LIGHT-GROWN PLANTS OF THE CUCUMBER LONG HYPOCOTYL MUTANT EXHIBIT BOTH LONG-TERM AND RAPID ELONGATION GROWTH RESPONSES TO IRRADIATION WITH SUPPLEMENTARY FAR-RED LIGHT

Seedlings of wild type (WT) and the lh mutant of Cucumis sativus were grown in white light and given supplementary far-red radiation either to the whole plant, or via fiber-optic probes directed at the apical region of the hypocotyl. In both WT and mutant seedlings, reducing the ratio of red to far-red radiation resulted in significant increases in extension growth. Direct measurement of extension rate by transducer techniques revealed that the mutant seedlings responded to additional far-red radiation by an increment in extension rate that was equivalent to that observed with the WT seedlings. Assuming that the lh mutant is deficient in phytochrome B (Kendrick and Nagatani, The Plant Journal 1: 133–139, 1991), these results indicate that although phytochrome B clearly has an important role in the induction of the shade avoidance syndrome, it is unlikely to be the sole responsible receptor.     

PHOTOTROPISM AND AXIS EXTENSION IN LIGHT-GROWN MUSTARD (Sinapis alba L.) SEEDLINGS‡

Abstract— The relationship between phototropism and axis extension was examined in light-grown mustard (Sinapis alba L.) seedlings using the low pressure sodium lamp (SOX)? technique to eliminate growth responses due to phytochrome. Addition of blue light caused no net inhibition of hypocotyl elongation, but plants showed a phototropic response. Curvature was caused by a simultaneous inhibition of growth on the illuminated side of the hypocotyl and an acceleration on the shaded side. Phototropism thus occurs independently of axis elongation and suggests that they are two separate processes. The results are inconsistent with the Blaauw theory of phototropism.     

PHOTORESPONSES OF Arabidopsis SEEDLINGS EXPRESSING AN INTRODUCED OAT phyA cDNA: PERSISTENCE OF ETIOLATED PLANT TYPE RESPONSES IN LIGHT-GROWN PLANTS

The photocontrol of hypocotyl elongation has been studied in etiolated and light-grown wild type (WT) Arabidopsis thaliana (L. Heynh) seedlings, and in two homozygous isogenic lines that have been transformed with the oat phy A gene coding sequence under the control of the cauliflower mosaic virus (CaMV) 35S promoter. For etiolated seedlings the inhibition of hypocotyl elongation by continuous broad band far-red light (FR) is saturated at much lower photon fluence rates in the transgenic seedlings compared with WT seedlings. Furthermore, whereas de-etiolation of WT seedlings leads to loss of responsiveness of the hypocotyls to prolonged FR, de-etiolated transgenic seedlings continue to show a pronounced FR-mediated inhibition of elongation. This may reflect the persistence of a FR-high irradiance response (HIR) mediated by the introduced oat phytochrome A. Although the hypocotyls of light-grown transgenic seedlings display a qualitatively normal end-of-day FR growth promotion, such seedlings display an aberrant shade-avoidance response to reduced red:far-red ratio (R:FR). These results are discussed in relation to the proposal that the constitutive expression of phytochrome A leads to the persistence of photoresponse modes normally restricted to etiolated plants.     

PHOTOMORPHOGENIC RESPONSES TO UV RADIATION. II: A COMPARATIVE STUDY OF UV EFFECTS ON HYPOCOTYL ELONGATION IN A WILD-TYPE AND AN aurea MUTANT OF TOMATO (Lycopersicon esculentum Mill.)

