Extracts from Zn-accumulating phloem tissue of blight-affected trees had higher Zn-binding capacity than extracts from healthy trees. A 4-kDa, highly anionic, zinc-binding fraction was isolated from the phloem of predecline and decline stage ‘Valencia’ sweet orange [ Citrus sinensis (L.) Osbeck] and ‘Marsh’ grapefruit (C. paradisi Macf.) trees grown on blight-susceptible rootstocks. Diethylaminoethyl (DEAE) Sephadex chromatography revealed the Zn-binding fraction contained at least four different anionic species. Two of the four species were present in healthy as well as predecline- and decline-stage trees. The other two species were found in only the predecline and decline trees. All Zn-binding factors were similar to the poly(γ-glutamyl-cysteinyl) glycine peptides in terms of size, anionic character, pH and temperature stability, capacity to bind other metals, and spectrophotometric properties.
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Taylor et al. (1988) studied this question.