Mechanism of manganese toxictty and tolerance of plants VII. effect of light intensity on manganese‐induced chlorosis

Abstract

Effect of light intensity on Mn‐induced chlorosiss was investigated with bush bean (Phaseolus vulgaris L.) and corn (Zea mays L.) seedlings. The seedlings were grown in nutrient solutions containing different concentrations of Mn in enclosures which transmitted different percentages of the total solar radiation. At high levels of Mn in nutrient solution, the increase in light intensity increased the Mn uptake by the plant and resulted in a decrease in the chlorophyll content of the leaves. Even at similar levels of Mn concentrations within the leaves, high light intensity increased the severity of Mn‐induced chlorosis. Photobleaching experiments were carried out with isolated chloroplasts suspended in media containing 0, 10^‐4 , 10^‐3,10^‐2 and 10^‐1 M Mn²⁺. Addition of Mn²⁺ to the medium decreased the extent of photobleacing of chlorophyll with increaing Mn²⁺ concentration up to 10^‐3 M . In concentrations of Mn²⁺ higher than 10^‐3 M, the extent of bleaching was increased again, accompanied by precipitation of oxidized manganese in the medium. It is suggested that high light intensity stimulates not only the Mn uptake by the plant but also the destruction of chlorophyll when Mn in excess.