Hypocotyl growth in etiolated seedlings of wild-type and an aurea mutant of tomato (Lycopersicon esculenturn Mill.), that appears to be deficient in labile phytochrome, is strongly inhibited by UV radiation in the region of 300–400 nm. The role of phytochrome in the UV-mediated inhibition of hypocotyl growth was studied using different experimental approaches: (1) by comparing the effectiveness of treatments of increasing duration of exposure to 692 nm and UV radiation; (2) by modifying the UV spectral range with specific cut-off filters. The experimental results suggest that the UV-induced inhibition of growth in wild-type tomato is mediated to a large extent by the longer wavelengths of the UV-A region and is mediated mainly by phytochrome. In contrast, at wavelengths < 305 nm a strong UV-B effect was found in the aurea mutant, suggesting a preeminent action of a specific UV-B absorbing photoreceptor that displays less action in the wild-type.     

Comparative photobiology of growth responses to two UV-B wavebands and UV-C in dim-red-light- and white-light-grown cucumber (Cucumis sativus) seedlings: physiological evidence for photoreactivation

We examined the influence of short-term exposures of different UV wavebands on the elongation and phototropic curvature of hypocotyls of cucumbers (Cucumis sativus L.) grown in white light (WL) and dim red light (DRL). We evaluated (1) whether different wavebands within the ultraviolet B (UV-B) region elicit different responses; (2) the hypocotyl elongation response elicited by ultraviolet C (UV-C); (3) whether irradiation with blue light-enriched white light (B/WL) given simultaneous with UV-B treatments reversed the effect of UV in a manner indicative of photoreactivation; and (4) whether responses in WL-grown plants were similar to those grown in DRL. Responses to brief (1-100 min) irradiations with three different UV wavebands all induced inhibition of elongation measured after 24 h. When WL-grown seedlings were irradiated with light containing proportionally greater short wavelength UV-B (37% of UV-B between 280 and 300 nm), inhibition of hypocotyl elongation was induced at a threshold of 0.5 kJ m(-2), whereas exposure to UV-B including only wavelengths longer than 290 nm (and only 8% of UV-B between 290 and 300 nm) induced inhibition of hypocotyl elongation at a threshold of 1.6 kJ m(-2). The UV-C treatment induced reduction in elongation at a threshold of <0.01 kJ m(-2) for DRL-grown plants and <0.03 kJ m(-2) for WL-grown plants. B/WL caused 50% reversal of the short-wavelength UV-B-induced inhibition of elongation in DRL-grown seedlings but did not reverse the effect of long-wavelength UV-B. B/WL caused 30% reversal of the UV-C-induced inhibition of elongation in WL-grown seedlings but did not affect the response to short-wavelength UV-B. Short-wavelength UV-B also induced positive phototropic curvature in both types of seedlings, and this was reversed 60% or completely in DRL-grown and WL-grown seedlings, respectively. The similarity of responses between the etiolated (DRL-grown) and de-etiolated (WL-grown) seedlings indicates that the short-wavelength specific response may be relevant to natural light environments, and the apparent photoreactivation implicates DNA damage as the sensory mechanism for the response.     

ACTION SPECTRA FOR INHIBITION OF HYPOCOTYL GROWTH OF WILD-TYPE PLANTS AND OF THE hy2 LONG-HYPOCOTYL MUTANT OF Arabidopsis thaliana L.

Abstract— An analysis was made by action spectroscopy, using the Okazaki Large Spectrograph, of the inhibition of hypocotyl elongation of wild-type plants and the hy2 mutant of Arabidopsis thaliana. Two day old etiolated seedlings were irradiated for 8 h with monochromatic light and left in the dark for 16 h before measurement of hypocotyl length. Spectrophotometric measurement showed that levels of phytochrome in the etiolated tissue of the hy2 mutant were less than 9% of those in the wild type. The action spectra of the wild type looked like those of high irradiance response and showed peaks at 375, 450, 625 and 725 nm, whereas the action spectra of hy2 showed only the peaks at 375 and 450 nm. Monochromatic light of wavelengths longer than 500 nm had no significant inhibitory effects on hy2 plants. Blue and UV-A light were about five times more effective in the wild type than in hy2 plants. Severe inhibitory effects were observed with UV-B light. It is concluded that inhibition of the growth of the hypocotyl involves combined actions of phytochrome and a putative blue/UV-A photoreceptor(s).     

