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.     

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.     

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.     

The influence of season on adenine nucleotide concentrations and energy charge in four marsh plant species

Unilateral pulse ultraviolet irradiation caused positively phototropic coiling (> 180°) and curvature (≤ 180°) in the growth zone of dark-grown broom sorghum ( Sorghum bicolor Moench, cv. Acme Broomcorn and Sekishokuzairai Fukuyama) first internodes. Coiling was induced by irradiation at 257 to 302 nm, and proceeded to develop almost linearly during 72 h or more involving new tissue produced after irradiation. Curvature, caused at 308 to 413 nm and by red light, developed rapidly during the first several hours then slowly to cease by about 24 h, and did not surpass 120° even at the optimum photon fluences. Action spectra were higher towards shorter wavelengths, having a shoulder at about 287 nm, and could not separate the two photoresponses. The curvature was partially but markedly suppressed by far-red following the ultraviolet irradiation, whereas the coiling was not affected. Possible involvement of a specific UV-B photoreceptor and phytochrome in curvature and of a UV-C photoreceptor in coiling is discussed.     

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.     

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.     

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.     

Are two photoreceptors involved in the flowering of a long-day plant?

The effect of daylength extension with narrow spectral bands on the flowering of a long-day plant, Brassica campestris L. cv. Ceres, was investigated to obtain clues to the identity of the photoreceptor involved. Extension of a 9 h photoperiod with 5 h of light pulses at various wavelengths resulted in maximal flowering occurring after irradiation at 710 nm, less at 730 nm, and none at 550, 660 and 750 nm. Flowering at 710 and 730 nm was negated by simultaneous exposures at 550 nm, but not at 660 nm. A short preirradiation at 660 nm enabled a following irradiation at 750 nm to induce flowering. This latter induction was prevented by 550 nm irradiation.
Short flashes of light at 710 nm induced flowering that was negated by a following flash at 550 nm but not at 660 nm. The negation by 550 nm radiation was prevented by subsequent flashes at 710 nm, indicating photoreversibility. A flash at 660 nm enabled subsequent light flashes at 750 nm to initiate flowering that was reversed by a following 550 nm flash.
From the results showing the necessity of red and far-red lights, it is proposed that flowering in this long-day plant is due to two photoreceptors - one is phytochrome and the other an unknown pigment with far-red, green photoreversible properties. By using fluence response data, it is deduced that the unidentified photoreceptor has weak absorption bands in the far-red, but has a strong absorption band in the green. Flowering is induced when effects of red light absorbed by phytochrome interact with effects of far-red light absorbed by the unidentified photoreceptor.     

Effects of 2,6-dichlorobenzonitrile on differentiation to tracheary elements of isolated mesophyll cells of Zinnia elegans and formation of secondary cell walls

Seeds of Caesulia axillaris Roxb. displayed an absolute light requirement for germination throughout the period of dry storage at 28°C. The seeds were found to show a gradual increase in percent germination with storage time - reaching a maximum value between 8-14 months and then a sharp decline. Percent water uptake and photosensitivity were at maximum after a 5-day imbibition period in the dark in both seedlots studied. Seedlot I, which was only marginally responsive to far-red light, showed a nearly complete red-far-red reversal effect in contrast to seedlot II. The latter also displayed a considerable promotion of germination in far-red light. Interestingly, a noticeable degree of heterogeneity, besides the one observed in both seedlots with reference to red light, was found to exist in seedlot II for far-red light. Exogenous application of nitrate and ammonium, at the levels occurring in soil during seed germination/seedling emergence phase of the plant in nature, promoted a considerable proportion of high Ø-requiring seeds to germinate under irradiation conditions establishing low Ø-value. The probable ecological implication of this reponse has been discussed. Little correlation was found between the requirement for an exogenous supply of nitrate and the endogenous nitrate level in the seeds in their response to far-red light.     

