Changes in phloem export of sucrose in leaves in response to phosphorus, potassium and magnesium deficiency in bean plants

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Abstract
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 lower 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 resup ply of Mg 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.

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