Optical properties of Lactuca and Taraxacum seed and fruit coats: Their role as light filters

The optical properties of seed and fruit coats were examined from several varieties of light-sensitive achenes. Taraxacum vulgare L. and Lactuca sativa L. cv. Grand Rapids achenes with dark fruit coats and L. sativa cvs Huvudsallat and Issallat with white fruit coats were examined. Transmission spectra varied among the different achenes: white fruit coats of Lactuca acted as neutral density filters between 450 and 780 nm, whereas Taraxacum transmitted 2–36% in this region. The ribbed fruit coat structure greatly affected transmission so that at different locations in the same coat, transmission varied between 20 to 80% at 660 and 730 nm. Fruit coats of Grand Rapids lettuce and Taraxacum transmitted more far-red than red light with T₆₆₀/T₇₃₀ ratios of 0.8 and 0.4, respectively. The relationship between the optical properties of fruit coats and light-stimulated germination is discussed.     

The photoreceptors in the high irradiance response of plants

Several studies show that the high irradiance response (HIR) of plants is probably due to two photoreceptors. One of the photoreceptors is phytochrome, and the other is an unidentified pigment provisionally named heliochrome. One of the functions of heliochrome is the synthesis of phytochrome, using far-red and blue radiations of high intensities, to replace the phytochrome destroyed by light. Another possible function could be an interaction of heliochrome with a substance produced by phytochrome. The data presented show that heliochrome is a pigment with different properties from phytochrome. It shows a far-red/green reversibility. Heliochrome has been shown to participate with phytochrome in such HIRs as leaf movement in Albizzia and flowering in a long-day plant. The first event initiated by phytochrome and by heliochrome could be the generation of a strong positive, electrostatic charge in the cell membrane.     

The effect of SAN 9789 and light on the germination of seeds from Taraxacum vulgare

Photoblastic seeds (achenes) of Taraxacum vulgare coll. were treated with a water solution of SAN 9789, 4-chloro-5 (methylamino) -2- (α,α,α-trifluoro- m -tolyl) -3(2H) pyridazinone. SAN-treatment increased the germination in darkness from 0 to 12%. An irradiation for 5 min with red light, giving a germination of 12% for seeds in water only, gave together with SAN treatment a germination of 60%. In both water and SAN, the effect of red irradiation could be reversed by a short irradiation (15 min) of far-red light. If far-red light was repeatedly given (5 min per h) it had hardly any effect on germination in water (4% germination), but for seeds in SAN solution, intermittent far-red light had a stimulating effect (63% germination). If far-red light was given continuously for 96 h, the germination in water was 1% and in SAN solution 17%. The results in the present paper indicate that SAN may broaden the concentration interval of P_fr for which germination is high.     

Effect of light on seed germination of eight wetland Carex species

BACKGROUND AND AIMS: In wetland plant communities, species-specific responses to pulses of white light and to red : far-red light ratios can vary widely and influence plant emergence from the seed bank. Carex species are the characteristic plants of sedge meadows of natural prairie wetlands in mid-continental USA but are not returning to restored wetlands. Little is known about how light affects seed germination in these species-information which is necessary to predict seed bank emergence and to develop optimal revegetation practices. The effects of light on germination in eight Carex species from prairie wetlands were investigated. METHODS: Non-dormant seeds of eight Carex species were used to determine the influence of light on germination by examining: (a) the ability of Carex seeds to germinate in the dark; (b) the effect of different lengths of exposures to white light on germination; (c) whether the effect of white light can be replaced by red light; and (d) whether the germination response of Carex seeds to white or red light is photoreversible by far-red light. KEY RESULTS: Seeds of C. brevior and C. stipata germinated >25 % in continuous darkness. Germination responses after exposure to different lengths of white light varied widely across the eight species. Carex brevior required <15 min of white light for > or =50 % germination, while C. hystericina, C. comosa, C. granularis and C. vulpinoidea required > or =8 h. The effect of white light was replaced by red light in all species. The induction of germination after exposure to white or red light was reversed by far-red light in all species, except C. stipata. CONCLUSIONS: The species-specific responses to simulated field light conditions suggest that (a) the light requirements for germination contribute to the formation of persistent seed banks in these species and (b) in revegetation efforts, timing seed sowing to plant community development and avoiding cover crops will improve Carex seed germination.     

The effects of SAN 9789 and light on phytochrome and the germination of lettuce seeds

Photoblastic seeds (akenes) of lettuce (Lactuca sativa (L.) cv. Grand Rapids) were treated with SAN 9789 [4-chloro-5-(methylamine)-2-a, a, a,-trifluoro-m-tolyl-3-(2H)-pyridasinone]. The seeds weere placed in Petri dishes on filter paper soaked with water or SAN solution. The treatment increased the germination in darkness from 17% for water to 78% for SAN treated seeds. An irradiation with 5 min red light gave a germination of 98% both in water and in SAN. In water the effect of red irradiation could be reversed with a short irradiation (8 min) of far red light (17% germination), while in SAN solution the far red reversibility was poor (92% germination). If the far red light was given repeatedly (5 min per h) it had a slightly larger effect. If given continuously for 24 hours, the germination in water was decreased to 0.3% and in SAN solution to 9%. Possible mechanisms for the SAN effect are discussed.     

