A rapid phytochrome-mediated growth response in etiolated Sinapis alba hypocotyls

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Abstract
Photocontrol of hypocotyl elongation in etiolated Sinapis alba L. seedlings was investigated using a high-resolution growth-measurement system. Different photoresponses were characterized by the dynamics of plant response to monochromatic irradiations at wavelengths ranging from 380 to 780 nm. Brief puises of 660- or 670-nm red light caused a large, rapid inhibition of growth rate after a lag of ca. 5 min. Growth rate remained depressed for several hours following a single red light pulse. If given 120 min or less after the red light, a 740-nm far-red pulse reversed the inhibitory effects of prior red light treatment after a lag of ca. 6 min. Most seedlings did not respond to single far-red irradiations in the 720- to 780-nm wave band, although some underwent small depressions in growth 5 to 10 min after the end or irradiation. Irradiation with 450-nm blue light caused a deeper inhibition than red light after a lag of only 1 min. Recovery from inhibition by blue light was rapid, unless the irradiation was prolonged. Removal of the plumule and cotyledons did not affect the dynamics of the rapid blue and red – far-red growth responses. The rapid response to red light occurred at wavelengths from 550 to 710 nm. The rapid blue response occurred only from 380 to 500 nm. Within each of these wave bands, the depth of photoinhibition was nearly constant. However, the duration of photoinhibition by red light declined from about 3 h after a 660-nm pulse to about 45 min after a 710-nm pulse. Elongation in etiolated Sinapis hypocotyls is thus controlled by a classical phytochrome-mediated induction-response system and by the blue light photoreceptor. Photoinhibition of hypocotyl elongation by red light required a minimum of about 5% of total phytochrome to be in the far-red absorbing form.