Seedling responses of three Australian tree species to toxic concentrations of zinc in solution culture

A frequently desired outcome when rehabilitating Zn toxic sites in Australi a is to establish a self-sustaining native ecosystem. Hence, it is importan t to understand the tolerance of Australian native plants to high concentra tions of Zn. Very little is known about the responses of Australian native plants, and trees in particular, to toxic concentrations of Zn. Acacia holo sericea, Eucalyptus camaldulensis and Melaleuca leucadendra plants were gro wn in dilute solution culture for 10 weeks. The seedlings (42 days old) wer e exposed to six Zn treatments viz., 0.5, 5, 10, 25, 50 and 100 muM. The or der of tolerance to toxic concentrations of Zn was E. camaldulensis > A. ho losericea > M. leucadendra, the critical external concentrations being appr oximately 20, 12 and 1.5 muM, respectively. Tissue Zn concentrations increa sed as solution Zn increased for all species. Root tissue concentrations we re higher than shoot tissue concentrations at all solution Zn concentration s. The critical tissue Zn concentrations were approximately 85 and 110 mug g(-1) DM for M. leucadendra, 115 and 155 mug g(-1) DM for A. holosericea an d 415 and 370 mug g(-1) DM for E. camaldulensis for the youngest fully expa nded leaf and total shoots, respectively. The results from this paper provi de the first comprehensive combination of growth responses, critical extern al concentrations, critical tissue concentrations and plant toxicity sympto ms for three important Australian genera, viz., Eucalyptus, Acacia and Mela leuca, for use in the rehabilitation of potentially Zn toxic sites.