Manganese is a vital plant macronutrient due to its involvement in oxidative stress protection, enzyme catalysis, and water splitting in photosystem II; however, it can be toxic to most plant species at high concentration (typically above 500 ppm) or when oxidized. Plants acquire manganese from soil. While manganese in soil is found as Mn(II), Mn(III), and Mn(IV), only Mn(II) is absorbed by plants. Furthermore, manganese is usually stored in tree leaves as Mn(II). This study investigated how tree type influenced the concentration, spatial distribution, and speciation of manganese in tree leaves from the lowland tropical Forest Reserve at EARTH University in Costa Rica. The six most abundant tree types were studied including Pentaclethra macroloba, Inga pezizifera, Goethalsia meiantha, Dendropanax arboreus, Protium confusum, and Socratea exorrhiza. Elemental analysis of digested leaf samples indicated Socratea exorrhiza had higher concentrations of manganese (average 1600 ppm) compared to the other five tree species. Manganese distribution and speciation were analyzed with micro x-ray fluorescence (µ-XRF) mapping and x-ray absorption near edge structure spectroscopy (XANES). Manganese µ-XRF maps indicated a similar concentration and distribution within different leaves of the same species. All six tree species contain primarily Mn(II) (55-76%), but localized examples of oxidized Mn were observed in some Socratea exorrhiza samples. A previous study has shown that Socratea exorrhiza was the only tree species in the area to have had an increased relative growth due to phosphorus fertilization. Recently, elevated manganese concentrations in leaves have been shown to indicate a root system that releases carboxylates to facilitate phosphorus uptake. Future experiments will test the hypothesis that excess manganese taken up by Socratea exorrhiza during concurrent phosphorus uptake is then oxidized so it is not toxic to the tree.

Oxidized manganese in Costa Rican tree leaves

255th National Meeting (2018)

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