Hydraulic conductance of root and shoot measured with the transient and dynamic modes of the high-pressure flowmeter

The hydraulic conductance (k) of shoots and root systems was measured using the transient and the dynamic modes of the high pressure flowmeter (HPFM). Measurements were conducted on Quercus robur and Fagus sylvatica plants grown on different substrates (forest soil, sand, Terra-green and vermiculite) and harvested at different times of the year. The values of k obtained by the transient mode were compared to those obtained by the dynamic mode. A tight 1: 1 correlation was observed for shoots and defoliated stems but several types of discrepancies appeared for root systems. The underestimation of k by the dynamic mode as compared to the transient mode could be explained by reverse osmosis at the endodermis. However the transient mode was not functional for some root systems. This problem occurred essentially in small plants harvested early in the year before budbreak had been completed. Nature and origins of problems are discussed.