Cellular-level distribution of manganese in Macadamia integrifolia, M. ternifolia, and M. tetraphylla from Australia

Macadamia integrifolia and M. tetraphylla, unlike M. ternifolia, are known for their edible nuts. All three species over-accumulate the trace metal nutrient manganese (Mn) in their shoots. This study seeks to examine tissue- and cellular-level distribution of Mn and other plant nutrients in the three Macadamia species. The distribution of Mn, calcium, iron, and potassium were investigated in whole leaves and cross-sections of roots, petioles, and leaves using synchrotron-based X-ray fluorescence microscopy (XFM) in M. integrifolia, M. tetraphylla, and M. ternifolia. The results show Mn sequestration primarily in the leaf and midrib palisade mesophyll cells of all three species. Leaf interveinal regions, root cortical cells, and phloem cells were also found to be Mn loaded. The current study confirms earlier findings but further reveals that Mn is concentrated in the vacuoles of mesophyll cells owing to the exceptional resolution of the synchrotron XFM data, and the fact that fresh hydrated samples were used. New insights gained here into Mn compartmentalization in these highly Mn-tolerant Macadamias expand knowledge about potentially toxic over-accumulation of an essential micronutrient, which ultimately stands to inform strategies around farming edible species in particular.