RELATIVE IMPORTANCE OF BLUE LIGHT AND LIGHT ABSORBED BY PHYTOCHROME IN GROWTH OF MUSTARD (Sinapis alba L.) SEEDLINGS*

Abstract— Hypocotyl straight growth in mustard (Sinapis alba L.) responds very strongly and in precisely the same way to low fluence rate red (RL) and white light (WL). The effect of weak light can be attributed fully to light absorption by phytochrome. Only with increasing fluence rate an effect of blue light (BL) comes into play which cannot be explained by the action of phytochrome. However, this specific action of BL can be demonstrated in hypocotyl growth of mustard seedlings only up to 5 days after sowing (25°C). With older seedlings control of hypocotyl growth seems to be exerted exclusively via phytochrome. Regarding the far-red light dependent “high irradiance reaction” (FR-HIR) it was found that it plays a dominant role in growth of mustard only during a relatively short period. It tends to disappear in favor of a RL-HIR between 3 and 4 days after sowing. It is concluded that the seedling exhibits a largely endogenous temporal pattern of responsiveness to light. Phototropism of the mustard seedling can be elicited by low fluence rates (< 1 mW m^?2) of unilateral BL. This same light has no effect on straight growth. It is concluded that BL-dependent phototropic growth response of a hypocotyl and the effect of BL on longitudinal growth of the hypocotyl are unrelated phenomena.     

PHOTOCONTROL OF HYPOCOTYL ELONGATION IN DE-ETIOLATED Cucumis sativus L. RAPID RESPONSES TO BLUE LIGHT

Abstract— Rapid inhibition of hypocotyl extension of de-etiolated Cucumis sativus L. by blue light is described, and compared with responses to white fluorescent light. Rapid inhibition of hypocotyl growth by blue light via the specific blue light photoreceptor requires a minimum Buence rate. Above this minimum value rapid modulations of growth rate accompany changes in blue light fluence rate. An initial response to blue light, or to a step-up in blue fluence rate takes less than 5 min. A drop from high to low fluence rate blue light (or to darkness) is followed by a recovery of the growth rate after about 20 min. A change from a low fluence rate to darkness elicits a more rapid recovery of growth rate (within 7 min). Similar responses were obtained in seedlings de-etiolated for a few hours and for several days.     

POTENTIAL ERRORS IN THE USE OF CELLULOSE DIACETATE AND MYLAR FILTERS IN UV-B RADIATION STUDIES

Abstract— The increase in UV-B radiation(290–320 nm) penetrating to the earth's surface as a result of the chemical depletion of the stratospheric ozone layer is an important environmental concern. In most studies using artificial UV-B sources, the determination of enhanced UV-B radiation effects on plants relies on equivalent UV-A radiation(320–400 nm) from the experimental UV-B fluorescent lamp source, filtered with either cellulose diacetate (CA) to create UV-B treatments, or with type S Mylar or polyester (PE) to create controls (no UV-B). The spectral irradiance in the UV-A was measured in the dark below lamps at two daily UV-B irradiance levels (14.1 and 10.7 W m^-2) with CA and PE at two ages. Highly significant differences in UV-A radiation (P 0.01) were measured below the treatment/control pairs at both fluence rates and filter ages. Filter aging was observed, which reduced the UV-A irradiance, especially for PE. The total daily ambient UV-A irradiance was also determined in the glasshouse at three seasons: the fall equinox, summer and winter, from which the total daily UV-A (lamp + ambient) irradiances were calculated. The addition of low to moderate ambient irradiance removed the treatment/control differences in the longwave UV-A(350–400 nm); however, the treatment/contro1 differences remained in the shortwave UV-A(320–350 nm), which was restricted by the glass, and in the total UV-A. The treatment/control differences persisted in the shortwave UV-A for the higher irradiance level, even under high summer ambient light. Also, spectral ratios (UVB:UV-A and shortwave: longwave UV-A) for all treatment groups decreased as the ambient UV-A radiation increased. Therefore, a range of experimental conditions exist where PE-covered lamps do not provide adequate control for UV-A irradiance, relative to the CA treatment, for glasshouse/growth chamber experiments. Potential complications in the interpretation of plant response exist for UV-B experiments conducted under low ambient light conditions (e.g. growth chambers; glasshouse in winter) or high daily UV-B irradiances (e.g. 14 kJ m^-2) for those plant responses that are sensitive to UV-A radiation.     

