Photoinduced Seed Germination of Oenothera biennis L: II. Analysis of the Photoinduction Period

The photoinduction period of Oenothera biennis L. seed germination was analyzed by varying the photoinduction temperature and by substituting red light pulses for continuous red light. At 24°C, seeds require 36 hours of continuous red light for maximal percent germination. The optimal photoinduction temperature is 32°C, with higher and lower temperatures being strongly inhibitory. A 30 minute exposure to far-red light, given immediately after a red light period of 1 to 36 hours, reduces germination by about 25%. Seeds escape from far-red inhibition with a half-time of 5 to 10 hours, depending on the length of the red exposure that precedes the far-red light. Periodic 15 minute pulses of red light can substitute for continuous red light in stimulating germination. Ted red light pulses, with 6 hours of darkness between successive pulses, cause maximal germination. The response to periodic red light is fully reversible by far-red light. Probit analysis of the periodic light response shows that as the length of the dark periods between successive pulses increases, less incident light is needed to induce germination but the population variance in light sensitivity remains constant. Probit analysis of the temperature response shows that as the photoinduction temperature increases from 16 to 32°C, less incident light is needed to induce germination and the population variance in light sensitivity also increases.     

Effect of red, far-red radiations on germination of cotton seed

Seeds of two cultivars, H-14 and J-34 of Gossypium hirsutumL. soaked in the dark for durations of 8, 12 and 16 hr wereexposed to red and far-red radiations alone and in differentsuccessions. While cv. J-34 was unresponsive to all light treatments,seed germinaiton in cv.H-14 was promoted by darkness and byexposure to far-red radiation and was inhibited by red andwhitelight.Seed germination depended upon what treatment was given at theend. The effect ofred light was reversed by far-red and viceversa. Although instances of light inhibition of seed germinationare known, the authors are not aware of any clear cut instancein the literature on far-red promotion of seed germination (Received November 4, 1970; )     

Photophysiology of Kalanchoë Seed Germination

The effects of irradiations with different proportions of red/farred light and of gibberellic acid on the phytochrome-mediated seed germination of Kalanchoë blossfeldiana cv. Feuerblüte, were studied. The seed coat transmits much more red than far-red light, and therefore the energy ratio between 660 nm and 730 nm is given only for the transmitted light. Decreasing this ratio from 65 to 1.0 caused only a very slight inhibition. If this ratio is further lowered to 0.64, a 10 min terminal irradiation after a 3-h white light photoperiod is inhibitory, but a 12-h photoperiod or continuous irradiation is not. If the ratio is decreased to 0.44 or 0.31, a 12-h photoperiod is now also inhibitory, although continuous irradiation and 10 min terminal irradiation are still more inhibitory. These results are discussed in terms of phytochrome phototransformations. Although gibberellic acid is unable to cause any germination in complete darkness, it can result in a very high germination percentage, if combined with treatments which by themselves do not induce any germination such as continuous far-red, terminal far-red after short photoperiods, or very short photoperiods at 25°C. These results point to a strong synergism between gibberellic acid and the so-called stabilized form of phytochrome, P*_FR.     

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.     

Germination characteristics of some plant species from calcareous fens in southern Germany and their implications for the seed bank

Seeds of 25 plant species from calcareous fen hay meadows were exposed to different experimental conditions and their germination was characterised. Constant temperature inhibited germination especially in Cyperaceae . Both gibberellic acid and potassium nitrate failed to terminate dormancy. Increased germination rates were found in dicot species after treatment with gibberellic acid. Temperature fluctuations increased germination of Cyperaceae as well as dicotyledons. Treatment with gibberellic acid removed the chilling requirement in some of the species. Dormancy of small seeds with thin seed coats was broken by the application of gibberellic acid or fluctuating temperature; large thick-coated seeds were unaffected by gibberellic acid. No obligatory darkness requirement was found in any species; three species germinated irrespective of light treatments. All other species achieved higher percentage germination in daylight or in red (670 nm) light. Permanent darkness and far-red light (730 nm) reduced germination drastically. The results indicate that germination characteristics of the species investigated can be related to their seed bank types.     

