Anthocyanin synthesis in radish seedlings: Effects of continuous far red irradiation and phytochrome transformations

Abstract

Seedlings of Raphanus sativus synthesize anthocyanin upon irradiation with FR in three distinct regions: cotyledons, hook and the basal part of hypocotyl. The time course of pigment accumulation under continuous FR is similar for cotyledons and hook, and the time between the onset of light and the first appearance of anthocyanin is about 6 hr. In hypocotyl however this lag-phase takes 15 hr. Five minutes R are sufficient to induce some pigment synthesis in cotyledons and hook as measured after 24 hr darkness. In hypocotyl a significant effect of R has been obtained only after pre-irradiation with at least 9 hr FR. In cotyledons, hook and hypocotyl the R effect is reverted by a subsequent 5 min FR signal, thus showing that phytochrome is present and can influence the anthocyanogenesis. The effect of continuous FR probably results from the maintenance in the tissues of a low but effective level of active phytochrome. In fact in hypocotyl a red-pre-irradiation reduces, after a dark period, the action of continuous FR. This inhibitory R effect, reversible by FR, is explainable on the basis of phytochrome destruction. Another response to FR in this material is the increase of the activity of the phenylalanine ammonia-lyase. Irradiation for 4 hr increases the activity of this enzyme by the end of the light period. Five minutes R followed by 4 hr darkness have no effect. Other data from pre-irradiation experiments also suggest the non-participation of the phytochrome system in this particular photoresponse. It appears therefore that FR can act with two different mechanisms and that phytochrome is clearly present in only one of them.