PHOTOCONTROL OF HYPOCOTYL ELONGATION IN LIGHT-GROWN Cucumis sativus L. PHOTOSYNTHETIC REQUIREMENT FOR A FLUENCE RATE DEPENDENT PHYTOCHROME RESPONSE

We investigated the role of photosynthesis in the photocontrol of extension growth of the hypocotyl of light-grown Cucumis sativus L. Previous work [Gaba and Black (1985b) Plant Physiol. 79 , 1011] demonstrated that the inhibition of cucumber hypocotyl elongation is a fluence rate dependent response in red light. However, the relative contributions of phytochrome and photosynthesis to the photon flux dependent inhibition response were not clear. Here we have shown that photoperception by the foliar cotyledons as well as the hypocotyl itself are responsible for fluence rate dependence in red light. The inhibitor of photosynthesis diuron [3-(3,4-dichlorophenyl)-1, 1-dimethylurea] reduced both the magnitude of inhibition and the fluence rate dependency in red light, indicating an involvement of photosynthesis. Furthermore, the growth of non-pigmented seedlings (treated with the herbicide norflurazon) was less inhibited by red light, with no fluence rate dependency. In particular, inhibition due to cotyledon photoperception was completely lost in non-pigmented (norflurazon-treated) plants, and much reduced by diuron treatment. Hypocotyl-perceived red light inhibition was only slightly reduced by treatment with norflurazon and diuron. Photosynthesis was compared directly to photo-inhibition of growth: the photon flux response curve of oxygen evolution of green Cucumis cotyledons was distinctly different from that of hypocotyl inhibition. In conclusion, photosynthesis is an essential requirement for the cotyledon-perceived inhibition, but the response itself is not due to photosynthesis.     

Phytochrome Control of Anthocyanin Biosynthesis in Tomato Seedlings: Analysis Using Photomorphogenic Mutants

Anthocyanin biosynthesis has been studied in hypocotyls and whole seedlings of tomato (Lycoperskon esculentum Mill.) wild types (WTs) and photomorphogenic mutants. In white light (WL)/dark (D) cycles the fri¹ mutant, deficient in phytochrome A (phyA), shows an enhancement of anthocyanin accumulation, whereas the tri¹ mutant, deficient in phytochrome Bl (phyBl) has a WT level of anthocyanin. Under pulses of red light (R) or R followed by far-red light (FR) given every 4 h, phyA is responsible for the non-R/FR reversible response, whereas phyBl is partially responsible for the R/FR reversible response. From R and blue light (B) pretreatment studies, B is most effective in increasing phytochrome responsiveness, whereas under R itself it appears to be dependent on the presence of phyBl. Anthocyanin biosynthesis during a 24 h period of monochromatic irradiation at different flu-ence rates of 4 day-old D-grown seedlings has been studied. At 660 nm the fluence rate-response relationships for induction of anthocyanin in the WT are similar, yet complex, showing a low fluence rate response (LFRR) and a fluence rate-dependent high irradiance response (HIR). The high-pigment-1 (hp-1) mutant exhibits a strong amplification of both the LFRR and HIR. The fri¹ mutant lacks the LFRR while retaining a normal HIR. In contrast, a transgenic tomato line overexpressing the oat PHYA3 gene shows a dramatic amplification of the LFRR. The tri¹ mutant, retains the LFRR but lacks the HIR, whereas the fri¹, tri¹ double mutant lacks both components. Only an LFRR is seen at 729 nm in WT; however, an appreciable HIR is observed at 704 nm, which is retained in the tri¹ mutant and is absent in the fri¹ mutant, indicating the labile phyA pool regulates this response component.     

