The influence of photoperiod on incorporation of precursors into tocopherols and plastoquinone in Xanthium strumarium L.

Radioactivity from tyrosine applied to leaves of Xanthium strumarium L. was incorporated into α- and γ-tocopherol and plastoquinone. Time-course studies on vegetative plants showed that γ-tocopherol was rapidly labelled and turned over, while α-tocopherol showed no sign of turnover. The same pattern was observed in the light and in darkness. Plastoquinone synthesis was rapid in the light but it was greatly reduced in the dark. Following floral induction the accumulation of radioactivity in γ-tocopherol was at first slower, but it later readied higher levels than in the vegetative plants. Total incorporation into α-tocopherol and plastoquinone was greatly reduced.