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

Abstract

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) darkgrown with a low total phytochrome level, (c) light-grown with chlorophyll, or (d) light-grown with no chlorophyll Itreated with 4-chloro-5-(methylamino)-2-(a,a,a-trifluoro-m-tolyl)-3(2H)-pyridazinone (San 9789)1.The resulting action spectra show that the blue, red, and far-red (716 to action spectrum, although this may be partially due to differences in irradiation time and mode (14,15). A loss of the response to FR light with increasing age or after a light pretreatment has also been reported (2,5,27). The identity of the photoreceptor(s) is not entirely clear. Candidates which have been considered are Chl/photosynthesis, phytochrome, and, for the B part of the spectrum, the "blue light receptor." Evidence suggests no direct role for photosynthesis (14), although it is possible that it may affect the final expression of responses in green plants. Hartmann (7-9) has shown that it is possible to explain the FR action peak on the basis ofphytochrome alone. The receptor(s) responsible for the B action maximum has not been identified.To clarify some of these points and also to determine the form of the action spectrum in green plants where screening by Chl will occur and total phytochrome will be lower than in dark-grown plants, action spectra for the inhibition of hypocotyl elongation in Sinapis alba L. seedlings have been measured. The measurements were carried out on (a) dark-grown plants with a high level of Ptot, (b) dark-grown plants with a low level of P,ot, (c) MATERIALS AND METHODSSinapis alba L. seeds (harvest 1975) were obtained from Asgrow Co., Freiburg-Ebnet, W. Germany, selected and sown on filter paper in plastic boxes as described by Mohr (19)

Phytochrome A null mutants of Arabidopsis display a wild-type phenotype in white light.

“…This observation is fully consistent with physiological studies indicating that light-labile phytochrome is responsible for the FR-mediated hypocotyl growth inhibition of etiolated seedlings (the so-called FR high-irradiance response) and that a light-stable phytochrome mediates the effect of R (e.g., Beggs et al, 1980). Thus, phytochrome A null mutants might be expected to exhibit an elongated hypocotyl when grown in FR.…”

Photophysiology of the Elongated Internode (ein) Mutant of Brassica rapa

“…Growth inhibition by prolonged FR probably reflects the operation of the high irradiance response (4), a response mode thought to be mediated by light-labile phytochrome A (21). Inhibition of hypocotyl elongation by prolonged R is thought to be, at least partly, mediated by a light-stable phytochrome species (4) and has been shown to be reduced in the phytochrome Bdeficient Arabidopsis hy3 mutant (11) and the phytochrome B-deficient cucumber lh mutant (2 putative phytochrome mutant, has also been reported to display aberrant phytochrome-controlled anthocyanin accumulation (6). Cotyledon expansion is phytochrome regulated in many etiolated seedlings, and light-dependent cotyledon expansion has been shown to be reduced in the phytochrome B-deficient cucumber Ih mutant (2).…”

Anthocyanin Production in Chl-Rich and Chl-Poor Seedlings

“…The differences in anthocyanin production be- (1,4,5). However, these inhibitors should not be used in physiological experiments without previous studies of their effects on the response/system combination: differences in the effects of the inhibitor can be quite large even for the same response in closely related systems, as shown in this report.…”

“…Streptomycin inhibits chloroplast development and Chl synthesis and enhances anthocyanin production in young cabbage seedlings (9). We used STM instead of Norflurazon, a more commonly used 'Chl-bleacher' (1,4,5), because the differences in the Chl levels of seedlings exposed to prolonged irradiations with different spectral regions are less pronounced in STM-treated than in Norflurazon-treated seedlings (7). Figure 1 are the means of 14 to 18 replicates in two independent experiments.…”

“…Large amounts of Chl are formed during the prolonged irradiations required to elicit maximum anthocyanin production (9,10). As a consequence of light attenuation due to screening by Chl, the photoconversion and cycling rates and the photoequilibrium ratios of phytochrome in vivo are different from those predicted on the basis of the spectral parameters of purified phytochrome and may cause distortions in the action spectra of photomorphogenesis (6) of seedlings have been used in photomorphogenesis research to decrease the effects of Chl screening on the state of phytochrome and to allow spectrophotometric assays of phytochrome in vivo in light-grown seedlings (1,4,5). We also wanted to check if a specific inhibitor action on the photoresponse studied might affect the interpretation of the results in terms of the expected differences between the states of phytochrome in Chl-rich and Chl-poor seedlings (6).…”