Partitioning of shoot and root dry matter and carbohydrates in bean plants suffering from phosphorus, potassium and magnesium deficiency

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Abstract
The influence of varied supply of 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) on the partitioning of dry matter and carbohydrates (reducing sugars, sucrose and starch) between shoots and roots was studied in bean (Phaseolus vulgaris) plants grown in nutrient solution over a 12 d period. Shoot and root growth were quite differently affected by low supply of P, K, and Mg. The shoot/root dry weight ratios were 4.9 in the control (sufficient plants), 1.8 in P-deficient, 6.9 in K-deficient and 10.2 in Mg-deficient plants. In primary (source) leaves, but not in trifoliate leaves, concentrations of reducing sugars, sucrose and starch were also differently affected by low nutrient supply. In primary leaves under K deficiency and, particularly Mg deficiency, the concentrations of sucrose and reducing sugars were much higher than in control and P-deficient plants. Magnesium deficiency also distinctly increased the starch concentration in the primary leaves. In contrast, in roots, the lowest concenfrations of sucrose, reducing sugars and starch were found in Mg-deficient plants, whereas the concentrations of sucrose and starch were particularly high in P-deficient plants. There was a close relationship between shoot/root dry weight ratios and relative distribution of total carbohydrates (sugars and starch) in shoot and roots. Of the total amounts of carbohyd rates per plant, the following proportions were parti tioned to the roots: 22.7% in P-deficient, 15.7% in control, 3.4% in K-deficient and 0.8% in Mg-deficient plants. The results indicate a distinct role of Mg and K in the export of photosynthates from leaves to roots and suggest that alteration in photosynthate partitioning plays a major role in the differences in dry matter distribution between shoots and roots of plants suffering from mineral nutrient deficiency.

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