Some reflections on the relationship between endogenous hormones and light-mediated seed dormancy

The relationships between phytochrome and endogenous hormones in the light-mediated control of seed dormancy are discussed. It is concluded that gibberellins are primarily involved in post-dormancy metabolic processes leading to embryo growth and radicle emergence, such as food reserve mobilisation and endosperm softening. Evidence is considered that germination inhibitors, particularly abscisic acid, are involved in the establishment and maintenance of primary dormancy. The role of cytokinins not fully elucidated but there is considerable evidence to suggest that phytochrome control may involve cytokinin effects on transmembrane ion fluxes. In terms of hormonal control, phytochrome mediated dormancy is a complex phenomenon. There is a need for molecular studies of processes controlled by phytochrome, GAs, CKs and ABA during dormancy and germination to unravel the complexities of the dormancy mechanisms. Such studies would be facilitated by the availability of CK-deficient mutants of classical light-sensitive species.     

光质对杉木种子萌发及幼苗生长的影响

光是影响森林早期天然更新(种子萌发和幼苗生长)的重要因子之一.但光质对杉木(Cunninghamia lanceolata(Lamb.) Hook)早期更新的影响尚不清楚.为探讨光质对杉木种子萌发及幼苗生长的影响机制,以杉木种子为研究对象,通过设置8h白光(W)、8h红光(R)、6h红光-2 h远红光(R-FR)、4h红光-2 h远红光-2 h红光(R-FR-R)、黑暗(D,对照)5种不同光质处理,观察种子萌发及幼苗生长特征.结果表明,不同光质处理对杉木种子萌发影响显著,D和R-FR-R处理促进杉木种子萌发,R处理抑制种子萌发.最后一次照光选择红光处理(R和R-FR-R)能够显著促进杉木幼苗根和子叶的伸长,促进生物量积累以及增加叶生物量分配比例,但抑制茎的伸长,远红光处理则促进茎的生长.杉木种子为光不敏感种子,黑暗处理下萌发率最高,且远红光间断处理能够相对促进其萌发,说明杉木种子可以较好地适应异质光环境.     

Photoregulation of the Greening Process of Wheat Seedlings Grown in Red Light*

Wheat seedling grown with their shoot bottom exposed to red light (400 μmol m⁻² s⁻¹) either with constant illumination or light-dark cycles did not accumulate chlorophyll. This near-etiolation response was manifested by a critical threshold intensity of red light and did not need continuous illumination. The inhibition of the greening process resulted from reduced synthesis of glutamate-1-semialdehyde and consequent reduction in tetrapyrrole precursor 5-aminolevulinic acid. Red light perceived by the shoot bottom down regulated the protein and/or gene expression of enzymes involved in the biosynthesis of tetrapyrroles. The contents of endogenous cytokinins, i.e., isopentenyl-adenosine and dihydrozeatinriboside, were reduced in seedlings grown in red light having their shoot bottom exposed. Application of exogenous cytokinin and its analogue to roots of seedlings grown in red light reversed the down regulation of the greening process. The reversal of red-light-induced near-etiolation morphogenesis by far-red (200 μmol m⁻² s⁻¹) or blue (25 μmol m⁻² s⁻¹) light suggests that it could be a very high red-irradiance response of phytochrome, in the meristematic layers of the shoot bottom, that works in concert with blue light receptor(s). This work was supported by a competitive grant from the Department of Science and Technology, Govt. of India (DST/SP/SO/A-49/95) to BCT. Suchi Sood Varsha Gupta: Equal contributors     

PKS1 plays a role in red-light-based positive phototropism in roots

Aerial parts of plants curve towards the light (i.e. positive phototropism), and roots typically grow away from the light (i.e. negative phototropism). In addition, Arabidopsis roots exhibit positive phototropism relative to red light (RL), and this response is mediated by phytochromes A and B (phyA and phyB). Upon light stimulation, phyA and phyB interact with the phytochrome kinase substrate (PKS1) in the cytoplasm. In this study, we investigated the role of PKS1, along with phyA and phyB, in the positive phototropic responses to RL in roots. Using a high-resolution feedback system, we studied the phenotypic responses of roots of phyA, phyB, pks1, phyA pks1 and phyB pks1 null mutants as well as the PKS1-overexpressing line in response to RL. PKS1 emerged as an intermediary in the signalling pathways and appears to promote a negative curvature to RL in roots. In addition, phyA and phyB were both essential for a positive response to RL and act in a complementary fashion. However, either photoreceptor acting without the other results in negative curvature in response to red illumination so that the mode of action differs depending on whether phyA and phyB act independently or together. Our results suggest that PKS1 is part of a signalling pathway independent of phyA and phyB and that PKS1 modulates RL-based root phototropism.