Effects of manganese toxicity on leaf CO2assimilation of contrasting common bean genotypes

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Abstract
Parameters related to leaf photosynthesis were evaluated in three genotypes of common bean (Phaseolus vulgarisL.) with contrasting tolerance to Mn toxicity. Two short‐term studies in solution culture were used to assess the effect of excess Mn on CO2assimilation in mature and immature leaves. Mn toxicity decreased total chlorophyll content only in immature leaves, with a consequent reduction of leaf CO2assimilation. Mature leaves that showed brown speckles characteristic of Mn toxicity, did not suffer any detriment in their capacity to assimilate CO2, at least in a 4‐day experiment. Stomatal conductance and transpiration were not affected by the presence of high levels of Mn in leaf tissue. Lower stomatal conductance and transpiration rates were observed only in leaves with advanced chlorosis. Differences among genotypes were detected as increased chlorosis in the more sensitive genotype ZPV‐292, followed by A‐283 and less chlorosis in the tolerant genotype CALIMA. Since CO2assimilation expressed per unit of chlorophyll was not different between high‐Mn plants and control plants, we conclude that the negative effect of Mn toxicity on CO2assimilation can be explained by a reduction in leaf chlorophyll content.