Zinc hyperaccumulation and cellular distribution in Arabidopsis halleri

Although Arabidopsis halleri (= Cardaminopsis halleri) is known as a Zn hyp eraccumulator, there have been no detailed studies on Zn accumulation, tole rance and cellular distribution in this species. In a hydroponic experiment , A. halleri grew healthily with Zn concentrations varying from 1 to 1000 m u M, without showing phytotoxicity or reduction in root or shoot dry weight s. The concentration of Zn in the shoots increased from 300 mu g g(-1) dry weight in the 1 mu M Zn treatment to 32 000 mu g g(-1) in the 1000 mu M Zn treatment. Approximately 60% of the total Zn in the shoots were water-solub le, and there was no evidence of Zn and P coprecipitation. Both citric and malic acid concentrations in the shoots were not significantly affected by the Zn treatments, whereas in the roots there was a positive response in bo th organic acids to increasing Zn in solution. Cellular distribution of Zn, Ca and K in frozen hydrated leaf tissues was examined using energy-dispers ive X-ray microanalysis. Zinc was sequestered in the base of trichomes, whe reas the middle and upper parts of trichomes were highly enriched with Ca. Mesophyll cells appeared to have more Zn than the epidermis, probably becau se the latter were very small in size. Similarities and differences between A, halleri and the other well-known Zn hyperaccumulator, Thlaspi caerulesc ens, are discussed.