Rapid Growth Responses of Dark-Grown Wheat Seedlings to Red Light-Irradiation: I. Kinetic Studies on the Short-Term Growth Responses of Intact Coleoptiles at Different Seedling Age

Continuous recordings of the effect of red light on intact darkgrown wheat seedlings (Triticum aestivum L. cv. Hatri) weremade at different times after sowing. When the coleoptile tipregion was irradiated 50, 70 or 90 h after sowing with red lightfrom two opposite fibre bundles a decrease in extension ratewas detectable after a latent period of 10 to 15 min. Growthrate reached a fluence dependent minimum at about 60 min, afterwhich growth acceleration towards the dark control rate wasobserved. When continuous red irradiation was started 50 or70 h after sowing the dark control rate was reached 2.5 h afteronset of irradiation and growth rate was little above this levelduring the next 2 h. With older coleoptiles (90 h after sowing)the growth rate recovery was only up to 50% of the dark controlrate and a second phase in growth inhibition became detectableabout 2.5 h after onset of red exposure, characterized by acontinuous decrease in extension rate. Under R/FR pulse irradiationboth the red-light-induced transient growth inhibition and thesecond phase of growth inhibition exhibit far-red reversibilityup to the level of far-red induced growth rate changes. (Received September 19, 1986; Accepted December 8, 1986)