Axial-to-radial water permeability of leaf major veins:: a possible determinant of the impact of vein embolism on leaf hydraulics?

The leaf hydraulic conductance (K-L) was measured in Prunus laurocerasus L. and Juglans regia L. in which previous measurements had revealed different impacts of dehydration on K-L. Leaves of P. laurocerasus lost 8% of their K-L at water potentials (Psi(L)) of -2.0 MPa. Leaflets of J. regia showed K-L losses of 40% at Psi(L) = -1.0 MPa. When major veins were blocked using cyanoacrylate to simulate their embolism, the K-L of P. laurocerasus was reduced by 57% but that of J. regia leaflets was reduced by 80%. Such differences were hypothesized to be due to different axial-to-radial permeabilites of major veins. Infiltration of leaves with Phoxine B revealed that P. laurocerasus major veins were largely leaky in the radial direction whereas those of J. regia leaflets showed prevailing axial water transport. Differences between species in terms of axial-to-radial water permeability were confirmed by measurements of changes of hydraulic resistance along the midrib. The two hydraulic models are discussed in terms of leaf vulnerability to embolism and plant adaptation to dry habitats.