The hydraulic conductivity of the xylem in conifer needles (Picea abies and Pinus mugo)

Main resistances of the plant water transport system are situated in leaves. In contrast to angiosperm leaves, knowledge of conifer needle hydraulics and of the partitioning of resistances within needles is poor. A new technique was developed which enabled flow-meter measurements through needles embedded in paraffin and thus quantification of the specific hydraulic conductivity (K(s)) of the needle xylem. In Picea abies, xylem K(s) of needle and axes as well as in needles of different age were compared. In Pinus mugo, resistance partitioning within needles was estimated by measurements of xylem K(s) and leaf conductance (K(leaf), measured via 'rehydration kinetics'). Mean K(s) in P. abies needles was 3.5×10(-4) m(2) s(-1) MPa(-1) with a decrease in older needles, and over all similar to K(s) of corresponding axes xylem. In needles of P. mugo, K(s) was 0.9×10(-4) m(2) s(-1) MPa(-1), and 24% of total needle resistance was situated in the xylem. The results indicate species-specific differences in the hydraulic efficiency of conifer needle xylem. The vascular section of the water transport system is a minor but relevant resistance in needles.