Germination and Dormancy in Immature and Fresh-mature Lettuce Seeds

The effects of red light, far-red light, Gibberellin A₃, andethephon were studied on the germination of lettuce seeds cv.Grand Rapids harvested at different stages of development. Seeds did not become capable of germination until 8 days afteranthesis. Red light promoted seed germination from the age of8–9 days following anthesis up to the newly mature stage.Ten or 11 days following anthesis, a large percentage of seedsbecame capable of germination in the dark and therefore couldbe considered not dormant. They were affected by far-red light,but less so than the mature seeds. The effect of light on the germination of developing seeds appearedto be similar to the known light effect on mature lettuce seedgermination. Gibberellin A₃ and ethephon had no effect on immatureand fresh seed germination. Lactuca sativa L., Lettuce, germination, dormancy, red light, far-red light, gibberellin A₃, ethephon     

Maternal and ambient environmental effects of light on germination in Plantago lanceolata: correlated responses to selection on leaf length

1. Seeds from artificial selection lines were exposed to different maternal and ambient conditions, simulating sunlight and vegetation shade.
2. Lines selected for longer leaves also produced larger seeds, indicating a positive genetic correlation between leaf length and seed size.
3. Light conditions during maturation had no large effect on seed size.
4. Seed germination was reduced by a low ratio of red to far-red light (R/FR ratio) in the ambient environment.
5. Seeds maturated under simulated vegetation shade germinated less readily and were more inhibited by a low ambient R/FR ratio than seeds maturated under full sunlight or R/FR-neutral shade. Thus, low R/FR-ratios in the maternal and ambient environment operated synergistically.
6. Large genotypic variation in the germination responses to both maternal and ambient light conditions was found among and within selection lines, indicating that such responses might have the potential to evolve in response to natural selection.
7. Artificial selection for leaf length had affected seed germination characteristics but correlated responses and thus genetic correlations largely depended on light conditions in the selective environment. Selection for longer leaves under a low R/FR ratio increased seed dormancy and plasticity of germination in response to the R/FR ratio. However, in the opposite selective environment selection for longer leaves reduced seed dormancy and plasticity to the R/FR ratio. It is argued that leaf length and seed germination characteristics are somehow linked by shared physiological mechanisms, which may facilitate concerted changes in shade avoidance responses.     

Photocontrol of seed germination in the hemiparasite Buchnera hispida (Scrophulariaceae)

Abstract Buchnera hispida, a facultative root parasite of grasses and graminaceous crops, has a light requirement for germination. Studies were carried out on the effects of varying photoperiods with or without preceding dark incubation, on seed germination. Buchnera seeds showed long-day behaviour, since they germinated at all photoperiods including continuous light, and longer photoperiods were more effective in triggering seed germination than shorter photoperiods. Also, effects of red and far-red light indicated that the phytochrome system is operative in the light-induced germination of Buchnera. Although dark incubation in water before illumination was not absolutely necessary for germination, it caused the seeds to respond more rapidly to light. The longer the time of the dark incubation the more responsive the seeds were to photoperiod except when 15 min light was given. The effectiveness of a preceding dark incubation in making Buchnera seeds sensitive to rapid light action was completely inhibited at 4°C. This is in agreement with the hypothesis that a reaction partner of the far-red absorbing form of phytochrome is produced during dark incubation of Buchnera seeds. Such an intermediate has also been reported in some positively photoblastic seeds of non-parasitic flowering plants.     

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.     

Release of reactive oxygen intermediates (superoxide radicals, hydrogen peroxide, and hydroxyl radicals) and peroxidase in germinating radish seeds controlled by light, gibberellin, and abscisic acid

Germination of radish (Raphanus sativus cv Eterna) seeds can be inhibited by far-red light (high-irradiance reaction of phytochrome) or abscisic acid (ABA). Gibberellic acid (GA3) restores full germination under far-red light. This experimental system was used to investigate the release of reactive oxygen intermediates (ROI) by seed coats and embryos during germination, utilizing the apoplastic oxidation of 2',7'-dichlorofluorescin to fluorescent 2',7'-dichlorofluorescein as an in vivo assay. Germination in darkness is accompanied by a steep rise in ROI release originating from the seed coat (living aleurone layer) as well as the embryo. At the same time as the inhibition of germination, far-red light and ABA inhibit ROI release in both seed parts and GA3 reverses this inhibition when initiating germination under far-red light. During the later stage of germination the seed coat also releases peroxidase with a time course affected by far-red light, ABA, and GA3. The participation of superoxide radicals, hydrogen peroxide, and hydroxyl radicals in ROI metabolism was demonstrated with specific in vivo assays. ROI production by germinating seeds represents an active, developmentally controlled physiological function, presumably for protecting the emerging seedling against attack by pathogens.     

After-ripening and phytochrome action in seeds of dormant lines of wild oat (Avena fatua)

The effects of a short exposure to red, far-red or alternate red/far-red light on the germination of seeds after-ripened for different periods of time were studied in dormant lines of wild oat ( Avena fatua L.). Three stages were distinguishable in the after-ripening period in the response of germination to light. Seeds stayed dormant and showed no response to light during stage I. Phytochrome-mediated germination was observed in seeds during stage II. The phytochrome action disappeared during stage III, i.e. seeds fully germinated following treatments of all light qualities. When the seeds were imbibed in polyethylene glycol solutions, dark germination was reduced and phytochrome again had an effect, which suggested the involvement of phytochrome in water uptake of the seed.     

