Characterization of Osmotic Restraints on Lettuce Fruit Germination

Germination responses of fruits of lettuce (Lactuca sativa L.cv. Arctic King) to solutions of mannitol or potassium chlorideof different osmotic potentials were examined over a range oftemperatures in both light and dark. The upper temperature limit(‘cut-off’) for germination was progressively loweredby solutions of increasingly negative osmotic potential. Thisobservation explained inhibition of germination by solutionsof low osmotic potential at single experimental temperatures,found in previous investigations. Solutions of the two compoundshaving equal calculated osmotic potentials produced an equalinhibitory effect. No evidence for solute absorption was seen.Solutions of lower osmotic potential were needed to producethe same inhibition of germination in light against total darknessand also at lower temperatures.     

Nitric oxide accelerates seed germination in warm-season grasses

The nitric oxide (NO) donor sodium nitroprusside (SNP) significantly promoted germination of switchgrass (Panicum virgatum L. cv Kanlow) in the light and in the dark at 25°C, across a broad range of concentrations. SNP also promoted seed germination in two other warm-season grasses. A chemical scavenger of NO inhibited germination and blocked SNP stimulation of seed germination. The phenolic (+)-catechin acted synergistically with SNP and nitrite in promoting seed germination. Acidified nitrite, an alternate NO donor also significantly stimulated seed germination. Interestingly, sodium cyanide, potassium ferricyanide and potassium ferrocyanide at 200 μM strongly enhanced seed germination as well, whereas potassium chloride was without effect. Ferrocyanide and cyanide stimulation of seed germination was blocked by an NO scavenger. Incubation of seeds with a fluorescent NO-specific probe provided evidence for NO production in germinating switchgrass seeds. Abscisic acid (ABA) at 10 μM depressed germination, inhibited root elongation and essentially abolished coleoptile emergence. SNP partially overcame ABA effects on radicle emergence but did not overcome the effects of ABA on coleoptile elongation. Light microscopy indicated extension of the radicle and coleoptiles in seeds maintained on water or on SNP after 2 days. In contrast, there was minimal growth of the radicle and coleoptile in ABA-treated seeds even after 3–4 days. These data indicate that seed germination of warm-season grasses is significantly influenced by NO signaling pathways and document that NO could be an endogenous trigger for release from dormancy in these species.     

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.     

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     

The role of phytochrome in an interaction with ethylene and carbon dioxide in overcoming lettuce seed thermodormancy

Ethylene and CO₂ were used to control induction of germination in thermodormant lettuce seed (Lactuca sativa L.). These experiments ultimately showed that germination depends on the presence of an active form of the phytochrome. The phytochrome system is functional and stable at 35 C, a temperature which completely inhibits germination. Phytochrome responses to red or far red light and darkness showed that this inhibition of germination under light must be due to some other block(s) rather than to a direct inactivation of the phytochrome system itself. A postred radiation increase in lettuce seed germination that is not reversed by far red light was observed. The CO₂ requirement for C₂H₄ action is not due to a change in the medium's pH; addition of C₂H₄ plus CO₂ at the start of imbibition did not result in as much germination as when they were added several hours after imbibition. This reduction in germination, when the gases are added at the start of imbibiton, is due to CO₂.     

Nitrate,abscisic acid and gibberellin interactions on the thermoinhibition of lettuce seed germination

Germination of lettuce seeds has obvious thermoinhibition, but the mechanism for thermoinhibition of seed germination is poorly understood. Here, we investigated the interactions of nitrate, abscisic acid (ABA) and gibberellin on seed germination at high temperatures to understand further the mechanism for thermoinhibition of seed germination. Our results showed that lettuce (Lactuca sativa L. ‘Jianye Xianfeng No. 1’) seeds exhibited notable thermoinhibiton of germination at ≥17°C in darkness, and at ≥23°C in light, but the thermoinhibited seeds did not exhibit secondary dormancy. Thermoinhibition of seed germination at 23 or 25°C in light was notably decreased by 5 and 10 mM nitrate, and the stimulatory effects were markedly prevented by nitric oxide (NO) scavenger 2-phenyl-4,4,5,5-tetramethylimidazoline-1-oxyl-3-oxide. The sensitivity of seed germination to exogenous ABA increased with increasing temperature. Thermoinhibition of seed germination was markedly decreased by fluridone (an inhibitor of ABA biosynthesis) and GA₃, and was increased by diniconazole (an inhibitor of the ABA-catabolizing enzyme ABA 8′-hydroxylase) and paclobutrazol (an inhibitor of GA biosynthetic pathway). The effect of fluridone in decreasing thermoinhibition of seed germination was obviously antagonized by paclobutrazol, and that of GA₃ was notably added to by fluridone, and that of nitrate was antagonized by paclobutrazol, diniconazole and ABA and was added to by GA₃ and fluridone. Our data show that thermoinhibition of lettuce seed germination is decreased by nitrate in a NO-dependent manner, which is antagonized by ABA, diniconazole and paclobutrazol and added by fluridone.     

