Continuous recordings of the effect of red light on intact dark grown wheat seedlings (Triticum aestivumL. cv. Hatri) were made at different times after sowing. When the coleoptile tip region was irradiated 50, 70 or 90 h after sowing with red light from two opposite fibre bundles a decrease in extension rate was detectable after a latent period of 10 to 15 min. Growth rate reached a fluence dependent minimum at about 60 min, after which growth acceleration towards the dark control rate was observed. When continuous red irradiation was started 50 or 70 h after sowing the dark control rate was reached 2.5 h after onset of irradiation and growth rate was little above this level during the next 2 h. With older coleoptiles (90 h after sowing) the growth rate recovery was only up to 50%of the dark control rate and a second phase in growth inhibition became detectable about 2.5 h after onset of red exposure, characterized by a continuous decrease in extension rate. Under R/FR pulse irradiation both the red-light-induced transient growth inhibition and the second phase of growth inhibition exhibit far-red reversibility up to the level of far-red induced growth rate changes.
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