Handbook for Irrigation of Wine Grapes in South Africa

Chapter 2

Grapevine training system: Due to the sprawling vegetative growth of untrained grapevines (bush vines), more leaves are exposed to the sun over the course of the day (Fig. 2.17A). Therefore, more water will transpire, compared to trellised grapevines with comparable leaf area, since leaves on the periphery of the canopies of trellised grapevines are partially shaded for a certain part of the day (Fig. 2.17B). Obviously, the extent of the shading, and the subsequent transpiration reduction, will depend on the row direction.

A

B

FIGURE 2.17. Over the course of the day, outer leaf layers of un-trellised grapevines (A) are more exposed to solar radiation, compared to those on vertical trellises (B).

Canopy orientation: As mentioned above, leaves on the periphery of grapevine canopies on a vertical trellis are partially in the shade for a certain period of the day (Fig. 2.18). Consequently, less leaves will be exposed to radiation, compared to grapevines on horizontal trellises. When canopies of equal total leaf area on vertical and horizontal trellises are compared, the vertical canopies would lose less water through transpiration (Fig. 2.19). Grapevines on the horizontal trellis will extract more water from the soil under the same conditions. The amount of water lost during a day from small vertical trained grapevines is ca. 1 to 2  per day and between 3 to 4  per day on larger VSP systems. On large horizontal training systems, transpiration may be up to 8  per grapevine per day. For a vineyard planted at 2.75 m × 1.2 m, and with a transpiration rate of 6  per day, the extraction of water from soil by transpiration can therefore be calculated as 6  ÷ 1.2 m ÷ 2.75 m = 1.8 mm water per day. The extent of the shading, and the subsequent transpiration reduction, will depend on the row direction.

IRRIGATION OF WINE GRAPES 43