Handbook for Irrigation of Wine Grapes in South Africa

Figure 6.14

Shiraz/99 R

Merlot/99 R Sauvignon blanc/99 R

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

Ȍ S (MPa)

30% RAW depletion 75% RAW depletion

FIGURE 6.14. Effect of RAW depletion on midday Ψ S in three grapevine cultivars during berry ripening in a sandy soil near Lutzville in the Lower Olifants River region (after Myburgh, 2011h). Figure 6.15

Stomata per mm 2

100

110

120

130

140

150

-1 0

y = 0.0104x - 2.7312 (R 2 = 0.9494) y = 0.0115x - 3.1219 (R 2 = 0.9973)

30% Depletion: 75% Depletion:

-1 2

-1 4

Ȍ S (MPa)

-1 6

-1 8

FIGURE 6.15. Relationships between midday Ψ S in Shiraz (  ), Merlot (  ) and Sauvignon blanc (  ) and stomatal density at two levels of plant available water depletion in a sandy soil near Lutzville (unpublished data). -2 0 6.3.2 ATMOSPHERIC CONDITIONS Since stomata respond to light, grapevine transpiration is closely related to net solar radiation. Consequently, radiation plays an important role in the grapevine’s water status, e.g. the diurnal water potential cycle. Any other interruption in

158 CHAPTER 6 – GRAPEVINE WATER STATUS