INVITED REVIEW: PART OF AN INVITED ISSUE ON TREE NUTRITION Wood formation of trees in relation to potassium and calcium nutrition

Potassium and calcium are essential for tree metabolism and various physiological processes related to growth. In recent years, special interest was therefore accorded to the effect of both cations on cambial activity and xylem development. Various studies revealed a distinct correlation between potassium as well as calcium nutrition and wood formation. When poplar trees were grown under low K or Ca regimes, the cambial activity as well as the seasonal rate of wood increment and the vessel size were significantly reduced. Molecular, biochemical and electrophysiological investigations indicate (i) a strong involvement of specific K channels in the regulation of xylem cell expansion and (ii) a significant influence of Ca on the onset of cambial reactivation after winter dormancy as well as on wood structure and chemistry. These studies highlight the important role of potassium as well as calcium in xylogenesis. Based on that knowledge, further research will be directed towards a better understanding of the mechanisms governing Kas well as Ca-dependent wood formation.