objective. Furthermore, measuring Ψ could be used in combination with soil based measurements to set refill lines for general irrigation scheduling. The correct protocol should be followed to obtain reliable and repeatable results. These include safety aspects and the correct operation of pressure chambers used to measure water potential. Furthermore, correct leaf selection, cutting and handling of leaves as well as observation of sap exudation, need careful consideration. It is quick and easy to measure Ψ using pressure chambers (Scholander et al. , 1965). More information about commercial pressure chambers is available on the internet. It is cheaper, however, to have pressure chambers made up locally. The pressure is usually obtained from cylinders filled with inert nitrogen gas, but compressed air can also be used. Although it is fairly simple to operate a pressure chamber, it is a dangerous apparatus since it contains gas at a high pressure. Before using pressure chambers, ensure that (i) there are no leakages; (ii) the lid fits properly and tightly; (iii) the pressure regulator and valves work properly and smoothly and (iv) the pressure gauges do not get stuck when the pressure increases. It is recommended to do trial runs before measurements. Since pressure gauge readings are sometimes very low before dawn, the pressure should increase by approximately 25 kPa per second to ensure that the water potential is not under- estimated. When Ψ is measured during the day, and readings are considerably higher, the pressure may increase at 50 kPa per second to save time. Care should be taken throughout that the pressure increase tempo remains constant while measurements are being taken. Predawn Ψ measurements should start between 03:00 and 04:00 depending on what time the sun rises and how many measurements need to be taken. Midday Ψ L and Ψ S must be measured between 12:00 and 14:00. All Ψ must be measured in undamaged, mature leaves on primary shoots in the bunch zone. The Ψ L is measured in leaves that are fully exposed to the sun. When the sun is at its highest, relatively few leaves on vertical trellises are exposed to the sun. In such cases, measurements may be done at 14:00 when the sides of the trellises are more exposed to solar radiation. When measuring Ψ S , leaves should be covered using aluminium foil bags to reflect solar radiation (Fig. 8.34). The inside of these homemade bags must be black to keep light out. The size of the bags will depend on the inside dimensions of the pressure chamber. Leaves must be rolled up and if necessary the tips folded over before being covered with the bags. Subsequently, the open side of the bag is folded over and secured with a hairpin or paper clip (Fig. 8.34).

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