Poster: Ecophysiology

Abs # 48: Mechanisms of manganese toxicity on the photosynthetic apparatus of maple seedlings in constrasting light environments

Presenter:	St.Clair, Samuel , sbs152@psu.edu
Authors	St.Clair, Samuel  (A)   Lynch, Jonathan  (A)
Affiliations:	(A): Penn State University

Photosynthetic sensitivity to nutrient imbalances appears to be an important factor contributing to forest decline in the northeastern US. Tree species growing on acidic non-glaciated soils prevalent in the eastern deciduous forest, accumulate foliar manganese (Mn) to levels that are several times greater than the critical toxicity threshold in most crop species. We hypothesized that in susceptible species excess foliar Mn decreases photosynthesis in response to high light, through photo-oxidative stress. To test this hypothesis, we assessed photosynthetic responses (C02 exchange and chlorophyll fluorescence) of sugar maple and red maple seedlings at variable Mn and light levels in greenhouse conditions. Rubisco appeared to be a key target of Mn toxicity, as indicated by decreased carboxylation efficiency in seedlings treated with excess Mn. Photochemical sensitivity to the interactive effects of Mn and light varied with leaf development. Mature sugar maple leaves were sensitive to excess Mn in high and low light. High light induced Mn toxicity in young leaves of sugar maple and mature leaves of red maple. Young red maple leaves were tolerant to Mn toxicity regardless of the light treatment. These results indicated that under most treatment conditions, sugar maple was photochemically more sensitive to Mn toxicity than red maple. Excess Mn and high light increased the PhiPSII /PhiCO2 of mature sugar maple leaves, a physiological marker indicating susceptibility to photo-oxidation. These results suggest that observed correlations between excess foliar Mn and sugar maple decline symptoms on the eastern deciduous forest have the potential to be mediated by Mn toxicity of the photosynthetic apparatus.