Summary The cambial K+ content of poplar increases during the growth period in a K+ supply dependent manner. Upon K+ starvation or application of tetraethylammoniumchloride (TEA+), a K+ channel blocker, the average vessel lumen and expansion zone area were significantly reduced. In search for the molecular basis of potassium‐dependent xylogenesis in poplar, K+ transporters homologous to those of known function in Arabidopis phloem‐ and xylem‐physiology were isolated from a poplar wood EST library. The expression profile of three distinct K+ channel types and one K+ transporter, Populus tremula K+ uptake transporter 1 (PtKUP1), was analysed by quantitative RT‐PCR. Thereby, we found P. tremula outward rectifying K+ channel (PTORK) and P. tremula K+ channel 2 (PTK2) correlated with the seasonal wood production. K+ transporter P. tremula 1 (KPT1) was predominantly found in guard cells. Following the heterologous expression in Xenopus oocytes the biophysical properties of the different channels were determined. PTORK, upon membrane de‐polarization mediates potassium release. PTK2 is almost voltage independent, carrying inward K+ flux at hyperpolarized potential and K+ release upon de‐polarization. PtKUP1 was expressed in a K+ uptake‐deficient Escherichia coli strain, where this K+ transporter rescued K+‐dependent growth. In order to link the different K+ transporters to the cambial activity and wood production, we compared the expression profiles to seasonal changes in the K+ content of the bark as well as xylem vessel diameter. Thereby, we found PTORK and PTK2 transcripts to follow the annual K+ variations in poplar branches. PtKUP1 was expressed at a low level throughout the year, suggesting a housekeeping function. From these data, we conclude that K+ channels are involved in the regulation of K+‐dependent wood production.

Seasonal variations in osmolality and components of xylem sap in tall birch trees were determined using several techniques. Xylem sap was extracted from branch and trunk sections of 58 trees using the very rapid gas bubble-based jet-discharge method. The 5-cm long wood pieces were taken at short intervals over the entire tree height. The data show that large biphasic osmolality gradients temporarily exist within the conducting xylem conduits during leaf emergence (up to 272 mosmol x kg(-1) at the apex). These gradients (arising mainly from glucose and fructose) were clearly held within the xylem conduit as demonstrated by (1)H NMR imaging of intact twigs. Refilling experiments with benzene, sucrose infusion, electron and light microscopy, as well as (1)H NMR chemical shift microimaging provided evidence that the xylem of birch represents a compartment confined by solute-reflecting barriers (radial: lipid linings/lipid bodies; axial: presumably air-filled spaces). These features allow transformation of osmolality gradients into osmotic pressure gradients. Refilling of the xylem occurs by a dual mechanism: from the base (by root pressure) and from the top (by hydrostatic pressure generated by xylem-bound osmotic pressure). The generation of osmotic pressure gradients was accompanied by bleeding. Bleeding could be observed at a height of up to 21 m. Bleeding rates measured at a given height decreased exponentially with time. Evidence is presented that the driving force for bleeding is the weight of the static water columns above the bleeding point. The pressure exerted by the water columns and the bleeding volume depend on the water-filling status of (communicating) vessels.

In previous studies, we have shown that annual expression profiles of cambial and wood tissue with respect to the Shaker K+ channel PTORK correlate with cambial activity. To follow PTORK-gene activity on the cellular level, we isolated the respective promoter regions and generated transgenic Arabidopsis plants expressing the GUS gene under the control of the PTORK promoter. Cross-sections of petioles showed PTORK-driven signals predominantly in the xylem parenchyma surrounding the vessels and in the phloem. Antibodies raised against a unique N-terminal region of PTORK in histo-immunochemical analyses recognised this K+-release channel in growth-active poplar plants only. PTORK labelling was found in differentiating xylem cells (young fibres) and mature xylem (vessel-associated cells of the ray parenchyma). Patch-clamp measurements on fibre cell protoplasts, derived from young poplar twigs, identified outward-rectifying K+ channels as the major K+ conductance of this cell type, which resembled the biophysical properties of PTORK when expressed in Xenopus oocytes.