Stomatal closure is the dominant factor limiting gas exchange during water deficit (e.g. reviews by Lösch & Schulze 1994; Davies & Gowing 1999). Stomata regulate transpiration so that sufficient carbon is gained while leaf water potential (Ψleaf) is prevented from becoming too negative and the break-down of the plants hydraulic system is avoided (Tyree & Sperry 1988; Jones & Sutherland 1991; Schultz & Matthews 1997). A decrease in stomatal conductance can correlate with a declining Ψleaf during soil drying, but can also occur before any measurable change in Ψleaf is recorded (Gollan, Turner & Schulze 1985; Trejo & Davies 1991). Differences in stomatal sensitivity during drought among cultivars or between species may serve to limit transpiration to compensate for differences in the vulnerability of xylem to cavitation (Jones & Sutherland 1991). Cultivated grapevine (Vitis vinifera L.) is a very heterogeneous species with an estimated 10–20 000 cultivars (Ambrosi et al. 1994) grown from the cool temperate 50° North latitude, through the dry mediterranean-type climates to the tropics. The diversity of this species with respect to its tolerance to drought seems large, yet it has been generally classified as ‘drought avoiding’ (Smart & Coombe 1983) or as ‘pessimistic’ following the ecological classification of Jones (1980) into ‘pessimists’ and ‘optimists’. The principal difference in strategies would be that ‘pessimists’ would modify their growth and physiology to conserve current resources and to control their demand for future resources whereas the ‘optimists’ use all the resources available to them in expectation of more arriving. This ecological classification is analogous to the physiological classification into isohydric and anisohydric plants (Stocker 1956; Tardieu & Simonneau 1998) and fundamentally linked to stomatal behaviour.
Barley and sunflower for instance belong to the anisohydric class of plants (Tardieu, Lafarge & Simonneau 1996; Tardieu & Simonneau 1998) that have a Ψleaf which markedly decreases with increasing evaporative demand during the day and is lower in droughted than in watered plants. In contrast, other species such as cowpea (Bates & Hall 1981), maize, poplar (Tardieu & Simonneau 1998) or sugarcane (Saliendra & Meinzer 1989) maintain a near constant Ψleaf during the day at a value which does not depend on soil water status. The classification into isohydric and anisohydric plants so far only applies to different species. However, some Vitis vinifera L. cultivars of contrasting genetic origin show very different responses of Ψleaf during water stress, which suggests that a similar classification within the same species may exist (Düring & Scienza 1980; Chaves et al. 1987; Winkel & Rambal 1993; Schultz 1996).
The apparent differences in stomatal control of isohydric and anisohydric plants are thought to be due to differences in the perception of abscisic acid [ABA] (Tardieu & Simonneau 1998), the chemical signal coming from the roots and the most likely candidate for root-to-shoot signalling in stomatal control (e.g. Davies & Zhang 1991). Such differences in stomatal behaviour may be related to the presence or absence of a sensitivity with respect to high evaporative demand and high temperature, frequent environmental cofactors of developing water deficit, or Ψleaf itself, which can modify the response of stomata to [ABA] in isohydric but not in anisohydric plants (Davies & Zhang 1991; Tardieu & Simonneau 1998).
[ABA] has been demonstrated to act on stomatal conductance (g) of grapevines (e.g. Loveys 1991; Correia et al. 1995; Stoll, Loveys & Dry 2000; Lovisolo, Hartung & Schubert 2002), yet a definite control has only be shown for mid-morning maximum g (gmax) (Correia et al. 1995). However, g as well as leaf water status undergo large diurnal fluctuations without substantial changes in soil water content and the controlling mechanism may not be directly related to [ABA] (Assmann, Snyder & Lee 2000). For instance the rapid decrease in Ψleaf in grapevines observed in the field after sunrise, even in well-watered plants (Naor & Wample 1994; Schultz 1996), suggests the development of substantial water potential gradients in the soil–plant system, which, despite the increased transpiration rates, indicate large hydraulic resistances in the water-conducting pathway (Schultz & Matthews 1988a). Differences in the diurnal behaviour of Ψleaf of different cultivars under drought could thus be related to the water-conducting capacity and stomatal behaviour may respond to a hydraulic signal (Fuchs & Livingston 1996; Hubbard et al. 2001; Comstock 2002). Jones & Sutherland (1991) have proposed that stomata act primarily to avoid damaging water deficits causing cavitation in the xylem (Tyree & Sperry 1989). Although at first glance this would only fit to isohydric behaviour (Jones 1998), depending on the hydraulic capacity and the proportion of conducting tissue which could be sacrificed for embolisms to maximize stomatal aperture and, hence, short-term productivity (Jones & Sutherland 1991), it would be applicable also to plants with anisohydric behaviour (Hubbard et al. 2001; Comstock 2002). Woody plants such as Quercus (Cochard, Bréda & Granier 1996), Juglans regia (Cochard et al. 2002) or Piper auritum (Schultz & Matthews 1997) can achieve control of cavitation by stomatal closure, and this has also been suggested for grapevines in pot studies (Lovisolo & Schubert 1998). There have also been observations of reduced g in response to reduced hydraulic conductance but at a relatively constant Ψleaf, suggesting a feedback link between g and some form of hydraulic signal (Meinzer & Grantz 1990; Sperry, Alder & Eastlack 1993; Saliendra, Sperry & Comstock 1995; Meinzer et al. 1995; Fuchs & Livingston 1996; Meinzer et al. 1999; Salleo et al. 2000; Nardini, Tyree & Salleo 2001). The mechanisms underlying such a response are not clear, but may be related to localized water potential differences as a result of the hydraulic properties of the water pathway (Nonami & Boyer 1993), or through pressure–volume changes in sensing cells or localized cavitations (Canny 1997; Salleo et al. 2000; Nardini et al. 2001).
The hypothesis that different cultivars of the same species have different stomatal sensitivities to drought and may thus act as isohydric or anisohydric plants depending on their genetic background was tested in the field on two grapevine varieties of different geographical origin. One, Grenache, originates from the Mediterranean basin and is largely planted in southern France and northern Spain, the other, Syrah, is of mesic origin from the Rhone valley. The importance of plant hydraulic conductance in the control of stomatal aperture during soil drying was assessed. The argument that apparent differences in stomatal sensitivity may be due to differences in plant developmental rate and thus leaf area formation and subsequent different rates of soil water depletion rather than true genetic differences in stomatal control was also addressed (Borel et al. 1997; Tardieu & Simonneau 1998).