Action Spectra for the Inhibition of Hypocotyl Growth by Continuous Irradiation in Light and Dark-Grown Sinapis alba L. Seedlings.

Action spectra for the inhibition by continuous (24-hour) irradiation of hypocotyl growth in 54-hour-old Sinapis alba L. seedlings were measured using seedlings which had had four different pretreatments. These seedlings were either (a) dark-grown with a high total phytochrome level, (b) dark-grown with a low total phytochrome level, (c) light-grown with chlorophyll, or (d) light-grown with no chlorophyll [treated with 4-chloro-5-(methylamino)-2-(alpha,alpha,alpha-trifluoro-m-tolyl)-3(2H) -pyridazinone (San 9789)].The resulting action spectra show that the blue, red, and far-red (716 nm) wavebands are most inhibitory for dark-grown plants with high phytochrome content, whereas hypocotyl growth in dark-grown plants with a low phytochrome content are only slightly inhibited by blue and far-red radiation. In light-grown plants, the effectiveness of blue and far-red light almost disappears. The position of red light effectiveness in chlorophyll-containing plants is shifted to lower wavelengths compared with those containing no chlorophyll.