Handbook for Irrigation of Wine Grapes in South Africa

Figure 2.18 Figure 2.18

A

B

FIGURE 2.18. Over the course of the day, outer leaf layers of (A) grapevines with horizontal canopies are more exposed to solar radiation, compared to (B) ones with vertical canopies.

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Horizontal canopies: y = 0.328x + 0.195 ( n = 3 7 ; R 2 = 0 .9 0 1 0 ; s. e. = 0 .6 3 ; p < 0 .0 0 1 ) Vertical canopies: y = 0.185x + 0.016 ( n = 3 8 ; R 2 = 0 .8 7 3 0 ; s. e. = 0 .3 5 ; p < 0 .0 0 1 )

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5

4

3

2

Sap flow (Ɛ grapevine d)

1

0

0 2 4 6 8 10 12 14 16

Crop load per grapevine: The amount of fruit on a grapevine will not make a significant difference to the water use of the grapevine. Research has shown that the transpiration tempo did not change after the removal of the grapes shortly before harvest (Myburgh, 1996). Furthermore, it was calculated that the volume of water accumulated in the bunches of a 40 t/ha crop at harvest amounts to ca. 6 mm of water. Since the latter only amounts to fractions of a millimetre per day, the water required by the bunches is therefore practically insignificant, compared to the volume of water lost by daily transpiration. Since transpiration by grapes is insignificantly small, the berries per se are unlikely to cause excessive transpiration Leaf area (m 2 /grapevine) FIGURE 2.19. The effect of canopy orientation and leaf area per grapevine on the daily transpiration of grapevines (Myburgh, 2016).

44 CHAPTER 2 – THE DYNAMICS OF WATER IN AND AROUND VINEYARDS