Stomatal closure is induced by hydraulic signals and maintained by ABA in drought-stressed grapevine

Abstract

Water saving under drought stress is assured by stomatal closure driven by active (ABA-mediated) and/or passive (hydraulic-mediated) mechanisms. There is currently no comprehensive model nor any general consensus about the actual contribution and relative importance of each of the above factors in modulating stomatal closure in planta. In the present study, we assessed the contribution of passive (hydraulic) vs active (ABA mediated) mechanisms of stomatal closure in V. vinifera plants facing drought stress. Leaf gas exchange decreased progressively to zero during drought, and embolism-induced loss of hydraulic conductance in petioles peaked to ~50% in correspondence with strong daily limitation of stomatal conductance. Foliar ABA significantly increased only after complete stomatal closure had already occurred. Rewatering plants after complete stomatal closure and after foliar ABA reached maximum values did not induced stomatal re-opening, despite embolism recovery and water potential rise. Our data suggest that in grapevine stomatal conductance is primarily regulated by passive hydraulic mechanisms. Foliar ABA apparently limits leaf gas exchange over long-term, also preventing recovery of stomatal aperture upon rewatering, suggesting the occurrence of a mechanism of long-term down-regulation of transpiration to favor embolism repair and preserve water under conditions of fluctuating water availability and repeated drought events.

Figure 1. Soil water content in the pots of Sangiovese and Montepulciano vines during the experiment.

Each point is the mean of three pots ± SE. Points with asterisk are different per P < 0.05 (t-student test).

Figure 2. Ψ_pd (A) and Ψ_stem at midday (B) of Sangiovese and Montepulciano grapevine during the experiment.

Each point is the mean of five vines ± SE. Points with asterisk are different per P < 0.05 (t-student test).

Figure 3. Stomatal conductance

(A), net CO₂ assimilation (A_n) (B) and leaf [ABA] (C) measured at midday during the experiment. Each point is the mean of five vines ± SE. Points with asterisk are different per P < 0.05 (t-student test).

Figure 4. Relationship between midday stomatal conductance

(A), midday net CO₂ assimilation (A_n) (B) and midday Ψ_stem in Sangiovese and Montepulciano vines.

Figure 5. Stomatal conductance vs leaf [ABA] in Sangiovese (y = 0.18 × e^−0.66x, R² = 0.48, P < 0.001) and Montepulciano (y = 0.14 × e^−0.67x, R2 = 0.42, P < 0.001) vines measured at midday during the experiment.

The insert depict stomatal conductance (g_s) vs foliar ABA per g_s > 0 in Sangiovese (R² = 0.09, P = 0.20) and Montepulciano (R² = 0.11, P = 0.15).

Figure 6. Daily course of Ψ_stem, stomatal conductance, foliar ABA in day 2 (A,D,G), 8 (B,E,H) and 15 (C,F,I) in Montepulciano and Sangiovese vines.

Each point is the mean of five vines ± SE. Points with asterisk are different per P < 0.05 (t-student test).

Figure 7. PLC of Sangiovese and Montepulciano petioles measured at midday during the experiment.

Each point is the mean of five vines ± SE. Points with asterisk are different per P < 0.05 (t-student test).

Figure 8. Relationship between midday stomatal conductance

(A), midday net CO₂ assimilation (A_n) (B), foliar ABA (C) and Ψ_leaf in Sangiovese and Montepulciano vines. The insert depict stomatal conductance (g_s) and foliar ABA when stomata closure occurred in the interval −1 MPa < Ψ_leaf < −1.5 MPa.