Effects of red light on the fluence–response relationship for pulse-induced phototropism of maize coleoptiles

Dark-adapted coleoptiles of maize (Zea mays L.) were treated with red light (3min at 10.5 μmol m^?2S^?1) and were Stimulated, after a dark interval, with a pulse of unilateral blue light to induce phototropism. Phototropic fluence-response curves were obtained in this way for different dark intervals. It was confirmed that the bell-shaped fluence-response curve for the first pulse-induced positive phototropism (FPIPP) shifts to higher fluences following the red-light treatment, the maximal shift being achieved at a dark interval of 2h. We found, however, that the two arms of the Fluence-response curve do not shift synchronously. The shift of the descending arm to higher fluences began at 15 min. The ascending arm showed a slight shift to lower fluences before a greater shift to higher flucnces. the change of the shift direction occurring at 30–40min. Accordingly, the fluence-response curve obtained for a 30 min dark interval was comparatively wide. Although dark-adapted coleoptiles showed only fPIPP, another bell-shaped fluence-response curve, representing the second pulse-induced positive phototropism (sPIPP), appeared gradually after the red-light treatment. These changes of the phototropic fluence– respnse curve following exposure to red light are likely to have adaptive values because they favour phototropism under brighter light.     

Penetration of light into soil and its role in the control of seed germination

Abstract Light transmitted through soil, and the leaf litter of two tree species, was measured using a spectroradiometer. In general, a greater penetration by longer wavelengths, especially far-red, was noted. This was most marked in a dry sand sample. The effect was less in the case of clay-loam aggregates. Moisture in the samples had an opposite effect in these two cases, causing an increase in transmission through sand but a decrease through clay loam aggregates. Reduction in particle size was found to reduce both the total light transmitted and the red/far-red ratio (R : FR). Red wavelengths were also more attenuated by a layer of freshly fallen oak (Quercus rotrur) leaves than were far-red wavelengths. A layer of Corsican pine (Pinus nigramaritima) needles however, was found to act as a neutral density filter over the 400 800 nm range. Prolonged exposure to soil-filtered light was found to affect the germination of seven species tested. Plantago major demonstrated an approximately linear decrease in germination with increasing depth. Rumex obtusifotius showed an apparent threshold response at 4 6 mm depth, as did Cecropia obtusifolia but at a slightly greater depth. Digitalis purpurea germinated very poorly in darkness, yet was extremely photosensitive with very high germination even at 10 mm depth. Galiutn aparine and Chenopodium album showed a two-phase response with germination reaching a peak at 2 mm depth. The implications for the function of the photoreceptor phytochrome in the control of germination are discussed in relation to soil-transmitted light and with regard to the results of the germination tests.     

Blue Light Reception in Phycomyces: Red Light Sensitization in madC Mutants

Sporangiophores of the zygomycete fungus Phycomyces blakesleeanus are sensitive to near UV and blue light. The quantum effectiveness of yellow and red light is more than 6 orders of magnitude below that of near UV or blue light. Phototropism mutants with a defect in the gene madC are about 10⁶ times less sensitive to blue light than the wild type. These mutants respond, however, to yellow and red light when the long wavelength light is given simultaneously with actinic blue light. In the presence of yellow or red light the photogravitropic threshold of madC mutants is lowered about 100-fold though the yellow and the red light alone are phototropically ineffective. A step-up of the fluence rate of broad-band red light (> 600 nm) from 6 × 10^?3 to 6W m^?2 elicits, in mutant C 148 madC, a transient deceleration of the growth rate. The growth rate of the wild type is not affected by the same treatment. The results are interpreted in terms of a red light absorbing intermediate of the blue light photoreceptor of Phycomyces. The intermediate should be short-lived in the wild type and should accumulate in madC mutants.     

The use of population parameters in the analysis of germination of lettuce seed

Probit analysis was used to determine the parameters of the germinating population of lettuce seed. Seeds were induced to germinate under different conditions and it was shown that with light induced germination the germinating populations were the same. However, when gibberellic acid (GA₃) was the inductive agent then the germinating populations were not the same being concentration dependent. – This type of analysis offers a rigorous comparison of different inductive treatments.     

