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This article is a highlight of the study by Stamenkovic and Hanelt in this issue of Photochemistry and Photobiology describing the high-irradiance photophysiology of several strains of freshwater chlorophyte microalgal genus, Cosmarium. These strains exhibit distinct differences in how combined thermal- high irradiance stress is managed which can be related to the temperature and irradiance conditions of their native habitat.

— The mixed epidermal cell lymphocyte reaction (MECLR) is a commonly used method to study the effects of ultraviolet B (UVB) radiation on the skin immune system. In UVB experiments dosimetry is very important. The influence of irradiance on the MECLR was studied in vitro using Philips FS40 lamps with variable UV intensities. Irradiation of isolated epidermal cells with high irradiance impaired the alloactivating capacity more than irradiation with low irradiance. In vivo, the influence of long-term UVB exposure on the MECLR was studied by treating normal healthy volunteers with suberythemagenic doses of UVB thrice weekly during 4 weeks. The first set of experiments, using low irradiance Sylvania UV-21 F75/85 W lamps, resulted in a decrease of MECLR responses of 83.1%. In the second set of experiments performed a year later, employing an identical protocol except for the use of high irradiance Waldmann UV-21 F85/100 W lamps, an increase of MECLR responses of 99.7% was observed. Volunteers of both sets of experiments received equal doses of UVB. In conclusion, this study shows that in vitro UVB-induced suppression of the MECLR is critically dependent on irradiance and therefore might explain contradictory results described in the literature. The in vivo data suggest that, comparable to the in vitro experiments, irradiance may influence the effects of UVB irradiation in vivo. Further experiments should prove whether this is indeed the case.

Sensitivity of plants to UV-B radiation (280–315 nm) is often reduced at high background irradiance. Interpretation of plant responses to potential increases in solar UV-B requires improved understanding of interactions between UV-B and other environmental parameters. In this study, photosynthetically active radiation (PAR, 400–700 nm) was kept approximately constant (38 mol m-2 per day) while the daily blue light fluence (BL, 400–500 nm) was varied between 0.23 and 2.68 mol m-2. Two lines of cucumber (cvs Ashley and Poinsett) with differential sensitivity to UV-B were compared. At low BL, 3 days of UV-B treatment (21 kJ m-2 biologically effective radiation per 10 h per day) caused severe inhibition of growth in a developing leaf in both cultivars. Growth effects were detectable sooner and were accompanied by chlorotic lesions in the sensitive cultivar (cv Poinsett). Supplemental BL progressively reduced symptoms, consistent with an important role for BL photoreceptor(s) in prevention or repair of UV-B damage. Ultraviolet-induced increases in UV-absorbing compounds on an area basis were significant within 24 h of the start of the treatment but were independent of BL fluence over the range tested, suggesting that bulk accumulation of screening pigments did not contribute to BL-dependent amelioration of UV damage. However, BL did stimulate net increases in extractable UV-absorbing compounds on a total leaf busis, while high-performance liquid chromatography analysis indicated that BL and UV-B acted synergistically to increase specific components. Thus, the data do not necessarily exclude UV-absorbing compounds from an important role in overall UV-B protection nor do they rule out some more specific function for these compounds (e.g. antioxidants). Finally, BL effects on UV-B alteration of leaf growth and accumulation of UV-absorbing compounds were not saturated under the conditions used here, suggesting that BL may contribute to interactions between UV-B and natural levels of background irradiance. Caution is urged in the interpretation of data on UV-B effects obtained under conditions of low BL irradiance.

