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...

Effects of supplying K+ at 2 or 10 millimolarity concentration on net carbon exchange and translocation of products of photosynthesis were studied in plants of Beta vulgaris L. (var. Klein E). Transport of K+ into and out of leaves was studied with 42K over a 3-day period. Increasing the K+ supplied to the roots from 2 milHimolarity, a level just sufficient to overcome obvious deficiency symptoms, to 10 milHimolarity resulted in a gradual accumulation of K+ per unit area and an increased export of K+ to sink regions. No significant increase in net carbon exchange was observed in leaves that had accumulated a high level of K+ per unit area. Initiation rate, total area, and total fresh weight of leaves of plants with K+ supplied at 10 millimolarity was similar to that for leaves fromu plants at a 2 milhimolarity level. Shoot/root ratio and dry weight accumulation, which are indicative of translocation and partitioning over the long term, were independent of K+ supply in the 2 to 10 milimolarity range. Accumulation of K+ by exporting leaves and its subsequent recirculation to sinks increased when K+ supply was increased in this range but did not appear to affect carbon nutrition even after a long period.Increased K4 supply to plants increases assimilate translocation (2, 7 and references cited therein). Possible mechanisms for this effect include increased NCE,2 phloem loading, sink unloading, and conversion in sink tissues. An increase in K4 supplied to source leaves appears to have a relatively immediateffect on export (3,5). Doman and Geiger (3) observed that K4 supplied through the leaf surface at a concentration of 5 to 30 mm causes an increase in export of products of photosynthesis. The treatment results in increased flux of "C-labeled sugar into the free space which is likely the cause of increased translocation. Subsequent studies revealed that increasing the concentration of K4 supplied to the roots from 2 to 10 mm does not cause an immediate increase in NCE or translocation (2). The increase in K4 supply results in a severalfold increase in arrival rate of K4 in the source leaf (2) but it is likely that the concentration of K4 in the free space is below that produced in the study in which K4 was applied to the leaf surface (3). MATERIALS AND METHODSPlant Culture. Sugar beet (Beta vulgaris L., var. Klein E) plants were raised in washed sand that was supplied three times daily until run-through with mineral solution (2, 3) containing 2 or 10 mM K+. All plants were raised at the 2 mm K4 level until the fiveleaf stage. In preliminary studies, it was determined that at concentrations of K4 below 2 mm deficiency symptoms began to appear; consequently, 2 mm was chosen as the lower concentration of K+. With the unrolling of the fifth leaf to appear after the cotyledons, plants ofsimilar appearance were selected, some plants were transferred to 10 mm K+, and growth measurements were begun. The mineral solutions had the following nutrient concentrations: 2 mm NaH2PO4, 0.5 mm KNO3, 2 mM MgSO4, 3 mM Ca(NO3)...