Silicon amelioration of manganese toxicity in Mn-sensitive and Mn-tolerant maize varieties

Differences in tolerance to Mn excess and amelioration by Si were evaluated in two maize varieties. Dry weight, callose accumulation, chloroplast ultrastructure, and photosynthesis parameters were used as stress indicators. Variety Kneja 605 was much more Mn-sensitive than variety Kneja 434. In Kneja 605 excess Mn caused severe chloroplast damage and enhanced carotenoid production, symptoms similar to those triggered by photoinhibiton. In Mn-tolerant Kneja 434, in contrast, a Mn-induced decrease of the carotenoid concentrations, and only slight alterations in the chloroplasts were observed. These effects were similar to light Fe-deficiency symptoms. The threshold tissue concentration for Mn-induced callose accumulation was much lower in Kneja 605 than in Kneja 434. Therefore tolerance to excess Mn in Kneja 434 was not due to more efficient exclusion but to more efficient detoxification and compartmentation of Mn. The constitutively thicker epidermal layers in Kneja 434 and the observation that Si-induced amelioration of Mn toxicity in Kneja 605 substantially increased the thickness of the epidermal layers suggest that Mn storage in non-photosynthetic tissue could be a Mn tolerance mechanism in maize.