Action Spectrum
An action spectrum is a graph of the rate of biological effectiveness plotted against wavelength of light.[1] It is related to absorption spectrum in many systems. Mathematically, it describes the inverse quantity of light required to evoke a constant response. It is very rare for an action spectrum to describe the level of biological activity, since biological responses are often nonlinear with intensity.
action spectrum
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Action spectra are typically written as unit-less responses with peak response of one, and it is also important to distinguish if an action spectrum refers to quanta at each wavelength (mol or log-photons), or to spectral power (W).
It shows which wavelength of light is most effectively used in a specific chemical reaction. Some reactants are able to use specific wavelengths of light more effectively to complete their reactions. For example, chlorophyll is much more efficient at using the red and blue regions than the green region of the light spectrum to carry out photosynthesis. Therefore, the action spectrum graph would show spikes above the wavelengths representing the colours red and blue.
The first action spectrum was made by T. W. Engelmann, who split light into its components by the prism and then illuminated Cladophora placed in a suspension of aerobic bacteria. He found that bacteria accumulated in the region of blue and red light of the split spectrum. He thus discovered the effect of the different wavelengths of light on photosynthesis and plotted the first action spectrum of photosynthesis.[2]
The photopigment in the human eye that transduces light for circadian and neuroendocrine regulation, is unknown. The aim of this study was to establish an action spectrum for light-induced melatonin suppression that could help elucidate the ocular photoreceptor system for regulating the human pineal gland. Subjects (37 females, 35 males, mean age of 24.5 +/- 0.3 years) were healthy and had normal color vision. Full-field, monochromatic light exposures took place between 2:00 and 3:30 A.M. while subjects' pupils were dilated. Blood samples collected before and after light exposures were quantified for melatonin. Each subject was tested with at least seven different irradiances of one wavelength with a minimum of 1 week between each nighttime exposure. Nighttime melatonin suppression tests (n = 627) were completed with wavelengths from 420 to 600 nm. The data were fit to eight univariant, sigmoidal fluence-response curves (R(2) = 0.81-0.95). The action spectrum constructed from these data fit an opsin template (R(2) = 0.91), which identifies 446-477 nm as the most potent wavelength region providing circadian input for regulating melatonin secretion. The results suggest that, in humans, a single photopigment may be primarily responsible for melatonin suppression, and its peak absorbance appears to be distinct from that of rod and cone cell photopigments for vision. The data also suggest that this new photopigment is retinaldehyde based. These findings suggest that there is a novel opsin photopigment in the human eye that mediates circadian photoreception.
1. Non-image forming, irradiance-dependent responses mediated by the human eye include synchronisation of the circadian axis and suppression of pineal melatonin production. The retinal photopigment(s) transducing these light responses in humans have not been characterised. 2. Using the ability of light to suppress nocturnal melatonin production, we aimed to investigate its spectral sensitivity and produce an action spectrum. Melatonin suppression was quantified in 22 volunteers in 215 light exposure trials using monochromatic light (30 min pulse administered at circadian time (CT) 16-18) of different wavelengths (lambda(max) 424, 456, 472, 496, 520 and 548 nm) and irradiances (0.7-65.0 microW cm(-2)). 3. At each wavelength, suppression of plasma melatonin increased with increasing irradiance. Irradiance-response curves (IRCs) were fitted and the generated half-maximal responses (IR(50)) were corrected for lens filtering and used to construct an action spectrum. 4. The resulting action spectrum showed unique short-wavelength sensitivity very different from the classical scotopic and photopic visual systems. The lack of fit (r(2) or =0.73). Of these, the best fit was to the rhodopsin template with lambda(max) 459 nm (r(2) = 0.74). 5. Our data strongly support a primary role for a novel short-wavelength photopigment in light-induced melatonin suppression and provide the first direct evidence of a non-rod, non-cone photoreceptive system in humans.
