Cation exchange capacity and aluminum–calcium–magnesium binding in roots of bananas cultivated in soils and in nutrient solutions

Bananas (Musa spp.), as many other crop species, accumulate aluminum (Al) in roots when grown in nutrient solution containing Al ions. Aluminum can compete with calcium (Ca) and magnesium (Mg) on the root exchange sites, which has been reported as a possible cause for Al toxicity to the plant. We measured the cation exchange capacity of roots (CECR) of 5 banana cultivars (Grande Naine, Agbagba, Obino l'Ewaï, Igitsiri, and Kayinja) and determined the composition of the root exchange sites in nutrient solutions with and without Al, and in two tropical soils. Aluminum, Ca, and Mg were extracted using a sequential procedure involving 0.01 M copper sulphate for Cu-extractable cations, 0.1 M hydrochloric acid and ash solubilization in nitric acid. The mean CECR values of the 5 cultivars were similar in the 2 soils and in the nutrient solution without Al (CECR=23.0 cmolc kg− 1), but significantly larger for bananas grown in nutrient solution with Al (CECR=32.6 cmolc kg− 1). The most Al-sensitive cultivar (Kayinja) had larger CECR than the others. Calcium was the dominant Cu-extractable cation in all cases. In the roots grown in nutrient solution with Al and in the two soils, Al amounted to 15–35% of the sum of Cu-extractable cations, but most Al was not extracted with Cu. The Cu-extractable Mg was drastically reduced in nutrient solution with Al, whereas the Cu-extractable Ca was little affected. Therefore we suggest that the Al/Mg ratio on root exchange sites could be a better indicator of Al toxicity than the Al/Ca ratio.