— The palmatcly-compound leaf lamina of Lupinus palaestinus Boiss. tracks the sun by independent diaphototropic movements of its individual leaflets, operated by the appropriate flexing of their pulvinules. The lamina also exhibits two additional, light-dependent nastic movements, that can operate concurrently with the diaphototropic response: (1) a nyctinastic response to light/dark transitions bv which all the leaflets fold down simultaneously and at equal rates, and which is reversed by a photonastic response to dark/light transitions by which all the leaflets unfold into a co-planar configuration; (2) a photonastic upward folding (“cupping”), which all the leaflets exhibit simultaneously and at equal rates in response to high irradiance. thereby modulating their diaphototropic response to reduce light interception and evade over-exposure. Although the individual leaflets can sense all these tight-signals and respond to them independently, they do so simultaneously, so that the entire lamina responds in a co-ordinated manner, as a unit. The photoreceptors for the two nastic responses are localized in the individual pulvinules and may or may not be the same. The photoreceptors for the diaphototropic perception are localized in the proximal segment of the lamina of the individual leaflets, adjacent to but outside the pulvinule. The separation of the sites of perception for the tropic and nastic responses allows these responses to operate concurrently. The capacity of the diaphototropic response to occur concurrently with one or the other of the two nastic responses may be accounted for by its inductive nature.

— The action of phytochrome on appearance of NADH-dependent glutamate synthase (NADH-GOGAT) and ferredoxin-dependent glutamate synthase (Fd-GOGAT) was studied in the cotyledons of the mustard (Sinapis alba L.) seedling. It was found that the [Pfr]-response curves are composed of two branches with two very different slopes (a1 a2). This explains the biphasic fiuence response curves reported previously. While a, is the same with both enzymes, a, is much higher in NADH-GOGAT than in Fd-GOGAT. However, the transition from the high (a1) to the low (a1) slope occurs at the same Pfr/Ptot ratio irrespective of the steepness of a1 While the appearance of NADH-GOGAT is very sensitive to small amounts of Prr (high a1), the response is not sensitive to the “High Irradiance Reaction” (HIR) of phytochrome. On the other hand, appearance of Fd-GOGAT (relatively low a1) exhibits the usual HIR. It is concluded that the presently available models of phytochrome action, including the dimeric model, are not adequate to account for the actual data. At present it appears that a multiplicity of primary actions of phytochrome exist, and that it is the availability of the primary reactant (X, 1,2,3) which determines transduction or non-transduction of the phytochrome signal.

The photoperiodic induction of bulbing in onion (Allium cepa L.) by long daylengths has an absolute requirement for far-red energy during the light period. Far-red light promotes bulbing most effectively during the middle portion of an 18-h inductive photoperiod, requiring about 5 times as much energy for the same response at the beginning and end of this photoperiod. The most effective wavelength for this promotion is 714 nm, suggesting that it is mediated through the high irradiance response (HIR) of phytochrome.

— The involvement of phytochrome in light-mediated anthocyanin synthesis in the mustard seedling (Sinapis alba L.) under inductive conditions (law of reciprocity valid) was shown previously (Drumm and Mohr, 1974). In the present paper the hypothesis (Hartmann, 1966) is checked that light-mediated anthocyanin synthesis in continuous high-irradiance far-red light (‘high-irradiance response’) is also due exclusively to phytochrome. The data indicate that the effectiveness of the far-red light is indeed a function of total phytochrome [Ptotal]* and therewith [Pfr]*. The data are not consistent with the suggestion (Schneider and Stimson, 1972) that photosynthesis (in particular, photosystem I) is involved in the ‘high-irradiance response’ of photomorphogenesis.

— In the two fungal species (Fusarium and Neurospora) carotenogenesis has been described as a blue light effect. Generally, phytochrome was believed to be absent in fungi. Recently, work was reported suggesting that this might not strictly be the case. Therefore, we very thoroughly tested these two organisms for any red light effect on carotenogenesis. No action of phytochrome was found, either as a ‘low irradiance response’ or a ‘high irradiance response’. Also, no sequential interaction between blue and red light and between UV and red light was observed. Therefore, we conclude that phytochrome is not involved in carotenogenesis in Fusarium and Neurospora.

— Spectral sensitivities for C16 rranshexadecenoic acid accumulation in etiolated leaves of Vigna sinensis L. show two maxima, one in the blue and one in the far red, indicating a control by the high irradiance reaction. Spectral sensitivities are different for rranshexadecenoic acid and chlorophyll accumulation indicating that the biosynthesis of these compounds must not be obligatory linked.