Do xylem vessels of a liana species behave like ideal capillaries?

Background/Question/Methods

Conducting capacity and cavitation safety are antagonistic characteristics of xylem, both of great importance in lianas like hop with thin stems but large leaf area. Although the wide and usually long vessels in lianas have been suggested to show a hydraulic behaviour like ideal capillaries with a high hydraulic conductivity (K), accurate quantification of their resistivity components (R = 1/K) remains little studied. We studied resistivity components of vessels with simple perforation plates in hop stems, including the effect of varying inner diameter along their longitudinal axis.

The contribution of vessel lumen resistivity (R_V) and end wall resistivity (R_W) to the difference between theoretical (R_T) and measured vessel resistivity was quantified in stems of Humulus lupulus (hop) by stem shortening experiments. Silicone injection was used to quantify the number of open vessels and changes in vessel diameter along the longitudinal axis of vessels.

Results/Conclusions

The measured hydraulic conductivity was on average 41 % (SD = 9%) of the predicted K based on the Hagen-Poiseuille equation. Assuming a circular cross-sectional shape of vessels, R_W contributed on average 2.8 times more to the total vessel resistivity than R_V. Theoretical resistivity calculated from varying inner vessel diameter was on average 18% higher than R_T based on the mean diameter.

The significant contribution of R_V and variation in vessel diameter along its longitudinal axis confirms that vessel lumina in hop do not behave like a straight-walled pipe approximation, but these parameters contribute to the whole vessel resistivity less than R_W.