Handbook for Irrigation of Wine Grapes in South Africa

Predawn Shade

Sun

Stem

-1 4 -1 2 -1 0 -0 8 -0 6 -0 4 -0 2 0 0

Ȍ (MPa)

FIGURE 6.9. Comparison between predawn and midday leaf water potential in shaded leaves and ones fully exposed to the sun, as well as midday stem water potential in Sauvignon blanc. Data are means for five grapevines (Myburgh, 2010). Vertical bars indicate ± one standard deviation.

6.2.2 CARBON ISOTOPE DISCRIMINATION Carbon isotope discrimination in grape juice sugar at ripeness can serve as indicator of the extent to which grapevines experienced water constraints during ripening (Van Leeuwen et al. , 2009 and references therein). The carbon isotope ratio ( 13 C/ 12 C) is usually stable. However, 12 C is preferentially used by enzymes during photosynthesis. Since 12 C is preferred over 13 C, this process is referred to as “isotope discrimination”. When grapevines experience no water constraints, the carbon isotope discrimination is less severe. Consequently, juice sugar contains more 13 C, compared to juice of grapevines that experience water constraints (Van Leeuwen et al., 2009). Based on the foregoing, the so-called δ 13 C index, i.e. according to the 13 C/ 12 C ratio in juice sugar, can serve as an indicator of cumulative grapevine water constraints during berry ripening (Fig. 6.10). The 13 C/ 12 C ratio is determined by means of mass spectrometry. It is compared to a standard and ranges between -27 ‰, i.e. indicating no grapevine water constraints, and -20 ‰, i.e. indicating severe water constraints (Van Leeuwen et al. , 2009).

152 CHAPTER 6 – GRAPEVINE WATER STATUS