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Potassium nutrition and its effect on wood formation in poplar

Abstract

Potassium is the most abundant cation in higher plants and essential for various physiological processes like enzyme activation, phloem transport and maintenance of cellular cation:anion balance. In addition, it mediates stomatal movements and osmoregulation during cell expansion. The uptake and transport mechanisms of potassium have been studied in detail in herbaceous species by various groups. After taken up by the roots, potassium is transported via the xylem towards the leaves, where it can be used metabolically or be loaded into the phloem and translocated to different sinks like young leaves, fruits and roots as well as developing wood. In contrast to herbaceous species our knowledge of potassium functions in trees is still incomplete and mainly focused on fertilization effects on phenological growth parameters. With regard to wood formation, previous work on spruce trees revealed a distinct correlation between potassium and cambial growth activity (Dünisch and Bauch 1994, Dünisch et al. 1998). In poplar, an important tree species in the field of wood biotechnology, we studied the role of potassium during xylogenesis and found a close correlation between cambial potassium content, osmotic potential and cambial activity (Wind et al. 2004). All parameters increased from spring to summer and decreased from summer to autumn. When poplar trees were grown under low potassium regimes, the cambial activity as well as the seasonal rate of wood increment and the vessel lumen were significantly reduced. Apart from the anatomical aspects the molecular and electrophysiological basis of potassium-dependent xylogenesis has been studied (Langer et al. 2002), indicating a strong involvement of specific ion channels. Two K+ ion channels, PTK2 and PTORK, have been identified and shown to correlate with seasonal wood production. While PTK2 carries inward K+ flux at hyperpolarized membrane potential and K+ release upon depolarization, PTORK mediates potassium release upon depolarization (Langer et al. 2002). Antibodies raised against PTORK recognized this channel in growth active trees only (Arend et al. 2005). PTORK labelling was found in young fibers of the developing wood and in ray parenchyma cells of mature wood. In the latter, it might be required for charge balance during uptake of metabolites from the vessels, whereas in differentiating fibers PTORK might be involved in the reduction of protoplast volume. Taken together, it can be concluded that potassium plays a key role in the regulation of wood formation. References Arend M., Stinzing A., Wind C., Langer K., Latz A., Ache P., Fromm J., Hedrich R. 2005. Polar-localised poplar K+ channel capable of controlling electrical properties of wood-forming cells. Planta 223: 140-148 Dünisch O., Bauch J. 1994. Influence of mineral elements on wood formation of old growth spruce (Picea abies L. Karst.). Holzforschung 48: 5-14 Dünisch O., Bauch J., Müller M., Greis O. 1998. Subcellular quantitative determination of K and Ca in phloem, cambium and xylem cells of spruce (Picea abies L. Karst.) during earlywood and latewood formation. Holzforschung 52: 582-588 Langer K., Ache P., Geiger D., Stinzing A., Arend M., Wind C., Regan S., Fromm J., Hedrich R. (2002) Poplar potassium transporters capable of controlling K+ homoeostasis and K+ dependent xylogenesis. The Plant Journal 32: 997-1009 Wind C., Arend M., Fromm J. 2004. Potassium-dependent cambial growth in poplar. Plant Biology 6: 30-37

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