Osmotic potential of Norway spruce [Picea abies (L.) Karst.] secondary phloem in relation to anatomy

Variations in water status of secondary phloem of Picea abies (L.) Karst., caused by (1) radial and (2) vertical differences within the tree, (3) seasonal influences, and (4) tree class, and their relation to bark anatomy were investigated. The water status parameters measured were the osmotic potential at full saturation [Psi(o) ((sat))], the in situ osmotic potential [Psi(o) ((in situ))], the in situ water content (C-in (situ)), and the in situ relative water content (R-in site). Psi(o) ((sat)) reached most negative values in the conducting part of the secondary phloem, whereas Psi(o) ((in situ)) was similar in conducting (P-C) and non-conducting secondary phloem (P-N). The remarkable discontinuity in the radial course of C-in situ and Psi(o) ((sat)) at the transition from P-C to P-N can be attributed to the degeneration of sieve cells and Strasburger cells. In P-C, the vertical decrease of Psi(o) ((sat)) towards the crown was compensated by an increase in R-in situ so that Psi(o) ((in situ)) did not change along the stem. With stem height, Psi(o) ((sat)) decreased and P-C width increased. The determining factor for vertical gradients in Psi(o) ((sat)) was the distance to the sources; similar gradients were also measured in P-N. Seasonal differences in Psi(o (sat)) could only be detected in P-C, where they corresponded exactly to changes of Psi(o (sat)) in needles. Suppressed trees showed less negative Psi(o (sat)) values in P-C, smaller annual secondary phloem increments and smaller radial lumen diameter of living sieve cells than predominant or dominant trees.