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

Abstract

The composition of phloem sap was investigated in R. communis var. gibsonii, grown for 2 wk 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, G6-P and fructose 6-phosphate. The osmotic pressure of the phloem sap was substantially increased in the higher K treatment. Experiments in which labeled 14CO2 was applied to 1 leaf showed that K had a favorable effect on the assimilation of CO2 and in particular promoted the export of photosynthates from the leaf. The higher rate of phloem-loading in the plants with the better K supply is probably 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.