Differences in hydraulic architecture account for near-isohydric and anisohydric behaviour of two field-grown Vitis vinifera L. cultivars during drought

A comparative study on stomatal control under water deficit was conducted on grapevines of the cultivars Grenache, of Mediterranean origin, and Syrah of mesic origin, grown near Montpellier, France and Geisenheim, Germany. Syrah maintained similar maximum stomatal conductance (g(max)) and maximum leaf photosynthesis (A(max)) values than Grenache at lower predawn leaf water potentials, Psi(leaf), throughout the season. The Psi(leaf) of Syrah decreased strongly during the day and was lower in stressed than in watered plants, showing anisohydric stomatal behaviour. In contrast, Grenache showed isohydric stomatal behaviour in which Psi(leaf) did not drop significantly below the minimum Psi(leaf) of watered plants. When g was plotted versus leaf specific hydraulic conductance, K-1, incorporating leaf transpiration rate and whole-plant water potential gradients, previous differences between varieties disappeared both on a seasonal and diurnal scale. This suggested that isohydric and anisohydric behaviour could be regulated by hydraulic conductance. Pressure-flow measurements on excised organs from plants not previously stressed revealed that Grenache had a two- to three-fold larger hydraulic conductance per unit path length (K-h) and a four- to six-fold larger leaf area specific conductivity (LSC) in leaf petioles than Syrah. Differences between internodes were only apparent for LSC and were much smaller. Cavitation detected as ultrasound acoustic emissions on air-dried shoots showed higher rates for Grenache than Syrah during the early phases of the dry-down. It is hypothesized that the differences in water-conducting capacity of stems and especially petioles may be at the origin of the near-isohydric and anisohydric behaviour of g.