Sensitivity of annual Medicago species to manganese toxicity as affected by calcium and pH

Abstract

Two annual Medicago species developed severe manganese toxicity symptoms at a low manganese concentration 0.3¦M) in complete nutrient solution when the pH was neutral and the calcium concentration in solution was low 250¦M) . Either increasing the calcium concentration to 2500¦M or decreasing the solution pH to 5.4 eliminated these symptoms, decreased manganese concentrations in the tops, and increased growth. Increasing the calcium concentration in solution alleviated manganese toxicity mainly by decreasing sorption, whereas decreasing the pH appeared to act by decreasing both manganese sorption and the transport of manganese from the roots to the tops. In contrast to the Medicago species, two cultivars of T. subterraneum developed no manganese toxicity symptoms. The greater sensitivity of annual Medicago species to manganese toxicity was associated with both a higher rate of manganese sorption per gram fresh weight of roots and a lesser retention of manganese in the roots. Sensitivity to manganese toxicity may be a factor contributing to the poor growth of Medicago species relative to T. subterraneum on acid and waterlogged soils.