The Effect of Segment Length on Conductance Measurements in Lonicera fragrantissima

Abstract

The effect of stem segment length on conductance measurements was determined by repeatedly shortening the transport distance in isolated stem segments of Lonicera fragrantissima. If vessel ends limit water flow, shorter segment lengths could result in higher readings of K_h (hydraulic conductance per unit pressure gradient). The mean K_h in the successively shortened segments showed no statistically significant change when shortened from 20 to 2 cm, even though maximum vessel lengths ranged from 6 to 14 cm. This suggests vessel ends may be less limiting to water flow than are vessel lumens. There was a statistically significant drop in K_h (19%, s.e.=0·29%) when segments were shortened from 2 to 1 cm, regardless of the position of the segment within the branch. The K_h remained constant with applied pressure in 2 cm segments while it was more variable with applied pressure in 1 cm segments. Based on the paint-infusion method mean vessel lengths were from 0·6 to 1·2 cm. The consistently lower K_h in 1 cm segments might be due to an end effect in segments with a high percentage of cut open vessels. Within a branch, distal and proximal stem segments had lower K_h values than segments from intermediate positions, but nodes had no measurable impact on K_h. Longer segments allow for more accurate and consistent measurements of K_h since they integrate localized variation in K_h while minimizing the segment end effect.

Vessel length, vessel end, hydraulic conductance, paint-infusion method, Lonicera fragrantissima