Response of Plantago major seeds to the red/far-red ratio as influenced by other environmental factors

Seeds of Plantago major L. ssp. major were exposed to continuous light of various red/far-red ratios (R/FR). Germination was strongly inhibited at low R/FR. The quantitative response of the seeds to the R/FR appeared to depend on various environmental factors. A low R/FR was less inhibitory after a stratification pretreatment, at higher temperatures, in the presence of nitrate and at high osmotic potentials. This is discussed in relation to the concept of a variable requirement of P_fr for germination, which depends on state of dormancy as influenced by pretreatment and on conditions during the germination test. The data are also discussed in relation to field emergence. The species is known to form a persistent seed bank, among other things due to its almost absolute light requirement. It is predicted that germination is effectively inhibited under a leaf canopy in summer after dispersal, and perhaps even in spring to some extent when R/FR is not so low but temperatures are low.     

Stimulation of lettuce seed germination by ethylene

Ethylene increased the germination of freshly imbibed lettuce (Lactuca sativa L. var. Grand Rapids) seeds. Seeds receiving either red or far-red light or darkness all showed a positive response to the gas. However, ethylene was apparently without effect on dormant seeds, those which failed to germinate after an initial red or far-red treatment. Carbon dioxide, which often acts as a competitive inhibitor of ethylene, failed to clearly reverse ethylene-enhanced seed germination. While light doubled ethylene production from the lettuce seeds, its effect was not mediated by the phytochrome system since both red and far-red light had a similar effect.     

An unusual effect of the far-red absorbing form of phytochrome: Photoinhibition of seed germination inBromus sterilis L.

Seeds ofBromus sterilis L. germinated between 80–100% in darkness at 15° C but were inhibited by exposure to white or red light for 8 h per day. Exposure to far-red light resulted in germination similar to, or less than, that of seeds maintained in darkness. Germination is not permanently inhibited by light as seeds attain maximal germination when transferred back to darkness. Germination can be markedly delayed by exposure to a single pulse of red light following 4 h inhibition in darkness. The effect of the red light can be reversed by a single pulse of far-red light indicating that the photoreversible pigment phytochrome is involved in the response. The response ofB. sterilis seeds to light appears to be unique; the far-red-absorbing form of phytochrome (Pfr) actually inhibiting germination.Abbreviations Pr red absorbing form of phytochrome - Pfr far-red absorbing form of phytochrome     

Photophysiology of Kalanchoë Seed Germination

Germination of Kalanchoé blossfeldiana seeds is absolutely light-requiring and needs repeated daily light periods. With increasing length of the photoperiod there was a gradual escape from the far-red inhibition. This escape depended also upon the duration of the far-red exposure: 10-second far-red caused a strong inhibition after a 10- to 30-minute photoperiod and did not inhibit after a 4-hour day, although the effect of the latter was completely suppressed by 5 minutes far-red. The action of a 12-hour photoperiod was not reversed by 10 minutes far-red but it was by 12 hours far-red. Light intensity and temperature during the photoperiod were two other important factors influencing the escape from far-red inhibition. The common features of this escape displayed in very different photomorphological responses are stressed. In order to explain our results in terms of phytochrome action, we distinguish two effects of white light: 1) on the initial photoconversion of the inactive to the active P_FR form 2) on the much slower transformation of P_FR to a reacted form P*_FR; the latter reaction can also proceed in darkness, but is enhanced by light and is dependent upon light intensity and temperature; this reacted phytochrome is not reversible by a brief far-red illumination.     

Photocontrol of spore germination in the fern Thelypteris kunthii

The light requirement for germination in spores of the fern Thelypteris kunthii (Desv.) Morton was fully satisfied by a long period of continuous red light or partially by intermittent, short periods of red light. Red light-potentiated spore germination was inhibited by brief far-red light irradiation, indicating phytochrome involvement. Repeated exposure of spores to prolonged red and short far-red irradiations, or exposure of red-potentiated spores to far-red light after an extended period in darkness, led to their escape from inhibition of germination by far-red light. Prolonged irradiation of spores with blue light before or after red light treatment partially antagonized the effect of red light.     

