Viticulture and Wine Science - Horticulture: Plants for People and Places - page 215

Plant Based Indicators of Water Stress
There is an array of indicators available to commercial grape growers wishing to
understand the water status of their vines for irrigation scheduling purposes. In most
cases, these indicators are used in complement of soil based methods. Where soil
based methods are excellent for coarse irrigation management, the nature of RDI
and PRD clearly requires a fine tuning of the understanding of plant water status
and response to irrigation.
Leaf Water Potential
Leaf water potential Ψ leaf is by far the most reported plant based water stress indi-
cator reported in the literature and is usually measured using a pressure chamber
(Waring and Cleary 1967 ; Scholander et al. 1965 ), although automated continuous
data acquisition is also possible using thermocouple psychrometers (Loveys 2005 ).
A considerable amount of literature reports studies using the leaf as the preferred
hydraulic component for determining plant water status. In grapevine studies, most
of the work has used Ψ leaf to characterise the degree of water stress and/or identify a
threshold value for the onset of irrigation. The diurnal course of leaf water potential
however typically displays a large variability (Loveys 2005 ) in particular, due to
the rapid response by leaf water potential to environmental changes in low stress
situations.
Pre-Dawn Leaf Water Potential
This measure is now commonly accepted as an indicator of water stress (Carbonneau
2004 a , b ; Yuste et al. 2004 , 1999 ; Quereix et al. 2001 ; Medrano et al. 2002 ). Dur-
ing the night, hydraulic gradients at the soil/root interface decrease and stabilise at
a value related to the soil water content. The vine's hydraulic system equilibrates
and pre dawn leaf water potential can be used as a surrogate indicator of rootzone
water status (Lebon et al. 2003 ; Riou and Lebon 2000 ) although not all authors are
in agreement. Furthermore, the relationship between Ψ PD and the soil water fraction
is not linear, and its strength varies as a function of Ψ PD (being weaker at values
closer to 0).
Xylem (Stem) Water Potential
Estimates of water potential in shoot xylem Ψ xylem are achieved from leaf petioles
after the leaves have been covered and bagged for at least one hour. This allows the
xylem water potential of the leaf petiole to equalise with that of the shoot (Chone
2001 ). The use of xylem water potential (also termed stem water potential) over leaf