Although many action spectra for plant blue-light responses exist in the literature, their interpretation is complicated by the fact that multiple plant photoreceptors in addition to cryptochromes are active in blue light. Unrelated blue-light photoreceptors, distinct from the cryptochromes, have recently been identified and shown to be flavoproteins, absorbing maximally at 450 nm (Briggs and Huala, 1999; Briggs et al., 2001). In addition, plant phytochrome photoreceptors, which respond principally to red/far-red light, show some absorption of blue light, rendering the interpretation of classical blue-light action spectra more difficult (Shinomura et al., 2000; Shinomura et al., 1996). In the case of higher plants, the situation is even further complicated by the observation that a signal from phytochrome is necessary for full activity of the cryptochrome blue-light photoreceptors in hypocotyl growth inhibition and anthocyanin accumulation (Ahmad and Cashmore, 1997). Therefore, it is not possible to infer that any given published blue-light action spectra represents the activity of cryptochrome.
A concern with the interpretation of action spectra in de-etiolated plant material is shading by non-photoreceptor pigments, in particular chlorophylls and carotenoids, both of which absorb in blue light. It had been previously shown for both Sinapis alba (Beggs et al., 1980) and Chenopodium rubrum (Holmes and Wagner, 1982) that chlorophyll significantly altered the action spectrum of hypocotyl growth inhibition in these plant species. To correct for possible shading artifacts, action spectra for seedling growth inhibition were repeated on petri plates containing the herbicide norflurazon, an inhibitor of carotenoid biosynthesis that also prevents chlorophyll accumulation in light-grown plants (Beggs et al., 1980; Holmes and Wagner, 1982).
In overall shape and fluence threshold, the data we present for wild-type seedlings are in agreement with the results from prior studies investigating hypocotyl growth in Arabidopsis as a function of wavelength, involving a comparison of wild-type with phytochrome-deficient hy2 mutant seedlings (Goto et al., 1993) and to blu1, an allele of hy4 (deficient in CRY1; Young et al., 1992). However, the action spectra obtained from norflurazon-treated seedlings, though similar in shape to those of untreated seedlings for wild type, showed significantly greater sensitivity at all light intensities, in agreement with prior observations for other plant species (Beggs et al., 1980; Holmes and Wagner, 1982).
Absorption spectra of purified blue-light photoreceptors. A, Absorption spectrum of CRY1 protein expressed in baculovirus-infected Sf9 insect cells. Characteristics of absorption spectra are as in published plant cryptochrome spectra (Malhotra et al., 1995). B, Absorption spectrum of amino terminal NPL1 or PHOT-2 protein fragment expressed in E. coli. Characteristics of absorption spectra are as in published NPH1 spectra (Christie et al., 1998; Salomon et al., 2000).
An alternative explanation for the lack of correspondence between absorption spectra and action spectra may be complex events downstream of the point of photoreception, which distort the action spectra. This is particularly a possibility with action spectra involving long periods of irradiation (HIR spectra). To determine whether distortion might occur over the period of total irradiation, we measured degree of growth inhibition after a shorter (30 h) growth period for seedlings in 450 nm light, but observed no significant distortion at least after the 1st d of growth (not shown). Nevertheless, only a much more rapid assay for cryptochrome function can definitively exclude the possibility of artifact in these action spectra. It would also be preferable to devise an assay for cryptochrome function in dark-grown seedlings, because in this way, indirect light effects on concentrations of other photoreceptors and/or signaling intermediates may be avoided.
Evidence of wavelength-specific effects on the cryptochrome photoreceptors is found in the differential stability of CRY2 protein. Under broad band blue-light conditions, CRY2 protein is rapidly degraded and accumulates to only very low concentrations in seedlings. This may be due to targeting by degradative enzymes upon activation of the photoreceptor, by analogy to the situation with phyA. However, under the narrow band light conditions used in these action spectra, CRY2 protein appears to be more stable and accumulates even at relatively high blue-light intensities. It is possible that cryptochromes may cycle between multiple conformations upon activation by light and that only one of these forms is recognized by protein degradative enzymes. It will be intriguing to examine whether combinations of monochromatic light of different wavelengths reduce the stability of CRY2 protein and whether the conformation of CRY2 photoreceptor under monochromatic light conditions differs from that under broad band blue light. 041b061a72