Photoinhibition of radish (Raphanus sativus L) seed germination: control of growth potential by cell-wall yielding in the embryo

The biophysical mechanism underlying photoinhibition of radish (Raphanus sativus L.) seed germination was investigated using three cultivars differing in sensitivity to continuous irradiation with far-red light (high-irradiance reaction of phytochrome). Sensitivity of germination to the inhibitory action of light was assessed by probing germination under osmotic stress (incubation in media of low water potentials adjusted with polyethylene glycol 6000) and expressed in terms of ‘germination potential’ (positive value of the water potential at which germination is inhibited by 50%). Far-red light decreases the germination potential to various degrees in the different cultivars, reflecting the light-sensitivity of germination in water. Removal of the seed coat increases the germination potential by a constant amount in darkness and light. It is concluded that germination depends on the expansive force of the embryo which can be drastically diminished by far-red light. Seed-coat constraint and expansive force of the embryo interact additively on the level of the germination potential. Photoinhibition of germination was accompanied by an inhibition of water uptake into the seed. Analysis of seed water relations showed that osmotic pressure and turgor assumed higher levels in photoinhibited seeds, compared to seeds germinating in darkness, while the water potential was close to zero under both conditions. Far-red light produced a shift (to less negative values) in the curve relating water-uptake rate to external water potential, i.e. a reduction in the driving force for water uptake. It is concluded that photoinhibition of germination results from the maintenance of a high threshold of cell-wall extensibility in the embryo.     

Seed germination traits of Trapella sinensis (Trapellaceae), an endangered aquatic plant in Japan: Conservation implications

We investigated the effects of cold stratification, temperature, light, and oxygen conditions on seed germination of Trapella sinensis Oliver, an endangered aquatic plant in Japan. Seeds had physiological dormancy, and final germination rate increased with an increasing period of cold stratification. Seeds of T. sinensis had an almost absolute requirement for aerobic conditions to germinate. Also, alternating temperatures significantly promoted germination regardless of light conditions, although final germination percentage was twice to four times higher in light under constant temperature conditions. Suitable sites for germination of T. sinensis appear to be shallow water with alternating temperatures and sufficient oxygen and sediment anoxia, caused by eutrophication of water bodies, may have prevented recruitment of T. sinensis from seed.     

Variation in temperature and light but not salinity invokes antioxidant enzyme activities in germinating seeds of Salsola drummondii

Seeds with efficient antioxidant defence system show higher germination under stress conditions; however, such information is limited for the halophyte seeds. We therefore studied lipid peroxidation and antioxidant responses of a leaf-succulent halophyte Salsola drummondii during seed germination under different salinity levels (0, 200 and 800 mM NaCl), temperature (10/20, 20/30 and 25/35°C) and light regimes. Seeds absorbed water and germinated in less than 1 h in non-saline control while increases in salinity decreased the rate of water uptake as well as seed germination. Non-optimal temperatures (10/20 and 25/35°C) and complete dark condition reduced seed germination in comparison to those seeds germinated under optimal temperature (20/30°C) and 12-h photoperiod, respectively. Generally, higher lipid peroxidation and antioxidant enzyme activities were observed in seeds at non-optimal temperature and in those seeds germinated in dark. Decrease in reduced ascorbic acid content was found in highest salinity and temperature treatments, while reduced glutathione content did not change significantly with changes in salinity, temperature and light regimes. These results indicate variation in temperature and light but not salinity enhances antioxidant enzyme activities in germinating seeds of Salsola drummondii.     

Effects of Photoperiod on Germination of Seeds of Talinum triangulare (Jacq.) Willd

Seed germination in Talinum triangulare as affected by photoperiod,with or without previous incubation in the dark in water at25 or 4 °C, was studied. The time course and quantity ofseed germination in photoperiods of 1 h and above were similarwith or without dark pretreatment, but the time to half maximumgermination was reduced from 12 days in non-dark pretreatedseeds to 4 days in seeds given 20 days in the dark at 25°C.A photoperiod of 0·25 h gave a lower rate and total germinationthan photoperiods of 1 h and above. Un-pretreated seeds required17 cycles of 24 h photoperiod for maximum germination as comparedwith 7 or less cycles if the seeds received more than 10 daysdark pretreatment at 25 °C. Both the rate and total germinationin light increased as the length of dark pretreatment at 25°C was increased from zero to 30 days. Incubation of theseeds in water in the dark at 4 °C for 5 to 30 days priorto illumination at 21 °C, reduced both the rate and quantityof seed germination in light as compared with those similarlyincubated in the dark at 25 °C. However, previous incubationin the dark for 30 days at 4 °C partially substituted forthe light requirement. The possible mechanism of breakage ofseed dormancy in Talinumis discussed in relation to these andother findings. Talinum triangulare (Jacq.), Willd, light, photoperiod, seed germination