CHANGES IN PHLOEM EXPORT OF SUCROSE IN LEAVES IN RESPONSE TO PHOSPHORUS, POTASSIUM AND MAGNESIUM-DEFICIENCY IN BEAN-PLANTS

The effect of varied phosphorus (10 and 250 mmol P m(-3)), potassium (50 and 2010 mmol K m(-3)) and magnesium (20 and 1000 mmol Mg m(-3)) supply on sucrose, reducing sugars, amino acids, P, K, and Mg in phloem exudate was studied in bean (Phaseolus vulgaris L;) plants over a 12 d growth period in nutrient solution. Phloem exudates were collected from detached primary leaves using the EDTA-promoted exudation technique. Compared with control nutrient-sufficient plants, sucrose export in the phloem exudate was drastically decreased by K deficiency and, particularly, by Mg deficiency, whereas P deficiency either had no effect or stimulated sucrose export. In Mg-deficient plants the rate of sucrose export was decreased to 10-20% of the control plants. There was a close inverse relationship between phloem export and leaf concentration of sucrose: higher leaf concentrations of sucrose were accompanied by fewer phloem export of sucrose. In contrast to sucrose, reducing sugars in the exudates were very low and not affected by P, K and Mg deficiency. The phloem export of amino acids was strongly depressed by Mg deficiency, but only slightly by P and K deficiency. Resupplying Mg to Mg-deficient plants for 12 h during the dark or light periods rapidly stimulated sucrose export. After resupply of Wig for 24 h and 48 h the rate of sucrose export was comparable with the rate in the control plants. The results demonstrate a key role for Mg in phloem loading and export of photosynthates from source leaves, especially sucrose. Inhibition of root growth and development of visual symptoms of chlorosis in Mg-deficient plants are suggested as consequences of impaired phloem loading. In agreement with this in P-deficient plants where phloem loading was not impaired, chlorosis was absent and root growth was maintained at a high level.