External K requirement of young plants of potato, sugar beet and wheat in flowing solution culture resulting from different internal requirements and uptake efficiency

Potato, wheat and sugar beet were grown in a growth chamber in tanks with flowing solutions containing K-concentrations of 1.5, 5, 15, 50 and 200 mu M to find reasons for different K contents of solution needed to reach 90% maximum dry matter accumulation (external K requirement). All species showed variation in dry matter accumulation with varying K levels. To reach 90% maximum dry matter accumulation potato needed 40 mu M K, whereas wheat and sugar beet needed 6.4 and 4.4 mu M K respectively. These differences in external K requirement could not be explained by differences in their internal K requirement since the internal requirement of potato (7.7% K) was about one and a half times of that of sugar beet (5.4%) but its external requirement was nine times of that of sugar beet. At low K supply potato plants showed a lower influx and a reduced root growth as compared with wheat and sugar beet.