RESPONSE OF LIGHT-GROWN WILD-TYPE and LONG HYPOCOTYL MUTANT CUCUMBER PLANTS TO END-OF-DAY FAR-RED LIGHT

Abstract— A long-hypocotyl mutant ( lh ) of cucumber ( Cucumis sativus L.) has been studied which has previously been shown to lack phytochrome control of growth in de-etiolated seedlings and thought to be modified with respect to the light-stable type of phytochrome. We have analyzed the response of lh mutant and isogenic wild-type (WT) plants to daily treatment with end-of-day far-red light (EODFR). Only the WT responded to this treatment resulting in a large increase in internode length; an increase in petiole length; changes in leaf development (increased area, decreased thickness and reduction in indentation); redistribution of dry matter from leaf blades to stem; increased apical dominance and promotion of tendril formation. There were only small or no significant effects on chlorophyll and total carotenoid content, chlorophyll alb ratio, soluble protein levels and net photosyn-thetic rates. The lh mutant failed to respond to EODFR treatment, and had the appearance of a shade-avoiding plant growing in extreme shade. The lh mutant appears to completely lack the phytochrome responses attributable to the type of phytochrome that is active in shade detection. A discussion of the possible roles of the stable and labile types of phytochrome in light grown plants follows.     

PHYSIOLOGICAL RELATIONSHIPS BETWEEN PHYTOCHROME EFFECTS ON INTERNODE EXTENSION GROWTH AND DRY MATTER ACCUMULATION INLIGHT-GROWN MUSTARD

The physiological relationships between the effects of phytochrome photoequilibrium (Pfr/P) on internode extension growth and dry matter accumulation were investigated in white light (WL)-grown Sinapis alba L. seedlings. After 11 days under continuous WL, the seedlings were exposed: (a) to pulses of light providing different Pfr/P, followed by 24 h darkness (D); (b) to pulses of light providing different Pfr/P, followed by 3 h D and 24 h continuous WL; (c) to continuous WL with or without supplementary far-red light (to reduce Pfr/P); or (d) to pulses of light providing different Pfr/P followed by D, in factorial combination with either water or a saturating (0.2 M) sucrose solution applied to one of the leaves. In D (“a” and “d”) low, compared to high Pfr/P increased both internode extension growth and dry weight to the same extent. Under WL (“b” and “c”) low PfrlP promoted internode extension growth but had no proportional effects on internode dry weight. Sucrose promoted internode extension growth with a lag of at least 8 h (compared to the rapid effect of low Pfr/P) and did not reduce the effect of low Pfr/P. These results indicate that Pfr/P effects on internode extension growth are not the consequence of changes in photoassimilate translocation from the leaves. Under WL, PfdP effects on internode length occur partially at the expense of internode dry matter per unit length.     

PHOTOREGULATION OF ROOT: SHOOT RATIO IN SOYBEAN SEEDLINGS*

Abstract— The partitioning of plant growth between shoot and root has the potential to affect diverse physiological processes including water and nutrient uptake, nitrogen fixation, light interception, and interactions between plant and soil microorganisms. Root: shoot ratio is determined both by genetics and developmental status as well as by availability of water, nutrients and light. It is shown here that relative root growth was modulated by photomorphogenetic treatments designed to affect phytochrome (supplemental far-red radiation given either as an end-of-day treatment or continuously during the photoperiod) or blue light photoreceptors (blue light-deficient low pressure sodium lamps ± low irradiances of supplemental blue [i.e. 5% of total photon flux: 25 μ.mol m^?2 s^?1]). Photomorphogenetic control of root: shoot ratio was apparent within1–2 days when light treatments were initiated at emergence, and did not necessarily involve changes in net seedling growth. On the other hand, shortened daylength inhibited early seedling growth but had little effect on partitioning. Changes in relative root dry matter induced by supplemental far red radiation or blue light deficiency were similar to those caused by low irradiances, suggesting that phytochrome or blue light photoreceptors may be involved in regulating the partitioning of growth between shoot and root as a part of adaptation to vegetation shade. The influence of spectral quality on root: shoot ratio should be considered when comparing plants grown under different types of lighting or with different spacing.     

