Handbook for Irrigation of Wine Grapes in South Africa

Chapter 2

2.4.2 TRANSPIRATION This is the process whereby the water that is absorbed by roots flows through the grapevine and escapes through the stomata on the ad-axial side (lower side) of leaves into the atmosphere (Fig. 2.14). At the single leaf level, transpiration rate (mmole/m 2 /s) is usually measured to assess stomatal activity. In contrast, stomatal resistance (s/cm) indicates to what extent partial stomatal closure occurs. In fact, stomatal resistance is the reciprocal of transpiration rate, i.e. stomatal resistance decreases as transpiration rate increases. It is also possible to quantify whole plant transpiration by measuring sap flow through the grapevine trunks. Figure 2.14

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FIGURE 2.14. An example of stomata on the ad-axial side of a grapevine leaf. The square on the left indicates the area where the specific stomata occurred (unpublished data).

Stomatal density, i.e. the number of stomata per unit leaf area, differs between cultivars, irrespective of the locality. Furthermore, stomatal density is lower at localities where the air temperatures are higher during the three months following budbreak as discussed in Chapter 1, Section 1.5. This adaption is probably to reduce transpiration under warm, dry conditions. Stomatal density also appears to be a function of the water supply to grapevines. In this regard, it was reported that the stomatal density of Pinotage/99 R subjected to water constraints under greenhouse conditions amounted to 109.5±6.2 pores per mm 2 , compared to 95.7±6.2 per mm 2 of frequently irrigated grapevines (Serra et al. , 2013). Furthermore, guard cell lengths of the stomata were 13.5±0.24 µm and 12.6±0.24 µm for grapevines with, and without, water constraints, respectively. In contrast to the temperature effect, it seems that grapevines experiencing water constraints may have more stomata to enhance a possible cooling effect.

2.4.2.1 FACTORS THAT AFFECT TRANSPIRATION 2.4.2.1.1 Viticultural aspects

Plant spacing: If the inter-row spacing is reduced, more grapevines can be planted per hectare which will increase the leaf area concomitantly. For example, if the plant spacing of grapevines bearing 9 m 2 leaf area is 3 m × 1.5 m, the leaf area will be

IRRIGATION OF WINE GRAPES 41