Symbiotic and physiological performances of Bradyrhizobium-nodulated cowpea plants subjected to varying concentrations of manganese and iron.

The objective of this study was to analyse the response of Bradyrhizbium-inoculnted cowpea plants to increasing Mn concentrations from 0.1 to 80 μM. For each Mn concentration, Fe was provided at a ratio of 0.5, 1 and 5, as Fe-EDTA. Concentrations of Mn from 0.1 to 20 μM did not enhance the Mn concentration in leaves. Only the lowest Fe:Mn ratio had a positive effect. Except for plants grown at 5 and 10 μM Mn, the leaf Fe concentration increased with increasing Fe:Mn ratio until 60 μM Mn. Concomitantly, plant growth, nodulation and the catabolism of ureides in leaves were hindered at 0.1 up to 2 μM Mn and above 10 μM Mn, at Fe:Mn ratios equal or higher than 1. A decrease in the total reducing sugar concentration in leaves and in the first internode of the main stem occurred between 0.1 and 2 μM Mn. Leaf chlorophyll contents showed a decrease at Mn concentrations higher than 2 μM, regardless of the Fe:Mn ratio. These results indicate the very narrow range for Mn and Fe fertilization in cowpea for optimal plant growth, nodulation and catabolism of ureides in leaves.