Handbook for Irrigation of Wine Grapes in South Africa

A

B

Water uptake

Water uptake

Figure 6.4 FIGURE 6.3. Schematic illustration where the vertical arrows indicate water potential gradients (A) during daytime and (B) in the night.

0.30

0.25

DG

0.20

DC

0.15

Trunk diameter (mm)

0.10

00:00

12:00 Day 1

00:00

12:00 Day 2

00:00

12:00 Day 3

FIGURE 6.4. Diurnal variation in grapevine trunk diameter where DC and DG indicate daytime contraction for Day 1 and daily growth for Day 2, respectively (adapted from Myburgh, 1996).

It must be noted that water potential actually reflects the suction by which water is held by the plant cells. Therefore, it has a negative numeric value. Under normal atmospheric conditions, the highest water potential, i.e. the least negative value, occurs during the predawn period around 04:00 irrespective of soil water content (Fig. 6.5). If the soil is wet, and high humidity and windless conditions prevail in the

148 CHAPTER 6 – GRAPEVINE WATER STATUS