Photodynamic effect of light-emitting diode light on cell growth inhibition induced by methylene blue

The aim of this study was to propose the use of red light-emitting diode (LED) as an alternative light source for methylene blue (MB) photosensitizing effect in photodynamic therapy (PDT). Its effectiveness was tested against Staphylococcus aureus (ATCC 26923), Escherichia coli (ATCC 26922), Candida albicans (ATCC 90028) and Artemia salina. The maximum absorption of the LED lamps was at a wavelength of 663 nm, at intensities of 2,4,6 and 12 J.cm-2 for 10, 20, 30 and 60 min of exposure, respectively. Assays with and without LED exposure were carried out in plates containing MB at concentrations of 7 to 140.8 (micro) M for microorganisms and 13.35 to 668.5 (micro) M for microorganisms or microcrustaceans.The LED exposure induced more than 93.05%, 93.7% and 93.33% of growth inhibition for concentrations of 42.2 (micro)M for S.aureus (D-value=12.05 min) and 35.2 (micro)M for E.coli (D-value=11.51 min) and C.albicans (D-value=12.18 min), respectively after 20 min of exposure. LED exposure for 1 h increased the cytotoxic effect of MB against A.salina from 27% to 75%.Red LED is a promising light device for PDT that can effectively inhibit bacteria, yeast and microcrustacean growth.     

The effect of blue light on energy levels in epidermal strips

Red light applied together with blue enhanced stomatal opening in epidermal strips of Commelina communis L. more than red light alone. In red light, stomatal opening was enhanced by exogenously applied ATP and was inhibited by 3-(3,4-dichlorophe-nyl)-l,l-dimethylurea (DCMU), while in the presence of blue light external ATP was almost without effect, and DCMU stimulated stomatal opening. Blue light increased the ATP levels in the epidermal strips. DCMU diminished the amount of ATP in both red light and red + blue light treatments, but did not abolish the stimulatory effect of blue light. Blue light also stimulated the respiration rate of the epidermal strips. Rotenone, which inhibited stomatal opening and respiration rate, abolished the effect of blue light in both processes. These results imply that blue light increases the ATP levels by stimulation of oxidative phosphorylation.     

Fiber optic studies of light gradients and spectral regime within Lactuca sativa achenes

Light gradients and spectral regime were measured in Lactuca sativa L. cv. Grand Rapids achenes using fiber optic microsensors. The distribution of scattered light across lettuce achenes was linear for 660 and 730 nm and non-linear for 450 nm light. Spectra for scattered light within intact achenes also showed a non-linear increase with wavelength. The preferential attenuation of blue light by the pericarp and seed explains in part the relative ineffectiveness of blue light with respect to red in triggering germination of lettuce. Calculated action spectra for phytochrome-stimulated germination agree closely in the red with experimentally derived action spectra; however, there is little agreement within the blue.     

Biphasic fluence-response curves for light induced germination of Arabidopsis thaliana seeds

Abstract With appropriate pretreatment of the seeds fluence-response curves for the induction of germination of Arabidopsis thaliana show two phases. A proportion of the population responds to very low fluence (VLFR), 10⁴–10^?2μmolm^?2 establishing 10^?4–10^?2% of the total phytochrome in the far-red absorbing form (Pfr) and a proportion of the population respond to low fluence (LFR), 1–1000 μmolm^?2, establishing 1–75% Pfr. The VLFR is nol normally seen because the pre-existing Pfr level satisfies the Pfr requirement or use of green safelight establishes more Pfr than necessary to saturate the VLFR. Endogenous Pfr was depicted by a 24 h 35°C treatment, presumably as a result of dark destruction and/or dark reversion to the red absorbing form of phytochrome (Pr), making it possible to visualize the VLFR. A short pulse of 35°C treatment in combination with an appropriate temperature regime is also able to sensitize a proportion of the seed population. The proportion of the population showing the VLFR is determined by the duration of the cold imbibition pretreatment as well as the duration of the 35°C treatment. Complex fluence-response curves were observed in which a proportion of the seeds being promoted in the VLFR range, were inhibited at higher fluences before being further promoted in the LFR range. This was particularly clear for seed batches being sensitized by a short 35°C treatment. The VLFR may be of significance in the natural environment, enabling seeds buried in the upper layer of the soil to germinate, where the fluence rate falls off sharply and the LFR is not satisfied. A model is presented to explain the two phases in the fluence-response curves.     

Involvement of a high irradiance response in photoinhibition of white clover seed germination at low water potential

Seeds of white clover ( Trifolium repens L., cv. Podkowa) were germinated at water potential ψ=−0.3 MPa in darkness, at 25°C. A short exposure to blue light (B) inhibited germination in a manner similar to that described earlier for red (R) and far‐red (FR) light (Niedźwiedź‐Siegień and Lewak 1989). No reversibility of B, R and FR effects was observed. Saturation irradiance and energy was the lowest for R and the highest for B. The reciprocity of irradiance versus time of exposure was observed only for non‐saturating irradiances of B, R and FR.     

