Light and temperature control of germination in Agropyron smithii seeds

In darkness, A. smithii seeds germinated poorly at constanttemperatures but well at alternating temperatures. Prolongedperiods on the high part of the temperature cycles reduced germination;the higher the temperature the shorter was the period requiredon the high part of the temperature cycles for optimum germination.Continuous, unfiltered, incandescent illumination and intermittentfar red at 15?–25?C alternation also inhibited germination;the inhibitory effects were similar to those caused by the highintensity reaction. Far red inhibited germination when appliedafter 1 and 2 complete 15?–25?C cycles in darkness butnot after 3 cycles. Less than 20% of the seeds were under phytochromecontrol at constant 20?C. When red light was applied directlyafter far red that was applied in intermittent cycles at 15?–25?C,however, 50% of the seeds caused to germinate by the alternatingtemperature were shown to be controlled by the reversible phytochromereaction. The induced high-temperature dormancy was overcome by gibberellicacid (GA₃) plus kinetin. The hormonal treatment was much moreeffective than light for breaking dormancy. Inhibition fromprolonged illumination was alleviated or eliminated by GA₃+kinetin.The failure of red light to promote good germination at 20?Cwas also overcome with GA₃+kinetin; effects of light plus thehormone treatments were more than additive. These data suggestthat optimum alternating temperatures facilitate a proper balanceand interaction of hormones, enzymes, substrates and possiblypreexistent P_fr so that the germination of A. smithii seedscan proceed without benefit of a light treatment. (Received July 7, 1976; )     

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.     

Phytochrome-mediated germination control of maize caryopses

Maize caryopses sown in water germinate equally well either in darkness or under any light regime. However, when they are imbibed in mannitol solutions, continuous far-red light proves to be strongly inhibitory on the final germination as compared to darkness. Similar but less pronounced inhibition is also exhibited by continuous red or blue light. Intermittent far-red light can partially substitute for continuous far-red light in inhibiting maize caryopsis germination, and its effect is reversed to the intermittent red light level when red light is given immediately after each far-red illumination. These results are interpreted as a proof of existence and involvement of phytochrome in the germination control of maize caryopses, though its manifestation is realized only under osmotic stress.Abbreviations D darkness - FR far-red - R red - B blue - c-FR, c-R, c-B continuous FR, R, B, resp. - i-FR, i-R intermittent FR, R, resp.     

Persistence of red light induction in lettuce seeds of varying hydration

Incubation of lettuce seeds (Lactuca sativa L. cv. Grand Rapids) in 0.3 m mannitol allows sufficient water uptake to make seeds fully sensitive to red light. But germination is possible only after lowering the osmotic potential of the incubation medium. The red light induction of these incompletely hydrated seeds can be reversed by far red light. Their reversibility declines with time at a slower rate than seeds incubated in water. About half the seeds in 0.3 m mannitol respond to far red light when all seeds in water have escaped control by far red light. Close to 100% of the seeds remain sensitive to far red exposure if 0.6 m mannitol is used as osmoticum. The retention of the original red light stimulus is inversely related to the concentration of the incubation medium.The fresh weight of viable seeds incubated with water or with an osmoticum increases rapidly during the first 5 hours, then remains stationary for about 12 hours. After that only germinating seeds experience a second increase in fresh weight. Heat-killed seeds do not show such a discontinuity in water uptake.     

Seed germination of Pilosocereus arrabidae (Cactaceae) from a semiarid region of south‐east Brazil

We evaluated the effect of temperature regimes (six constant and four alternating temperatures), light qualities (five red : far red ratios) and water potentials (ΨW; seven NaCl and polyethylene glycol 6000 [PEG] solutions) on the percentage and germination rate, as well as the post‐seminal development morphology, that allow Pilosocereus arrabidae seeds to germinate in a hot semiarid climate on the south‐eastern Brazilian coast. The results showed that seeds germinated similarly between constant and alternating temperatures, with an optimal germination at 25/20°C and 20°C. Pilosocereus arrabidae seeds were photoblastic positive and the final germination percentage was inhibited at low red : far red ratios. Maximum germination was obtained in distilled water (0 MPa) and decreases of Ψ_W in the solutions reduced the germination, which was lower in NaCl than in iso‐osmotic PEG solutions. Germination inhibition appears to be osmotic because the recovery response was high when non‐germinated seeds from both iso‐osmotic solutions were transferred to water. Seeds of P. arrabidae are small and germination is phaneroepigeal. Despite the slow growth typically seen in seedlings and adults of Cactaceae, germination in this species depends on the ability of the seeds to appropriately sense and react to environmental cues that correlate with times and places under low‐risk growth conditions.     

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.     

