Specific distribution of gibberellins, cytokinins, indole-3-acetic acid, and abscisic acid in radish plants closely correlates with photomorphogenetic responses to blue or red light

Abstract

Summary Ontogenetic patterns of dry matter accumulation and contents of cytokinins, gibberellins, indole-3-acetic acid (IAA), and abscisic acid (ABA) in shoot and hypocotyl were studied in radish (Raphanus sativus L.) plants grown under blue (BL) or red (RL) light. BL promoted the development of storage organ (swollen hypocotyl), whereas RL favoured petiole and stem growth. RL did not prevent tuber formation, but delayed tuberization, which started on days 14 and 20 in BL- and RL-grown plants, respectively, and severalfold reduced tuber weight. About 10-times higher (as compared with BL-grown plants) levels of gibberellins were found in shoots of 11-day-old RL-grown plants, and the difference between RL and BL-grown plants increased with age. The 2.5-times higher level of IAA was found in shoots of 11-day-old BL-grown plants compared with RL-grown ones. IAA levels in shoot abruptly decreased with age regardless of light quality. Compared with RL, BL increased the contents of IAA and cytokinins (zeatin plus zeatin riboside and isopentenyladenine plus isopentenyladenosine) severalfold in hypocotyls of 11- and 14-day-old plants. By day 21, IAA content in hypocotyls decreased in BL, while it increased under RL. Unlike IAA, an increase in cytokinins of hypocotyls continued by day 21, which was much more pronounced in RL-grown plants. These findings indicate that both IAA and cytokinins are involved in tuber initiation regardless of light quality, while cytokinins appear to stimulate the assimilate flow to developing storage tissue. Specific morphogenetic responses of radish plants to BL or RL seem therefore to be closely associated with the specific action of light quality on phytohormones in shoot and hypocotyl: RL enhances sink strength of aboveground organs (petioles and stem) by stimulating gibberellins, whereas BL stimulates assimilate flow to hypocotyls by an increase in IAA, cytokinins, and, probably, ABA. Mechanisms by which BL- or RL-grown plants maintain contrasting levels of different hormones in shoots and hypocotyls are discussed.