Mechanisms of storage and detoxification of Al in two tropical mistletoes

Passovia ovata (Pohl ex DC.) Kuijt and Struthanthus polyanthus Mart. are hemiparasitic mistletoes that can grow on Al-accumulating and Al-excluding woody species in the Brazilian Cerrado. It is unclear to what extent this hemiparasitic lifestyle implies facultative Al accumulation by the mistletoes and if so, what are the Al tolerance mechanisms in these species. Using a field-work experiment composed by two facultative Al-accumulating mistletoe species (P. ovada and S. polyanthus) infecting Al-accumulating (Miconia albicans (Sw.) Steud.) and Al-excluding (Byrsonima verbascifolia (L.) DC.) hosts, we (1) investigated Al accumulation and leaf nutritional status of the mistletoes, (2) characterized the storage sites and the forms of Al accumulated in their leaves, and (3) determined differences in levels of simple organic acids associated with Al detoxification. Leaf nutrients and Al accumulation in mistletoes followed the host elements availability. In both mistletoe species infecting the Al-accumulating host M. albicans, Al was mainly allocated to the phloem fibres, and Al-citrate was the main form of Al. Contrastingly, in the hosts' leaves Al was present mainly in the form of oxalate complexes. When growing on B. verbascifolia, the Al excluding host, the two mistletoes neither showed symptoms of Al storage nor formation of Al-organic acid complexes. In conclusion, mistletoes growing on Al-accumulating trees tolerate high Al tissue levels by allocating Al in phloem fibres and by its chelation with citrate.