Photomanipulation of phytochrome in lettuce seeds

Seeds of lettuce (Lactuca sativa L. cv. Grand Rapids) were imbibed and given either short irradiation with red or far red light prior to drying or dried under continuous red or far red light. Seeds treated with either short or continuous red germinate in darkness, whereas seeds treated with either short or continuous far red require a short exposure to red light, after a period of imbibition, to stimulate germination. Irradiation of dry red seeds with far red light immediately before sowing results in a marked inhibition of germination. This result was predicted since far red-absorbing form phytochrome can be photoconverted to the intermediate P650 (absorbance maximum 650 nm) in freeze-dried tissue. A similar far red treatment to continuous red seeds is less effective and it is concluded that in these seeds a proportion of total phytochrome is blocked as intermediates between red-absorbing and far red-absorbing form phytochrome, which only form the far red-absorbing form of phytochrome on imbibition. The inhibition of dry short red seeds by far red light can be reversed by an irradiation with short red light given immediately before sowing, confirming that P650 can be photoconverted back to the far red-absorbing form of phytochrome. The results are discussed in relation to seed maturation (dehydration) on the parent plant.     

Photoinhibition of white clover seed germination at low water potential

Photosensitivity of germination of white clover ( Trifolium repens L. cv. Podkowa) seeds was studied under water deficit (low water potential) conditions at 25°C. The seeds showed negative photoblastism, which was most pronounced at -0.03 MPa polyethylene glycol solution. Inhibition was observed at two different wavelength bands with maxima at 660 nm (R) and around 730 nm (FR). Red light acted identically to white light (maximum inhibition ca 50%). The effect of far-red illumination was less inhibitory (20–30%). The photoresponse required long illuminations (3 h exposures); saturation level was at 0.1 W m⁻², independently of the light quality. White clover seed germination showed no reversibility of the effects of R and FR light. Prolonged illumination with R and FR increased the inhibition, and intermittent illumination had a higher effect than a continuous one. It was concluded that the photoinhibition of germination of seeds of Trifolium repens involves a reaction dependent on the rate of phytochrome interconversion, a property that is characteristic for the high irradiance reaction.     

A Role for the Surrounding Fruit Tissues in Preventing the Germination of Tomato (Lycopersicon esculentum) Seeds : A Consideration of the Osmotic Environment and Abscisic Acid

During tomato seed development the endogenous abscisic acid (ABA) concentration peaks at about 50 d after pollination (DAP) and then declines at later stages (60-70 DAP) of maturation. The ABA concentration in the sheath tissue immediately surrounding the seed increases with time of development, whereas that of the locule declines. The water contents of the seed and fruit tissues are similar during early development (20-30 DAP), but decline in the seed tissues between 30 and 40 DAP. The water potential and the osmotic potential of the embryo are lower than that of the locular tissue after 35 DAP also. Seeds removed from the fruit at 30, 35, and 60 DAP and placed ex situ on 35 and 60 DAP sheath and locular tissue are prevented from germinating. Development of 30 DAP seeds is maintained or promoted by the ex situ fruit tissue with which they are in contact. Their germination is inhibited until subsequent transfer to water, and germination is normal, i.e. by radicle protrusion, and viable seedlings are produced, compared with 30 DAP seeds transferred directly to water; more of these seeds germinate, but by hypocotyl extension, and seedling viability is very poor. Isolated seeds at 35 and 60 DAP re-placed in contact with fruit tissues only germinate when transferred to water after 7 d. At 30 DAP, isolated seeds are insensitive to ABA at physiological concentrations in that they germinate as if on water, albeit by hypocotyl extension. At higher concentrations germination occurs by radicle protrusion. Osmoticum prevents germination, but there is some recovery upon subsequent transfer to water. Seeds at 35 DAP are very sensitive to ABA and exhibit little or no germination, even upon transfer to water. The response of the isolated seeds to osmoticum more closely approximates that to incubation on the ex situ fruit tissues than does their response to ABA. This is also the case for isolated 60 DAP seeds, whose germination is not prevented by ABA, but only by the osmoticum; these seeds are inhibited when in contact with ex situ fruit tissues also. It is proposed that the osmotic environment within the tissues of the tomato fruit plays a greater role than endogenous ABA in preventing precocious germination of the developing seeds.     

Phytochrome and Seed Germination. I. Temperature Dependence and Relative P(FR) Levels in the Germination of Dark-germinating Tomato Seeds

Germination of the dark-germinating seeds of 3 varieties of tomato is controlled by the phytochrome system. Germination is inhibited by far red radiation and repromoted by red applied after far red. At low temperatures, 17 to 20°, a single, low energy far red irradiation is sufficient to inhibit germination in all 3 varieties. At higher temperatures far red is less effective in the inhibition of the germination of the tomato seeds. The phytochrome fraction present as P_FR in the dark-germinating seeds of the Ace variety is about 40% of the total phytochrome present.     

