Viticulture and Wine Science - Horticulture: Plants for People and Places - page 216

Table 7.3 Summary of thresholds for various energy status measured using Pre Dawn water
potential (Ψ PD ), midday leaf water potential (Ψ leaf ) and midday xylem water potential (Ψ xylem ).
Modified from Ojeda ( 2007 ), Williams and Araujo ( 2002 ) and Carbonneau ( 2002 )
Stress intensity Ψ PD (MPa) Ψ leaf (MPa) Ψ xylem (MPa)

Mild − 0.4 − 0.8 − 1.1

High − 0.6 − 1.1 − 1.4

Severe − 0.8 − 1.4 − 1.6

water potential is based on a stronger correlation with transpiration in cases of mild
water stress. Such estimates are also less variable than those of Ψ leaf (Lopes 1999 ;
Lopes et al. 1999 ; Silvestre et al. 1999 ; Sipiora and Lissarrague 1999 ; Flexas et al.
1998 ; Chone et al. 2000 ; Chone et al. 2001 ; Naor et al. 1997 ).
By contrast, Carbonneau et al. ( 2004 ) reported that Ψ leaf represented stomatal
behaviour better than Ψ PD at lower values of Ψ leaf . Both authors reported a gradient
in measured xylem water potential along the shoot.
Table 7.3 compares the threshold values of the various expressions of energy
status (Ψ PD , Ψ leaf and Ψ xylem ) as reported in the literature. One must note that the
relationship between Ψ PD and

Vapour Pressure Deficit

Although not a plant based indicator, the measurement of VPD represents an alter-
native to estimating water use directly from stomatal conductance. The relationship
between VPD and stomatal conductance reported in the grapevine literature may be
linear (Williams and Baeza 2007 ) or curvilinear (Lu et al. 2003 ). The range of re-
ported correlative equations suggests that these results are cultivar and site specific
even in non limiting situations. A global relationship to reflect the degree of feed
forward and feedback stomatal control should therefore be normalised for water
stress level using Ψ leaf as an intermediate variable.
Various studies have used VPD as a predictor variable for the computation of
the Crop Water Stress Index in grapevines with various degree of success (Riou and
Lebon 2000 ; Anconelli and Battilani 2000 ).

Canopy Temperature

A side effect of the necessary plant transpiration is the moderation of temperature
in the canopy microclimate. Indeed, the state of the stomata can be derived math-
ematically when the canopy temperature and environmental parameters are known.
Measuring leaf temperature became more convenient with the development of por-
table field radiometers. Reasons for the poor uptake of this technology have not
yet been reported but this methodology suffers from the fact that errors can be
introduced when measuring a surface's temperature including non transpiring tissue