Does homeostasis or disturbance of homeostasis in minimum leaf water potential explain the isohydric versus anisohydric behavior of Vitis vinifera L. cultivars?

Due to the diurnal and seasonal fluctuations in leaf-to-air vapor pressure deficit (D), one of the key regulatory roles played by stomata is to limit transpiration-induced leaf water deficit. Different types of plants are known to vary in the sensitivity of stomatal conductance (gs) to D with important consequences for their survival and growth. Plants that minimize any increase in transpiration with increasing D have a tight stomatal regulation of a constant minimum leaf water potential (Ψleaf); these plants are termed as ‘isohydric’ (Stocker 1956). Plants that have less control of Ψleaf have been termed as ‘anisohydric’ (Tardieu and Simonneau 1998). Isohydric plants maintain a constant Ψleaf by reducing gs and transpiration under drought stress. Therefore, as drought pushes soil water potential (Ψsoil) below this Ψleaf set point, the plant can no longer extract water for gas exchange. Anisohydric plants allow Ψleaf to decrease with rising D, reaching a much lower Ψleaf in droughted plants relative to well-watered plants (Tardieu and Simonneau 1998), so this strategy produces a gradient between Ψsoil and Ψleaf that allows gas exchange to continue over a greater decline in Ψsoil. Thus, anisohydric plants sustain longer periods of transpiration and photosynthesis, even under large soil water deficit, and are thought to be more drought tolerant than isohydric species (McDowell 2011). In practice, the distinctions between isohydric and anisohydric strategies are often not clear (Franks et al. 2007), even among different cultivars of the same species. For example, cultivars of poplar (Hinckley et al. 1994) and grapevine (Schultz 2003, Lovisolo et al. 2010) have been shown to exhibit both contrasting hydraulic behaviors. A third mode of behavior was also suggested by Franks et al. (2007), in which the difference between soil and midday water potential (Ψsoil − Ψleaf) is maintained seasonally constant but Ψleaf fluctuates in synchrony with soil water availability (isohydrodynamic behavior). The lack of a clear distinction between these two strategies and the complex and variable responses of stomata to D under high and low soil moisture is depicted in two papers in this issue (Rogiers et al. 2012 and Zhang et al. 2012), showing that even typically anisohydric grape (Vitis vinifera L.) cultivars (Semillon and Merlot, respectively) may constrain gs during periods of extremely low Ψsoil. The same individuals can switch from an isohydric-like behavior when transpiration is low to an anisohydric-like behavior with increasing water demand. Interestingly, both studies indicated that classifying species as either isohydric or anisohydric is a simplistic view of stomatal functioning and does not represent well the complex stomatal behavior under drying soil, and Zhang et al. (2012) also reported an isohydrodynamic behavior. Both studies suggested that when soil water is limited, gs is aimed at protecting the integrity of the hydraulic system, whereas as soil water content increases, stomata regulate transpiration less. The results of Zhang et al. (2012) indicated that under limited soil moisture the decrease in gs with increasing D was proportional to reference gs (gs at D = 1 kPa); which is in agreement with the stomata-sensitivity model developed by Oren et al. (1999) for isohydric species (see xeric line in Figure 1A). However, a significant departure from this theoretical model was observed under high soil moisture (see wet and mesic lines in Figure 1B). Similarly, in this issue Rogiers et al. (2012) showed that under Tree Physiology 32, 245–248 doi:10.1093/treephys/tps013