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.     

Germination of seeds in Jussiaea suffruticosa

Seeds of Jussiaea suffruticosa reach high germination percentagesonly when exposed to long periods of continuous illumination.The light reaction may be repeatedly reversed by short exposuresto red and far red light, thus being mediated by the phytochromesystem. Seeds also germinate at high percentages if exposedto various cycles of 1 hr light and 24 hr of darkness at 20°C.If the temperature in the periods of darkness is raised up to30°C or lowered to 10°C the promotive effect of lightis inhibited. High temperatures (35°C) during imbibitionhave a promotive effect, whereas a pure O₂ atmosphere decreasesthe response to light. KNO₃ and kinetin enhance the responseto light but do not provoke germination in the dark. Only ifseed coats are punctured or removed does germination in thedark occur. (Received January 14, 1969; )     

Phytochrome control of skotodormancy release in Grand Rapids lettuce achenes

Light-requiring Grand Rapids lettuce ( Lactuca sativa L.) achenes develop skotodormancy when imbibed in darkness for 7 days at 25°C. Redried skotodormant achenes maintain this type of dormancy upon subsequent rehydration. At 25°C full germination of skotodormant achenes can be induced by continuous and intermittent red light illumination as well as by several brief red irradiations given daily. One brief (10 min) red light irradiation can partly break skotodormancy at 20°C, while at lower temperatures the same treatment results in full induction of germination. Phytochrome control of the release from skotodormancy is proven by a) the dependence of the germination response on the relative sequence of red and far-red light in cyclic irradiations, and b) the reversion of red action by subsequent far-red irradiation. The time course of germination of skotodormant achenes treated with intermittent red light depends upon the length of dark interval between the light pulses. Germination is considerably delayed compared to that of non-skotodormant ones, induced by a single brief red light treatment. This fact in combination with the requirement, over a long period of time, of P_fr action for full manifestation of germination, indicates that skotodormancy is a deeper form of dormancy. It is concluded that the germination of lettuce achenes may always be subjected to phytochrome control.     

The problem of reduced nicotinamide adenine dinucleotide oxidation in glyoxysomes

Phototransformation of phytochrome in lettuce seeds (Lactuca sativa L. var. Grand Rapids) was examined by testing germination responses of seeds irradiated at various temperatures. Temperature variations from 0 to 50 C had no influence on the germination of partially hydrated seeds (about 15% water content) irradiated with either red or far red light prior to imbibition. At −15 C far red light more effectively retarded germination than red light promoted it. No effective phototransformation was detected at −79 C or −196 C.     

Photocontrol of germination in Amaranthus caudatus

Summary Germination of Amaranthus caudatus is inhibited by light, far-red being the most effective part of the spectrum. At temperatures of 25° and below there is a low final germination percentage under continuous far-red whereas above 25° there is only a delaying effect. In the presence of a saturating concentration of gibberellic acid (GA₃) at 25° seeds germinate under continuous far-red although they are delayed. At 25° seeds exposed to 48 hr far-red fail to germinate when transferred to darkness. This induced dormancy can be broken by a single short exposure to red light given at any time after the far-red illumination. This effect of short red can be reversed by a subsequent short period of far-red indicating that the seeds are phytochrome controlled. Although most seeds have escaped from the reversing effect of short far-red after an intervening dark period of 5 hours, germination is greatly reduced by continuous far-red at this time. Results of exposing seeds to varying periods of far-red before and after dark imbibition are interpreted in terms of a continual production of phytochrome in its active P _fr form and a requirement for P _fr action over a long period of time. Effects of intermittent and continuous low intensity far-red on the inhibition of germination provides further evidence for a low energy photoreaction involving phytochrome. Effects on Germination Index of continuous illumination with various light sources maintaining different P _fr /P _total ratios have been investigated. The results suggest that the proportion of phytochrome in the P _fr form is the most important factor in the regulation of germination. A scheme for the phytochrome control of germination in Amaranthus caudatus is presented and possible explanations for the dependence on P _fr /P _total ratio are discussed.Holder of a Science Research Council Studentship.     