REGULATION OF CHLOROPLAST DEVELOPMENT BY RED AND BLUE LIGHT

There are specific differences between red and blue light greening of etiolated seedlings of Hordevm vulgare L. Blue light results in a different prenyl lipid composition of chloroplast as compared to red light of equal quanta density. This is documented by a much higher prenylquinone content, higher chlorophyll a/b ratios, and lower values for the ratio xanthophylls to carotenes (x/c). The photosynthetic activity of “blue light” chloroplasts (Hill reaction) is higher than that of “red light” chloroplasts. These differences in prenylquinone composition and Hill-activity are associated with a different ultrastructure of chloroplasts. “Red light” chloroplasts exhibit a much higher grana content than “blue light” chloroplasts. The difference in thylakoid composition, photosynthetic activity and chloroplast structure found between blue and red light greening are similar to those found between sun and shade leaves and those between plants grown under high and low light intensities.     

BLUE AND ULTRAVIOLET-B LIGHT PHOTORECEPTORS IN PARSLEY CELLS

Abstract— Ultraviolet-B (UV-B) and blue light photoreceptors have been shown to regulate chalcone synthase and flavonoid synthesis in parsley cell cultures. These photoreceptors have not yet been identified. In the present work, we studied UV-B photoreception with physiological experiments involving temperature shifts and examined the possible role of flavin in blue and UV-B light photoreception. Cells irradiated with UV-B light (0.5–15 min) at 2°C have the same fluence requirement for chalcone synthase and flavonoid induction as controls irradiated at 25°C. This is indicative of a purely photochemical reaction. Cells fed with riboflavin and irradiated with 6 h of UV-containing white light synthesize higher levels of chalcone synthase and flavonoid than unfed controls. This effect did not occur with blue light. These results indicate that flavin-sensitization requires excitation of flavin and the UV-B light photoreceptor. The in vivo kinetics of flavin uptake and bleaching indicate that the added flavin may act at the surface of the plasma membrane. In view of the likely role of membrane-associated flavin in photoreception, we measured in vitro flavin binding to microsomal membranes. At least one microsomal flavin binding site was solubilized by resuspension of a microsomal pellet in buffer with high KPi and NaCl concentrations and centrifugation at 38000 g. The 38000 g insoluble fraction had much greater flavin binding and contained a receptor with an apparent K_D of about 3.6 μM and an estimated in vivo concentration of at least 6.7 × 10–⁸M. Flavin mononucleotide, roseoflavin, and flavin adenine dinucleotide can compete with riboflavin for this binding site(s), although each has lower affinity than riboflavin. Most microsomal protein was solubilized by resuspension of the microsomal pellet in non-denaturing detergents and centrifugation at 38 000 g ; however, this inhibited flavin binding, presumably because of disruption of the environment of the flavin receptor. The parsley microsomal flavin binding receptor(s) have a possible role in physiological photoreception.     

Effect of ultraviolet-B radiation on salt marsh vegetation: Trends of the genus Salicornia along the Americas

