Handbook for Irrigation of Wine Grapes in South Africa

20 000 m 2 per hectare. If the row width is reduced to 2.5 m, the leaf area will be 24 000 m 2 per hectare, i.e. 20% more. If the spacing in the row is reduced, e.g. from 1.5 m to 1.2 m, the number of grapevines per hectare will also increase. However, the leaf area per grapevine usually decreases as the cordon length decreases. Therefore, the leaf area per unit row length will tend to remain the same. This implies that row width will have a more pronounced effect on leaf area per hectare than spacing in the grapevine rows. Leaf area per grapevine: The volume of water that is lost due to transpiration at a certain stage is largely determined by the amount of leaves per grapevine (Fig. 2.15). Over-irrigation and/or -fertilization, particularly excessive nitrogen, causes vigorous grapevine growth which increases leaf area. In such cases, vigorous growth will definitely increase transpiration due to higher leaf area (Fig. 2.16A). Leaves on sucker shoots will also cause unnecessary transpiration losses, and should therefore be removed as soon as possible (Fig. 2.16B).

500

Leaf area = 3.2 m 2 /grapevine Leaf area = 9.6 m 2 /grapevine Leaf area = 13.7 m 2 /grapevine

400

Overcast

300

Sap flow rate (m  /grapevine/h) 01 Dec 200 100 0

02 Dec

03 Dec

04 Dec

05 Dec

Date

FIGURE 2.15. Transpiration based on sap flow rates through three grapevines having different leaf areas (Myburgh, 2016).

A

B

FIGURE 2.16. Examples where (A) too vigorous vegetative growth and (B) sucker shoots may cause unnecessary transpiration losses.

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