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.
Highlights
Acoustic emissions produced by stems and leaves[9]
Recent studies have reported that near-isohydric cultivars and anisohydric cultivars display different vulnerabilities to xylem cavitation, and it has been suggested that the different stomatal behaviour of grapevine genotypes under water limitation might be primarily related to their different hydraulic vulnerabilities, tailoring to abscisic acid (ABA) production and accumulation patterns a secondary role[36]
Our hypothesis was that in V. vinifera stomatal closure is primarily triggered by water potential decrease and coordinated with xylem vulnerability to embolism formation, with leaf ABA content eventually increasing after severe reduction of stomatal conductance to prevent sudden stomatal reopening upon transient plant rehydration
Summary
Acoustic emissions produced by stems and leaves[9]. Taking into account that stomatal conductance to water vapour is generally co-ordinated with liquid phase hydraulic conductance of the soil-to-leaf pathway[10,11,12], these findings have suggested that stomatal aperture is primarily modulated by hydraulic signals[13,14]. Brodribb and McAdam[24] suggested that stomatal closure under leaf water deficit has evolved from a passive hydraulic process mediated by water potential changes to an active process controlled by the extrusion of anions from guard cells In support to this view, the ancient lineages of lycophytes and ferns lack of active control mechanism to modulate stomatal closure[24], while in the gymnosperm Metasquoia glyptostroboides ABA effect is additive to the passive hydraulic influence on stomatal closure[25]. Recent studies have reported that near-isohydric cultivars and anisohydric cultivars display different vulnerabilities to xylem cavitation, and it has been suggested that the different stomatal behaviour of grapevine genotypes under water limitation might be primarily related to their different hydraulic vulnerabilities, tailoring to ABA production and accumulation patterns a secondary role[36]. In this study we used Sangiovese and Montepulciano, two of the most important grapevine cultivars, that are considered anisohydric and isohydric, respectively
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