Mechanisms for minimizing height-related stomatal conductance declines in tall vines

The ability to transport water through tall stems hydraulically limits stomatal conductance (g_s), thereby constraining photosynthesis and growth. However, some plants are able to minimize this height-related decrease in g_s, regardless of path length. We hypothesized that kudzu (Pueraria lobata) prevents strong declines in g_s with height through appreciable structural and hydraulic compensative alterations. We observed only a 12% decline in maximum g_s along 15-m-long stems and were able to model this empirical trend. Increasing resistance with transport distance was not compensated by increasing sapwood-to-leaf-area ratio. Compensating for increasing leaf area by adjusting the driving force would require water potential reaching −1.9 MPa, far below the wilting point (−1.2 MPa). The negative effect of stem length was compensated for by decreasing petiole hydraulic resistance and by increasing stem sapwood area and water storage, with capacitive discharge representing 8–12% of the water flux. In addition, large lateral (petiole, leaves) relative to axial hydraulic resistance helped improve water flow distribution to top leaves. These results indicate that g_s of distal leaves can be similar to that of basal leaves, provided that resistance is highest in petioles, and sufficient amounts of water storage can be used to subsidize the transpiration stream.