Hydraulic conductivity of grapevines: effects of shoot inversion and partial rootzone drying (PRD)

The xylem component of the plant hydraulic conductance changes as a result of interruption of the water column in the vessels (embolism) or modifications of the size of the xylem vessels. In the grapevine water stress induces embolism and loss of function of the vessels and induces a decrease in the average diameter of grapevine vessels and a decrease of xylem hydraulic conductance, as predicted by the Poiseuille's law. Shoot inversion mimics water stress effects on shoot hydraulic conductivity, causing growth slackening, known in viticulture practices. Downward-growing grapevine shoots have smaller and more frequent vessels and, as a consequence, a lower hydraulic conductivity. By manipulating shoot growth orientation in double-bent single-shooted grapevines (i.e. N-shaped vines whose central downward shoot portion is of increasing length) it is possible to induce increasing reductions of shoot hydraulic conductivity. These N-shaped plants show reduced shoot conductivity and water flow, according to a reduction in xylem development; however, the reduction in hydraulic conductivity has no effects on xylem development and growth in the apex growing upwards, located xylem-downstream the two bents. When the apex grows downwards, auxin accumulates in apical internodes, resulting in a higher density of xylem vessels with smaller vessel lumina. These results suggest that the downward orientation induces accumulation of auxin in the apex which in turn affects the density and the size of the xylem vessels, causing reduction of hydraulic conductivity, reflecting on growth. In the grapevine the changes in water availability affect both hydraulic conductance and stomatal conductance. As leaf specific conductance (the hydraulic conductance related to the leaf area distal the point of measurement) decreases, stomatal conductance is initially linearly correlated with leaf specific conductance, while excessive decrease of hydraulic conductance is avoided by stomatal closure. Vessel size modifications contribute to regulate water flow throughout the plant at relatively low water stress levels, while at lower leaf water potential the regulation of water flow is taken over by stomata. Besides hydraulic (both vessel-growth induced regulation and embolism formation), hormonal (ABA) signals regulate stomatal closure. Several authors report that [ABA] act on stomatal conductance of grapevines, however, the relative importance of hydraulic and ABA message has been debated in the grapevine for a long time. To test whether plant hydraulic conductance is reduced under drought conditions via an ABA-related mechanism a water stress experiment was carried out using split-rooted grapevines, as those used in PRD systems. In addition in that experiment, the inversion of shoot growth orientation was imposed to reduce plant hydraulic conductance independently of soil water availability and thus of the putative ABA root-generated stress message. As expected, water stress imposed to split roots affected ABA accumulation. ABA drought-stress message negatively affected stomatal conductance and transpiration, but it modified neither leaf and stem water potentials, nor whole-plant hydraulic conductance. When hydraulic conductance is reduced in split-rooted shoot-inverted grapevines (HSD), leaf and stem water potentials decrease, without changes in ABA accumulation, stomatal conductance and transpiration rate. Neither ABA drought-stress message modifies plant hydraulic conductance, nor plant hydraulic conductance impaired by the shoot growth inversion decreases ABA delivery to the leaves. However, leaf growth is depressed in HSD grapevines. The fact that no interactions between ABA stress messages, caused by split-root technique, and hydraulic constraints to sap flow, caused by shoot inversion, are necessary to impair leaf growth suggests that the target of ABA and hydraulic-limitation effects on leaf expansion are not the same.