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 CO2 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 Psi(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 Psi(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.