Handbook for Irrigation of Wine Grapes in South Africa

Chapter 8

with ordinary magnifying glasses. The unit which is used internationally for water potential, is mega Pascal (MPa). If the reading on the pressure gauge, usually in kilo Pascal (kPa), is e.g. 1 450 kPa, the water potential is reported as -1.45 MPa.

FIGURE 8.38. Sap bubbling through the petiole of a leaf in a pressure chamber. It is important that readings must be taken when the first sign of sap appears. When bubbles appear as in this illustration, it is too late to take the reading. Experience has shown that water potential varies little among grapevines in the same trial site or in a uniform vineyard. Good correlation was also found between the replications of a specific irrigation strategy in field trials. If water potential has to be measured in a large number of blocks, one leaf per irrigation block will suffice. By so doing, measurements will be completed more rapidly before the water potential changes too much, i.e. due to its diurnal behaviour. Remember that midday measurements can only be made between 12:00 and 14:00. 8.4.2.2 DIURNAL TRUNK SHRINKAGE AND EXPANSION The concept of diurnal shrinkage of grapevine organs is discussed in Chapter 6. Since the daily shrinkage increases as the soil dries out, trunk diameter can be measured to schedule irrigation (Myburgh, 1996). For example, if a vineyard needs to be irrigated at 75% PAW depletion, water must be applied when the daily trunk contraction is ca. 0.09 mm (Fig. 8.39). The diurnal shrinkage and expansion are only hundredths of a millimeter, but is measurable by using sensitive equipment. One method is by using a linear variable differential transformer. The latter basically consists of a cylindrical coil with a freely movable steel pin in its core (Fig. 8.40A). The LVDT is mounted on a base plate which is clamped to the grapevine trunk by means of a back plate and four invar shafts (Fig. 8.40B). Invar is nickel-iron alloy with an almost zero thermal expansion coefficient. This is to minimize the effect of diurnal temperature fluctuations. The device is held in place by means of a steel

IRRIGATION OF WINE GRAPES 271