Germination Inhibitors in Fruit Extracts of Red Beet (Beta vulgaris cv. rubra

Germination inhibitors in methanol and water extracts of redbeet fruits (Beta vulgaris cv. rubra L.) have been studied usinglettuce and red beet seed germination as bioassays. The methanolextracts contained substances which inhibited lettuce seed germination,but had no effect on the germination of red beet seeds. Germinationof both lettuce seeds and of water-leached or sulphuric acid-treatedred beet seed balls were inhibited by the water extracts. Theconcentrations of ammonia, ferulic acid, and oxalate in thewater extracts were much lower than required for inhibitionof red beet seed germination. The water extracts contained,however, large amounts of inorganic ions, and the results clearlydemonstrated that the inhibitory effect of the water extractson red beet seed germination was mainly due to the content ofsuch inorganic ions.     

Photomorphogenesis of the Gynophore, Pod and Embryo in Peanut, Arachis hypogaea L.

Darkened excized gynophores ceased to elongate after 8–10days in vitro and started to form a pod. Gynophore elongationwas inhibited to a greater extent in total darkness than underlow irradiance, while pod and embryo growth was stimulated indarkness only. Intact gynophores, enclosed in transparent vials containingglass beads, continued to elongate in both light and darkness.In light the elongating gynophores thickened as they penetratedbetween the glass beads, forming a seedless pod at the bottomof the vials. In the dark the elongating gynophores producedsmall pods in which the seeds had started to grow. Excized gynophores elongated in vitro under continuous whitelight at a rate similar to that of intact exposed gynophores.The rate of elongation in vitro, was lower under continuousblue or red-enriched light, than under white light, and wasfurther reduced under continuous far-red irradiation. Pods didnot form during any of the continuous irradiation treatmentsbut only after transfer to darkness, the largest pods formingafter continuous far-red irradiation. As little as 10 min daily exposure to red or far-red irradiancehad the same effect on gynophore elongation as continuous irradiation.Pods formed only when the daily periods of far-red irradiationwere 30 min or less. Reducing the daily exposures to 2 min decreasedthe time to onset of pod formation from 30 to 16 days. Far-redfollowing red irradiation was effective in inhibiting gynophoreelongation stimulated by red irradiation. Pod formation in red/far-redirradiation was only 50 per cent of that observed in far-redirradiation. The involvement of light in continual gynophoreelongation and in the concomitant inhibition of proembryo growthis discussed. Arachis hypogaea L., peanut, gynophore, photomorphogenesis, embryo development, pod development, proembryo     

Light and temperature interaction in the photocontrol of germination of seeds in Ludwigia octovalvis

Summary The germination response of seeds of Ludwigia octovalvis (Jacq.) (=Jussiaea suffruticosa L.; Raven, 1963), to continuous light and to various types of intermittent irradiations is determined at 25, 30, 35 and 40° at different light intensities. At 25 and 40° intermittent irradiations are an effective substitute for continuous ones, while at 30 and 35° no promotion of germination is observed in the intermittent irradiations tested, except if the dark interval between light pulses is reduced to 1 min. Previous results obtained in the photocontrol of germination in this species are confirmed and extended to a variety of light cycles and intensities, indicating that the response to temperature is bimodal unless light is supplied continuously with high intensity, and that germination depends on light intensity more markedly in the temperature region of minimum germination (30–35°). As germination in this species is controlled by the red far-red system, hypothesis to account for the temperature dependence are based on the interaction between temperature and the reactions in which phytochrome is involved.Abbreviations L light - D darkness     

Phytochrome and Seed Germination. III. Action of Prolonged Far Red Irradiation on the Germination of Tomato and Cucumber Seeds

Prolonged irradiation with continuous or intermittent far red prevents the germination of tomato and cucumber seeds. The inhibitory efficiency of intermittent far red decreases with the lengthening of the interval between successive irradiations, and with the increase of temperature. If each far red irradiation is followed by red, germination is restored. Intermittent far red is less inhibitory than intermittent red-far red when red is given immediately before each far red. This effect is more evident when the interval between successive irradiation becomes longer.     

The effect of some growth regulators on light-induced germination of Paulownia tomentosa seeds

Grubišié, D., Konjevié, R. and Neškovié, M. 1988. The effect of some growth regulators on light-induced germination of Paulownia tomentosa seeds. - Physiol. Plant. 72: 525–528.
The germination of Paulownia tomentosa Steud. (Empress tree) seeds can be induced either by red light (R) or by exogenously applied gibberellic acid (GA₃). The R induced germination is completely suppressed by far red irradiation, abscisic acid (ABA) or by growth retardants such as ancymidol, tetcyclacis and paclobutrazol, though not by AMO 1618 or chloro-choline chloride (CCC). The inhibition caused by far red light (FR) and growth retardants can be overcome by GA₃, while the ABA-induced inhibition can be reversed by fusicoccin (FC), which does not reverse the inhibition caused by FR or growth retardants. It is noteworthy that the germination of light insensitive wheat, corn, alfalfa and mung bean seeds is not inhibited by growth retardants.