Foliar nutrition and metal accumulation in two sympatric tree species endemic to northeastern Australia

DR Fernando, PT Green and AT Marshall

La Trobe University, Bundoora VIC 3086

The recently described tree genus Gossia (Myrtaceae) comprises 48 species almost entirely restricted to the metal-enriched substrates of eastern Australia and New Caledonia. It is taxonomically delineated by an affinity for the heavy metal manganese (Mn), with some species additionally accumulating other metals; traits manifest in elevated foliar concentrations. While eastern Australian soils and climate enhance soil-Mn bioavailability that drives plant Mn toxicity in field cultivations; native plant systems are highly adapted to these inherent conditions. Mineral nutrient analyses of Gossia leaf material procured from herbaria, and, collected via ecological field samplings have established that this Mn-accumulating group includes several species exhibiting the rare and extreme trait of Mn hyperaccumulation. Field studies conducted to date on just a few Australian Gossia species have revealed unusual foliar nutritional balance and novel metal detoxification strategies; for example, whereby metabolically important mesophyll cells serve as primary sites for Mn disposal. Vacuolar Mn concentrations exceeding 0.5 M have previously been detected in leaf palisade mesophyll cells of G. bidwillii. This is in sharp contrast to commonly observed foliar metal detoxification strategies in hyperaccumulators of other metals, where epidermal and/or apoplastic shoot tissues serve as primary disposal sites. This ecophysiological field study evaluates foliar nutrition and metal sequestration in two sympatric Gossia species, G. grayi and G. shepherdii, whose global distribution is limited to North Queensland. They were previously known only from herbarium sample analyses to have elevated foliar Mn concentrations. This study employs analytical cryo-electron microscopy to examine in vivo foliar mineral nutrient distribution patterns in fresh leaf tissues.