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Abstract
Two bush bean cultivars (Phaseolus vulgarls L. cv. ‘Wonder Crop 2’ and ‘Green Lord'), previously classified as sensitive or tolerant to excess Mn, were grown in a growth chamber for 12 days, in Hoagland No.2 nutrient solution containing 0.05, 0.2, 0.5 or 1.0 ppm Mn as MnCl2 · 4H2O, with 1 ppm Fe as Fe‐EDTA, at an initial pH 5.0. The cultivars were characterized with respect to Mn toxicity symptoms, growth and distribution of Mn and Fe within the plants.
‘Wonder Crop 2’ ('WC‐2') was much more sensitive to Mn toxicity than ‘Green Lord’ ('GL'). With 1.0 ppm Mn in the solution, Mn‐sensitive ‘WC‐2’ showed a 31% growth reduction in total dry weight in comparison to plants grown at 0.05 ppm Mn. However, Mn‐tolerant ‘GL’ showed no growth reduction under the same conditions. Dry matter distribution in the primary leaves and steins of Mn‐sensitive ‘WC‐2’ significantly increased with increasing Mn concentration in the solution, but that of the roots significantly decreased. In Mn‐tolerant ‘GL’, Mn in the nutrient solution had no significant effect on dry matter distribution.
In both cultivars, Mn concentration in plant tissues increased in proportion to Mn level in the nutrient solution. However, a significantly higher concentration of tissue Mn was required to produce Mn toxicity symptoms in ‘GL’ than in ‘WC‐2’. The Fe concentration in the leaves was not affected by Mn treatment in either cultivar. The Mn‐sensitive ‘WC‐2’ tended to accumulate more Mn and less Fe in younger leaves and more Fe in roots than did Mn‐tolerant ‘GL’.
Manganese inhibited the stem exudate production in Mn‐sensitive ‘WC‐2’ but not in Mn‐tolerant ‘GL’. However, the total amount of Fe transported per unit root mass was significantly higher in Mn‐tolerant ‘GL’ than that in Mn‐sensitive ‘WC‐2’.
The overall evidence indicated that the superior Mn tolerance of ‘GL’ bush bean, compared with ‘WC‐2’, is associated with lower transport of Mn and higher transport of Fe to young leaves, and a greater internal tolerance to a given Mn concentration within the leaves.