The effects of natural UV-B radiation on growth, photosynthetic and photoprotective pigment composition of different Salicornia species were analyzed in salt marshes at three different sites along the Americas (Puerto Rico, southern Brazil and Patagonia, Argentina). Plants were exposed to different levels of UV-B radiation for 1-2 years in situ as well as in outdoor garden UV-B exclusion experiments. Different UV-B levels were obtained by covering plants with UV-B opaque (blocked 93-100% of ambient UV-B) and UV-B attenuating (near-ambient) filters (reduced 20-25% of UV-B). Unfiltered plants were exposed to natural irradiance. UV-B filters had significant effects on temperature and photosynthetic pigments (due to changes in PAR; 400-700 nm). The growth of Salicornia species was inhibited after 35 to 88 days of exposure to mean UV-B radiation dosages between 3.6 and 4.1 kJ m(-2) day(-1). The highest number of branches on the main shoot (S. bigelovii and S. gaudichaudiana) and longest total length of the branches (S. gaudichaudiana) were observed in the UV-B opaque treatment. Salicornia species responded to increasing levels of UV-B radiation by increasing the amount of UV-B absorbing pigments up to 330%. Chromatographic analyses of seedlings and adult S. bigelovii plants found seven different UV-B absorbing flavonoids that are likely to serve as UV-B filtering pigments. No evidence of differential sensitivity or resilience to UV-B radiation was found between Salicornia species from low-mid latitudes and a previously published study of a high-latitude population.     

Gradient formation of anthocyanin in seedlings of Fagopyrum and Sinapis unilaterally exposed to red and far-red light

Abstract— Anthocyanin synthesis was measured in hypocotyl halves excised from Fagopyrum and Sinapis seedlings irradiated unilaterally or equilaterally with red or far-red light. Although no phototropic curvatures are produced by red or far-red exposure, a significant gradient in anthocyanin formation was observed for Fagopyrum seedlings and the trend for gradient formation is present in Sinapis seedlings. The gradient production in Fagopyrurn is correlated with this tissue's greater optical density. Since the intensities used do not inhibit elongation completely (through the phytochrome system) and an intensity gradient is present in the tissue (as evidenced by anthocyanin formation) it is concluded that (a) a steeper light gradient is required to induce phototropic curvatures or (b) a diffusible material affecting elongation growth prevents any differential from being established across the tissue.     

MODE OF COACTION OF PHYTOCHROME AND BLUE LIGHT PHOTORECEPTOR IN CONTROL OF HYPOCOTYL ELONGATION

Abstract The rate of hypocotyl longitudinal growth in seedlings of Sesamum indicum L. is strongly inhibited by continuous blue light (cBL)† and slightly by continuous far-red light while continuous red light (cRL) or red light pulses are hardly effective from 60 h after sowing onwards. Between 36 and 60 h after sowing the growth rate responds to red light pulses the effect of which is fully reversible by long wavelength far-red light. When seedlings are kept in cBL for 3 days and then treated with red light hypocotyl growth rate responds strongly. However, RL effectiveness decreases with time after transfer from BL to RL. BL → darkness transfer experiments with different levels of P_fr established at the beginning of darkness show that after a BL pretreatment phytochrome (P_fr) alone is capable of fully controlling growth rate. When white light (WL) is given no BL effect is detectable in weak WL. Only high light fluxes maintain a typical BL growth rate. At medium WL fluxes elongation rate returns gradually to the dark rate. The simplest explanation of the data is that light absorbed by a separate BL photoreceptor is necessary to maintain responsivity to P_fr. With increasing age of the seedlings the requirement for BL increases strongly. On the other hand, brief light pulses—given to demonstrate photoreversibility of phytochrome—remain equally effective provided that responsivity to P_fr exists.     

THE EFFECT OF PROLONGED LIGHT EXPOSURE ON THE EFFECTIVENESS OF PHYTOCHROME IN ANTHOCYANIN SYNTHESIS IN TOMATO SEEDLINGS*

Abstract— The hypocotyl of the tomato ( Lycopersicon esculentum ) seedling synthesizes large amounts of anthocyanin if exposed to prolonged light. Single light pulses are totally ineffective. The involvement of phytochrome can be shown by light pulse treatments following a prolonged light exposure. It is predominantly the action of blue/UV light which leads to a high responsiveness of anthocyanin synthesis towards phytochrome. Moreover, the data suggest a phytochrome-independent action of blue/UV light, in particular of UV-B, on anthocyanin synthesis.