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Vein recovery from embolism occurs under negative pressure in leaves of sunflower (Helianthus annuus)




Leaf veins undergo cavitation at water potentials (Ψleaf) commonly experienced by field-growing plants. Theoretically, embolism reversal should not be possible until xylem pressures rise by several kilopascals of atmospheric pressure, but recent evidence suggests that embolized conduits can be refilled even when surrounded by others at substantial tension (novel refilling). The present study reports ‘novel refilling’ occurring in leaf veins of sunflower (Helianthus annuus L.) while at Ψleaf = −0.33 MPa. Sixty per cent loss of vein hydraulic conductance (Kvein) was recorded at Ψleaf < −0.65 MPa, while stem hydraulic conductance (Kstem) was unaffected even at Ψleaf = −1.1 MPa. Loss of Kvein was accompanied by stomatal closure. Water-stressed plants (Ψleaf = −1.1 MPa) were rehydrated overnight to different target water potentials achieved by using PEG at different concentrations as irrigation medium. Kvein recovered by 50% at Ψleaf = −0.47 MPa and vein refilling was complete at Ψleaf = −0.33 MPa, i.e. well below the theoretical limit for conduit refilling (−0.05 MPa as calculated for sunflower minor veins). Mercurials supplied to detached leaves had no effect on the refilling process. Upon rehydration, recovery of Kvein was not paralleled by recovery of whole-plant hydraulic conductance or leaf conductance to water vapour (gL), as a likely consequence of hydraulic failure of other components of the water pathway (root system or extravascular leaf compartments) and/or root-to-leaf chemical signalling. This is the first study providing experimental evidence for ‘novel refilling’ in a herbaceous dicot and highlighting the importance of this process in the leaf.