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

Leaf veins undergo cavitation at water potentials (Psi(leaf)) 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 Psi(leaf) = -0.33 MPa. Sixty per cent loss of vein hydraulic conductance (K(vein)) was recorded at Psi(leaf) < -0.65 MPa, while stem hydraulic conductance (K(stem)) was unaffected even at Psi(leaf) = -1.1 MPa. Loss of K(vein) was accompanied by stomatal closure. Water-stressed plants (Psi(leaf) = -1.1 MPa) were rehydrated overnight to different target water potentials achieved by using PEG at different concentrations as irrigation medium. K(vein) recovered by 50% at Psi(leaf) = -0.47 MPa and vein refilling was complete at Psi(leaf) = -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 K(vein) was not paralleled by recovery of whole-plant hydraulic conductance or leaf conductance to water vapour (g(L)), 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.

Heterogeneity of gas exchange rates over the leaf surface in tobacco: an effect of hydraulic architecture?

Spatial heterogeneity of gas exchange rates in the leaves of Nicotiana tabacum L. (tobacco) was investigated. Leaf conductance to water vapour was higher (by about 18%) at the apical regions of leaves than at the basal ones. Local, small-scale measurements of pressure-volume (PV) parameters and water status (performed with a dewpoint hygrometer) revealed that bulk leaf water potential, osmotic potential, turgor pressure and bulk modulus of elasticity were not significantly different in the leaf apex or base. Hydraulic measurements showed that the apical regions of the leaf blade were about 30% more conductive than the basal regions. Such differences were explained by analogous differences in terms of venation patterns. In fact, vein density turned out to be higher (by about 13%) near the leaf apex with respect to the leaf base. On the contrary, stomatal density was the same both in the apical and basal leaf portions. Our data suggest that spatial stomatal heterogeneity may arise from heterogenous distribution of local hydraulic resistances and would be addressed to maintaining local water potential above critical values, possibly triggering vein cavitation.