Potassium nutrition and translocation in sugar beet

The effect of increased net foliar K(+) accumulation on translocation of carbon was studied in sugar beet (Beta vulgaris, L. var. Klein E and US H20) plants. Net accumulation of recently absorbed K(+) was studied by observing arrival of (42)K(+) per unit area of leaf. Labeled K(+) was added to give an initial concentration at 2 or 10 millimolar K(+) in mineral nutrient solution. Because the newly arrived K(+) constitutes a small part of the total leaf K(+) in plants raised in 10 millimolar K(+), export of (42)K(+) by phloem was negligible over the 2- to 3-day period; consequently, accumulation is a measure of arrival in the xylem. In leaves from plants in 2 millimolar K(+), export by the phloem was estimated to be of the same order as import by the xylem; K(+) per area was observed to remain at a steady-state level. Increasing the supply of K(+) to 10 millimolar caused arrival in the xylem to increase 2- to 3-fold; K(+) per area increased gradually in the mature leaves. Neither net carbon exchange nor translocation of sugar increased in response to a faster rate of arrival of K(+) over a 6- to 8-hour period. In the absence of short-term effects, it is suggested that K(+)-promoted increase in synthetic metabolism may be the basis of the increased carbon assimilation and translocation in plants supplied with an above-minimal level of K(+).