Potassium Nutrition and Translocation in Sugar Beet

Abstract

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 42K' 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 mi_molar K+, export of 42K' 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 KV.promote import into sinks. Haeder and Mengel (9) found that increasing K+ supplied to the roots caused increased translocation of carbon to tomato fruits. They observed that increased K+ promoted conversion of soluble metabolites to an insoluble form in tomato fruits. Mengel and Viro (16) cited this metabolic conversion in sink tissues as a possible mechanism responsible for the long-term increase in translocation of assimilates observed in plants under increased K+ fertilization.The present study was undertaken to determine if any of the four mechanisms cited above are likely to be means by which K+ regulates translocation of assimilate. Increasing the K+ available to the roots caused an immediate increase in delivery of K+ to the leaves but did not produce a concomitant increase in net CO2 exchange or export of carbon. The data lead us to conclude that increased K+-supply does not promote net carbon exchange or translocation over the short term. Instead, we favor the hypothesis that increased accumulation of K+ in sinks and older source leaves promotes synthetic metabolism, which in some manner leads to an eventual increase in net carbon exchange and in export of products of photosynthesis. MATERIALS AND METHODSVarious studies provide evidence that K+ promotes the translocation of products of photosynthesis in plants (1,2, 5,9,10,12,13,16,17). This promotion will occur if a higher level of K+ nutrition causes one or more of the following to increase: (a) net carbon exchange, (b) phloem loading, (c) transport into cells in sinks, and (d) metabolic conversion of sucrose in sink tissues.A number of studies have shown that net carbon exchange increases as a result of increased K+ fertilization of plants. Low K+ ap...

Effect of carbon assimilation on dry weight production and partitioning during vegetative growth

“…Overall, the present study provides evidence that K starvation globally decreases the translocation of photosynthetic products in plants by acting directly on the phloem loading and/or transport into cells in sinks as already reported by other authors (Conti and Geiger 1982;Cakmak et al 1994b;Marschner et al 1996;Pettigrew 1999). Phloem loading dysfunction under K-deficiency has been confirmed by labelled CO 2 studies (Mengel and Viro 1974b).…”

Influence of Environmental Stress on Biomass Partitioning in Transgenic Tobacco Plants Expressing the Movement Protein of Tobacco Mosaic Virus

“…A reduced root-to-shoot ratio in low-K*-grown plants has been reported previously (Cakmak et al, 1994a). This effect might be related to the enhancement of photoassimilate export, which occurs when high levels of Kt are present in source leaves (Conti and Geiger, 1982;Cakmak et al, 1994b). Plants grown under low-K+ levels would likely have reduced K* concentrations in their source leaves, which could retard the rate of phloem loading, thereby leading to the observed reduction in export in K+-stressed tobacco plant lines 306 and 277.…”

Foliar chemical composition of two oak species grown in a free-air enrichment system with elevated O₃ and CO₂

“…High level of K may accelerate the various functions in photosynthesis, such as net carbon exchange, phloem loading, metabolic conversion of sucrose, etc. (Conti and Geiger, 1982). Despite the fact that some previous researches indicated eO 3 and eCO 2 treatment decreased foliar K in potato and beech (e.g.…”