Degradation of phytochrome A and the high irradiance response in Arabidopsis: a kinetic analysis

The ability to respond to far‐red‐rich light is essential for seedlings germinating below dense canopies. Physiological and genetic studies have demonstrated that phytochrome A is the only photoreceptor mediating responses to far‐red light. However, all phytochromes including phytochrome A are believed to be activated by red light and to be inactivated by far‐red light. To address the fundamental question of why phytochrome A has its highest physiological activity at presumably inactivating wavelengths, we analysed light‐induced degradation of phytochrome A in Arabidopsis. Rate constants were obtained for all reaction events in a two‐step model of degradation. Based on biochemical data, the model includes a tagging mechanism preceding degradation. The parameterized model describes Pr accumulation, wavelength dependencies of degradation kinetics and steady‐state levels as well as Pfr‐induced Pr degradation. Subsequently, experimentally derived fluence rate response curves, action spectrum and response curves to dichromatic irradiation were compared to simulations based on the model of degradation. Two kinetically defined phytochrome subspecies, untagged Pfr and tagged Pr, have steady‐state levels closely matching the physiological response curves. Therefore, sensing of far‐red light by phytochrome A can be quantitatively explained based exclusively on regulated protein degradation.     

Blue light-regulation of cell division in Adiantum protonemata: an approach with pulse stimulation

Abstract Cell division of the single-celled Adiantum protonemata produced by red-light (RL) incubation of germinated spores is induced by transfer to darkness and is stimulated by blue light (BL). It is known that the cellular process leading to this cell division includes one cell cycle and the BL response results from shortening of the Gl phase. The authors studied this BL regulation of cell cycle by giving a pulse of BL after RL termination and measuring changes in the proportion of divided cells. To minimize phytochrome responses arising from BL irradiation, the plants were kept in continuous far-red light instead of total darkness after the RL incubation. The response to a pulse (10–100 s) approached saturation with increasing rluences in a manner that reciprocity is valid. The sensitivity to BL, investigated by measuring the response to a saturating pulse, showed an increase in the first several hours after RL termination, followed by a sustained sensitivity for 20 h. Time courses of the pulse-induced responses showed a lag of about 12 h, which was considerably shorter than in the non-stimulated control; the lag was approximately independent of the strength of BL stimulation or the timing of BL application after RL termination, and the major difference occurred in the slope. It is concluded that the sensitivity to BL is retained during the time span in which the dark-dependent Gl phase progresses, and that the BL response is initiated independently of the reactions involved in the dark-dependent Gl phase. A minimal reaction model of Gl phase is suggested to unify the results.     

The early time course of the inhibition of stem growth of etiolated pea seedlings by fluorescent light

The stem elongation responses of etiolated peas (Pisum sativum L.) to fluorescent light (35–45 mol.m^t-2.s^-1) were recorded using high resolution position transducers. Continuous fluorescent light decreased growth by 70% within 9 min. The growth rate declined to 5% of the control over the next 2 h and remained at this level for 7 h. Pulses of fluorescent light ranging from 8 s to 34 min led to partial suppression of growth and resulted in a complex kinetic response. The distinctive kinetics of blue and red light inhibition were apparent as components of the responses to non-saturating levels of fluorescent light. The rapid suppression of growth by blue light was not affected by concomitant red light. The lag time for the onset of red light inhibition was not affected by concomitant blue, but the rate of inhibition appeared accelerated.     

Rewiring of hormones and light response pathways underlies the inhibition of stomatal development in an amphibious plant Rorippa aquatica underwater

1. Download : Download high-res image (233KB)
2. Download : Download full-size image

     

Stem extension-growth responses to blue light require Pfr in tomato seedlings but are not reduced by the low phytochrome levels of the aurea mutant

The effects of blue light (B) on stem extension growth were investigated in wild-type (WT) and aurea (au ) mutant seedlings of tomato. The au mutant has reduced phytochrome levels. Etiolated seedlings were grown under background red light (R) or far-red light (FR) with or without B. Hypocotyl growth was inhibited by B added to R but not by B added to FR, both in WT and au seedlings. The levels of B and/or R reaching the stem of fully de-etiolated seedlings grown in a glasshouse were reduced by means of collars around it. Both in WT and au -mutant seedlings the responses to B were larger at high than at low R/FR quantum ratios. In etiolated and light-grown au seedlings, changing the levels of phytochrome-absorbable radiation did not cause the same effect as changing B levels, indicating the action of specific BL/UV-A photoreceptor(s) (BAP). The responses to B are reduced by the low calculated levels of Pfr established by light treatments but not by the low levels of phytochrome present in the au mutant. The au mutant appears to be deficient in a phytochrome pool that is not essential for the interdependent co-action observed between phytochrome and BAP in the control of stem extension growth in tomato.