Could rapid diameter changes be facilitated by a variable hydraulic conductance?

Abstract

Adequate radial water transport between elastic bark tissue and xylem is crucial in trees, because it smoothens abrupt changes in xylem water potential, greatly reducing the likelihood of suffering dangerous levels of embolism. The radial hydraulic conductance involved is generally thought to be constant. Evidence collected about variable root and leaf hydraulic conductance led us to speculate that radial hydraulic conductance in stem/branches might also be variable and possibly modulated by putative aquaporins. We therefore correlated diameter changes in walnut (Juglans regia L.) with changes in water potential, altered by perfusion of twig samples with d-mannitol solutions having different osmotic potentials. Temperature and cycloheximide (CHX; a protein synthesis inhibitor) treatments were performed. The temperature response and diameter change inhibition found in CHX-treated twigs underpinned our hypothesis that radial hydraulic conductance is variable and likely mediated by a putative aquaporin abundance and/or activity. Our data demonstrate that radial water transport in stem/branches can take two routes in parallel: an apoplastic and a cell-to-cell route. The contribution of either route depends on the hydraulic demand and is closely linked to a boost of putative aquaporins, causing radial conductance to be variable. This variability should be considered when interpreting and modelling diameter changes. This study shows that the radial hydraulic conductance between the xylem and the bark in tree branches and trunks can change and be controlled by aquaporin abundance and/or activity. The contribution of the protein-mediated cell-to-cell pathway to the overall radial water transport could be greater than generally thought and appears to be activated by environmental stimuli that increase transpiration. © 2011 Blackwell Publishing Ltd.