Cellular ion interactions in two endemic tropical rainforest species of a novel metallophytic tree genus

Gossia, a small genus of physiologically novel rainforest trees restricted to the Western Pacific and belonging to a key neotropical Southern Hemisphere family, the Myrtaceae, is characterized by high foliar manganese (Mn) concentrations. This field study provides a quantitative in planta snapshot detailing cellular localization of foliar Mn and other mineral nutrients in sympatric Gossia grayi N. Snow & Guymer and Gossia shepherdii N. Snow & Guymer endemic to far northeastern Australia, and previously not examined. Elements localized in the cells of fresh hydrated leaf tissues were quantified via in vivo cryo-scanning electron microscopy energy dispersive spectroscopy, a non-invasive method that effectively immobilizes cell contents. Leaf anatomical differences were found between species, along with foliar Mn spatial distribution patterns. Localized cellular Mn concentrations exceeding 600 mmol kg-1 were detected in G. shepherdii, whose Mn accumulation across different mesophyll cell types was heterogeneous compared with G. grayi. In both species there was little evidence to support previous findings on other Gossia species of carboxylate association with excess Mn. The analytical X-ray data strongly implicated chloride as a counter-ion to Mn in the two species examined here. The key findings align with the hypothesis that Mn disposal in the mesophyll is a generic trait in Gossia. This research has forged an emerging view of Gossia as being characterized by unusual cellular metal and mineral accumulation patterns that vary at the species level. It contributes to current limited knowledge about generic plant metallophyty, highlighting that assimilating a broader perspective of the phenomenon demands evaluation of individual taxa through field studies.