Intrachloroplast Localization of65Zn and63Ni in a Zn-tolerant Plant,Ocimum basilicumBenth

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Abstract
Intrachloroplast localization studies of 65Zn and 63Ni were made in a Zn-tolerant plant Ocimum basilicum var. purpurascens Benth. in order to investigate the mechanism and specificity of metal tolerance. The isotopes were supplied in solution to the roots 14 d before fractionation. It was observed that 65Zn activity was comparatively greater in the chloroplast envelope membranes and stroma than the 63Ni; and 63Ni was largely found in the lamellar and stroma fractions. Further analysis of lamellae revealed that photosystem II (PS II) particles were richer in radioactivity than photosystem I (PS I) particles. The photochemical events of photosynthesis were less affected in Zn-treated plants than in the Ni-treated plants. The changed levels of the electron transport chain intermediates including cytochromes, plastocyanin and ferredoxin provide supporting evidence for the localization studies. The activity of carbonic anhydrase, a zinc metalloprotein, was increased in Zn-treated plants with increase in nutrient Zn concentration, indicating the binding of zinc to a protein moiety in the chloroplasts.