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Effect of Potassium Supply on the Rate of Phloem Sap Exudation and the Composition of Phloem Sap of Ricinus communis

^a Agricultural Research Station Büntehof, D-300 Hannover 71, West Germany

The composition of phloem sap has been investigated in Ricinus communis var. gibsonii, grown for 2 weeks on nutrient solution of low and high potassium content (K₁ and K₂). Diagonal cuts were made in the bark of the stem resulting in the exudation of clear droplets which mainly consisted of phloem sap. Although the plants at low K (0.4 mM) and high K (1 mM) did not differ in growth, leaf area, height, or stem circumference, the rate of exudation of the high K plants was about twice as high as that of the plants with the lower K supply. This promoting effect of K on exudation did not result in a dilution of organic (sucrose, UDP-glucose, ATP, UTP) and inorganic constituents of the phloem sap. For the following compounds, even significantly higher concentrations in the exudate were observed in the K₂ plants: potassium, raffinose, glucose 6-phosphate, and fructose 6-phosphate. Also, the osmotic pressure of the phloem sap was substantially increased in the higher K treatment. Experiments in which labeled ¹⁴CO₂ was applied to one leaf showed that K had a favorable effect on the assimilation of CO₂, and in particular promoted the export of photosynthates from the leaf. It is suggested that the higher rate of phloem-loading in the plants with the better K supply is due to the higher CO₂ assimilation rate and especially to a better provision of ATP required for phloem loading. Higher phloem-loading rates result in higher osmotic pressure in the sieve tubes which probably gave rise to the higher flow rates observed in the plants with improved K supply.

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