Handbook for Irrigation of Wine Grapes in South Africa

Figure 8.36

A

B

Figure 8.37 FIGURE 8.36. Examples of pressure chambers mounted on quad bikes where (A) the rider remains on the bike and (B) the rider has to dismount.

FIGURE 8.37. Example of a pressure chamber mounted on a trolley for easier transport and water potential measurements.

Pressure readings are recorded as soon as the first signs of sap appear on the cut surface of the petiole. If the sap begins to bubble (Fig. 8.38), it is too late to take a reading since the water potential will be under-estimated. The first sap exudation is hard to spot with the bare eye, and should preferably be observed through a magnifying glass. During the day, the pressure chamber should also be turned towards the sun so that sap appearance is more obvious. In the dark, magnifying glasses with built-in flashlights, or LED headlamps may be used in conjunction

270 CHAPTER 8 – PRACTICAL IRRIGATION SCHEDULING