Diurnal depression of leaf hydraulic conductance in a tropical tree species

Diurnal patterns of hydraulic conductance of the leaf lamina ( K leaf ) were monitored in a field‐grown tropical tree species in an attempt to ascertain whether the dynamics of stomatal conductance ( g s ) and CO 2 uptake ( A leaf ) were associated with short‐term changes in K leaf . On days of high evaporative demand mid‐day depression of K leaf to between 40 and 50% of pre‐dawn values was followed by a rapid recovery after 1500 h. Leaf water potential during the recovery stage was less than −1 MPa implying a refilling mechanism, or that loss of K leaf was not linked to cavitation. Laboratory measurement of the response of K leaf to Ψ leaf confirmed that leaves in the field were operating at water potentials within the depressed region of the leaf ‘vulnerability curve’. Diurnal courses of K leaf and Ψ leaf predicted from measured transpiration, xylem water potential and the K leaf vulnerability function, yielded good agreement with observed trends in both leaf parameters. Close correlation between depression of K leaf , g s and A leaf suggests that xylem dysfunction in the leaf may lead to mid‐day depression of gas exchange in this species.