Handbook for Irrigation of Wine Grapes in South Africa

Chapter 8

Time (h) Figure 8.35

0 5

1

2

4

8

24

0 0

-0 2

-0 4

Ȍ s (MPa)

-0 6

-0 8

-1 0

FIGURE 8.35. Effect of covering leaves for different periods before midday Ψ S was measured in Sauvignon blanc at Nietvoorbij on 2 February 2004 (after Myburgh, 2010). Vertical lines indicate standard deviation.

Petioles are cut using a sharp blade through the thickening against the shoots. Surgical blades obtainable from pharmacies should be used. These blades ensure a clean, smooth cut making the sap exudation easier to see. The blades are razor sharp and must be handled with the utmost care to prevent injury. It is advisable to attach the blades to a scalpel. The cut through the thickening should be done at a right angle across the length of the petiole. It is more difficult to see sap exudation on a slanted cut. Leaves should be cut once only, i.e. do not make a second or third cut to get the surface even or smooth. As soon as a leaf is cut off, the sap withdraws back into the leaf. If the petiole is cut again, less pressure is required to squeeze out the sap. This may cause the water potential to be over-estimated (Ritchie & Hinckley, 1975). Once cut, the leaves must be placed in pressure chambers immediately to limit transpiration losses. Measurements must preferably be done at the grapevine where the leaf has been cut. Leaves should not be taken to a pressure chamber outside the vineyard. Ordinary plastic bags could be used to cover leaves just before cutting them to limit transpiration losses (Williams & Araujo, 2002 and references therein). In such instances, or when measuring Ψ S , the leaf must remain in the bag when it is placed in the pressure chamber. If bags are removed beforehand, the water potential values will be too low (Koide et al. , 1989). It is easy and quick to do measurements in most vineyards using pressure chambers mounted on motor cycles or quad bikes (Fig. 8.36). Researchers of the Soil and Water Science Division at Nietvoorbij have also designed a trolley for easy transportation of pressure chambers between measuring stations in field trials (Fig. 8.37). It is also more convenient to stand instead of crouching when taking the measurements.

IRRIGATION OF WINE GRAPES 269