Leaf-lamina conductance contributes to an equal distribution of water delivery in current-year shoots of kudzu-vine shoot, Pueraria lobata

Leaf-lamina resistance, R-L, accounts for a large fraction of branch resistance across a wide range of plant species. This work hypothesized that large R-L is essential for distributing water equally to leaves on the shoot, and tested this hypothesis through theoretical analyses and measurements using over 10-m-long current-year shoots of kudzu vine, Pueraria lobata [Willd.] Ohwi. First, the hydraulic architecture and the distribution of the motive force achieving equal distribution of water delivery were theoretically obtained by simulating water flow through a hypothetical shoot comprising an axial pathway and several lateral pathways as a stem and leaves, respectively, in a kudzu-vine shoot. The model predicts that large resistance of the lateral pathway relative to that of the axial pathway is associated strongly with small variation in the hydraulic conductance of a pathway from the base of the axial pathways to the lateral pathway among the nodes, rendering water delivery to each lateral pathway equal under small variation in motive force for water flow. For the kudzu-vine shoot, the measured ratio of the lateral (a petiole) to the axial (a stem) resistance was 115. When R-L was added to the lateral pathway, the ratio increased to 1136. According to the model prediction, these values imply that the hydraulic conductance of a pathway comprising a stem and a petiole, K-BP, is favored strongly at the basal nodes, while the hydraulic conductance of a pathway including a stem, a petiole and a lamina, K-SL, is slightly different across the nodes. For the shoots with leaf lamina, the diurnal change in transpiration rate was not different between the leaves on the three nodes dividing the shoot into four parts. K-SL was not related significantly to node number. Conversely, KBP at the distal node was similar to 0.06-fold that at the basal node. Furthermore, the motive force for water flow should vary by 6.64-fold among nodes to compensate for the favored distribution of K-BP, which is an unrealistic value. These results indicate that R-L contributes largely to an equal distribution of water delivery in a shoot, supporting our hypothesis.