Sap flow through petioles and petioles reveals leaf-level responses to light and vapor pressure deficit in the tropical tree Tabebuia rosea (Bignoniaceae)

Continuous measurements of sap flow have been widely used to measure water flux through tree stems and branches. However, these measurements lack the resolution necessary for determining fine-scale, leaf-level responses to environmental variables. We used the heat ratio method to measure sap flow rates through leaf petioles and leaflet petiolules of saplings of the tropical tree Tabebuia rosea (Bignoniaceae) to determine how leaf and leaflet sap flow responds to variation in light and vapor pressure deficit (VPD). We found that in the morning sap flow rates to east-facing leaves increased 26 minutes before adjacent west-facing leaves. Although leaves had higher integrated sap flow than their