Abstract
1. The effects of light on 18O 2 incorporation by epoxy xanthophylls in spinach leaves ( Tetragonia expansa) were investigated. The major epoxy xanthophylls incorporated 18O 2 more rapidly in the light than in the dark. Incorporation was highest in antheraxanthin, less in violaxanthin and only slight in neoxanthin. This light-stimulated incorporation which occurred without large net changes in pigment concentrations shows that the major epoxy xanthophylls derived their epoxy oxygen from O 2 and confirms recent interpretations of kinetic data that the violaxanthin cycle takes up O 2 through light-induced reactions. 2. The rate of 18O 2 incorporation under 3770 lux light and 33% O 2 atmosphere was determined. The rates of 18O 2 incorporation by antheraxanthin, violaxanthin and neoxanthin were 7, 100 and 5 mμmoles epoxy oxygen/μmole chlorophyll per h, respectively.
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