Modeling Within-Plant Water Distribution in Current Year Shoots of the Climbing Vine Kudzu Pueraria lobata
The purpose of this study was to investigate hydraulic limitations associated with long, tall stems. Longer stems may lead to decreased water delivery to leaves due to increased friction against the walls of the conduit, and taller stems lead to decreased water delivery due to increased hydrostatic pressure at greater heights. Leaves that receive less water must close stomata to avoid cavitation, and thus limit uptake and growth rates. Although both of these limitations have been demonstrated to lead to reduced growth in trees, there is evidence that some vines can compensate. I hypothesize that despite growing long stems, kudzu is able to maintain high rates of gas exchange and photosynthesis regardless of leaf position. This is accomplished through hydraulic compensations that manifest in structural changes of the conducting tissue along the pathway.
I observed only a slight (11%) decline in stomatal conductance as expected, but was unable to predict the small decline with height based on structural and hydraulic changes to the vine. Across treatment groups, leaf area increases with path length, so equal water delivery to each leaf would result in less water per unit leaf area to distal leaves, suggesting kudzu does not compensate by adjusting the sapwood to leaf area ratio. To compensate by increasing the driving force, leaf water potential would need to fall below the wilting point. To compensate by decreasing hydraulic resistance, kudzu stems would need to be two orders of magnitude more conductive than measured. Since the total amount of water storage in kudzu based on volume is too small to maintain water supply for 20 minutes, leaves cannot rely on capacitance to compensate for slow delivery through the stem. I am unable to describe the mechanism that allows kudzu to maintain high gas exchange and growth rates in distal leaves.
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