Stomatal behaviour of irrigated Vitis vinifera cv. Syrah following partial root removal

We examined stomatal behaviour of a grapevine cultivar (Vitis vinifera L. cv. Syrah) following partial root removal under field conditions during progressively developing water deficits. Partial root removal led to an increase in hydraulic resistances along the soil-to-leaf pathway and leaf wilting symptoms appeared in the root-pruned plants immediately following root removal. Leaves recovered from wilting shortly thereafter, but hydraulic resistances were sustained. In comparison with the non-root pruned vines, leaves of root-pruned vines showed an immediate decrease in both pre-dawn (psi(PD)) and midday (psi(leaf)) leaf water potential. The decline in psi(PD) was unexpected in as much as soil moisture was not altered and it has been shown that axial water transport readily occurs in woody perennials. Only similar to 30% of the functional root system was removed, thus leaving the system mainly intact for water redistribution. Stem water potential (psi(Stem)) and leaf gas exchanges of CO2 (A) and H2O(E) also declined immediately following root pruning. The lowering of psi(PD), psi(leaf), psi(Stem), A and E was sustained during the entire growing season and was not dependent on irrigation during that time. This, and a close relationship between stomatal conductance (g(s)) and leaf-specific hydraulic conductance (K-plant), indicated that the stomatal response was linked to plant hydraulics. Stomatal closure was observed only in the root-restricted plants and at times of very high evaporative demand (VPD). In accordance with the Ball-Berry stomatal control model proposed by Ball et al. (1987), the stomatal sensitivity factor was also lower in the root-restricted plants than in intact plants as soil water availability decreased. Although psi(PD), psi(Stem) and psi(Leaf) changed modestly and gradually following root removal, g(s) changed dramatically and abruptly following removal. These results suggest the involvement of stomatal restricting signals being propagated following removal of roots.