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

Continuous measurements of sap fow have been widely used to measure water fux through tree stems and branches. However, these measurements lack the resolution necessary for determining fne-scale, leaf-level responses to environmental variables. We used the heat ratio method to measure sap fow rates through leaf petioles and leafet petiolules of saplings of the tropical tree Tabebuia rosea (Bignoniaceae) to determine how leaf and leafet sap fow responds to variation in light and vapor pressure defcit (VPD). We found that in the morning sap fow rates to east-facing leaves increased 26 minutes before adjacent west-facing leaves. Although leaves had higher integrated sap fow than their largest leafet, this difference was not proportional to the difference in leaf area, which could be due to lower conduit area in petiolules than in petioles. In contrast to measurements on main stems, integrated daily sap fow was negatively correlated with daily mean VPD. Furthermore, leaves exhibited previously undescribed patterns of hysteresis in the sap fow-VPD and sap fow-PAR relationships. When hysteresis in the sap fow-PAR relationship was clockwise, the sap fow-VPD relationship was also clockwise; however, when hysteresis in the sap fow-PAR relationship was counterclockwise, the sap fow-VPD relationship displayed an intersected loop. These pattern differences highlight how substantially leaf-level processes may vary within a canopy and how leaf-level processes may not scale predictably to the stem level. 2 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48