Gonzalez, A.; Lynch, J.P.


Physiologia Plantarum, Volume 101, Issue 4, p.872-880 (1997)

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Parameters related to leaf photosynthesis were evaluated in three genotypes of Phaseolus vulgaris with contrasting tolerance to Mn toxicity. Two short-term studies in solution culture were used to assess the effect of excess Mn on CO 2 assimilation in mature and immature leaves. Mn toxicity decreased total chlorophyll content only in immature leaves, with a consequent reduction of leaf CO 2 assimilation. Mature leaves that showed brown speckles characteristic of Mn toxicity, did not suffer any detriment in their capacity to assimilate CO 2 during 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. SinceCO 2 assimilation expressed per unit of chlorophyll was not different between high-Mn plants and control plants, it was concluded that the negative effect of Mn toxicity on CO 2 assimilation can be explained by a reduction in leaf chlorophyll content.