The effect of growth retardants on phytochrome-induced lettuce seed germination

Experiments were carried out to explore the involvement of the plant hormone gibberellin (GA) in the light-induced germination of lettuce seeds. Three growth retardants known to be inhibitors of GA biosynthesis were tested for their effect on red-light-induced germination. Chlormequat chloride (CCC) and AMO-1618 had no effect, but ancymidol was strongly inhibitory. Moreover, the inhibition caused by ancymidol was completely overcome by GA₃. CCC and AMO-1618 inhibit the formation ofent-kaurene, while ancymidol blocks the oxidation ofent-kaurene toent-kaurenoic acid. Ancymidol also was found to inhibit GA-induced dark germination of lettuce seeds, and this inhibition was partially reversed by higher levels of GA. Therefore, the results suggest two possibilities for the relationship between phytochrome and GA in this system: first, the rate-limiting step in the germination of light-sensitive lettuce seeds, that which is regulated by phytochrome, is the oxidation ofent-kaurene toent-kaurenoic acid. Alternatively, red-light treatment may result in the release of active GAlike substances which, in turn, induce germination. In either case the results presented here support the view that phytochrome exerts its effect on lettuce seed germination by means of GA rather than via an independent pathway.     

Induction of phenylalanine ammonia-lyase (PAL) in germinating lettuce seeds (Lactuca sativa)

Dry lettuce seeds (achenes of Lactuca sativa L. cv. Grand Rapids) contain no detectable phenylalanine ammonia-lyase (PAL, EC 4.3.1.5) activity. Enzyme activity could be detected in these seeds within 4 h of imbibition under white light. The specific activity of PAL increased rapidly during the next 12–16 h of imbibition. Far-red light completely suppressed germination as well as the development of PAL. Gibberellic acid (GA₃, 0.1 m M ), although effective in causing almost 100% germination in dark, did not induce proportionate increases in PAL. Seed germination as well as PAL activity were substantially inhibited by cis -4-cyclohexene-l, 2-dicarboximide (CHDC, 1.0 m M ) both in light and dark. Both GA₃ and benzyladenine (BA, 0.1 m M ) retarded radicle elongation in light. Concomitantly, a decrease in PAL activity was observed. Benzyladenine was able to reverse the effects of CHDC on germination but PAL activity was still highly reduced, probably due to the inhibitory effects of BA on elongation of the radicle. More than 95% of the extractable PAL was found to be present in the radicle. When seeds incubated in white light for 10 h were transferred to FR, further increases in PAL activity as well as the growth of the radicle were severely inhibited. It is suggested that the induction of PAL in light-sensitive lettuce seeds is coincidental with the germination of seeds, and the amount of PAL per germinated seed is related to the extent of elongation of the embryonic axes.     

Characterization of the Germination Responses to Temperature of Lettuce (Lactuca sativa L.) Achenes

Germination tests were done on 23 cultivars of lettuce (Lactucasativa L.) comprising a variety of different morphological formsselected for cultivation at various seasons. Significant differences at the upper limit of temperature tolerancewere found between different cultivars: maximum temperaturesfor 50 per cent germination ranged from 23 to 32 °C, andsusceptibility to the induction of secondary dormancy by hightemperatures varied widely from one cultivar to another. Nocorrelations were established between germination responsesand heading type, achene colour or growing season The germination responses of all cultivars at temperatures fromjust above 0 to 20 °C were closely similar and results froma large number of experiments were used to produce a standardgermination reference curve foti culvars of L. sativa. Departfuresrom this curve were found to arise predominantly from variationsin seed quality or test conditions rather than from the genotypeof the cultivar. Detailed comparisons of germination at all possible combinationsof alternate and constant day/night temperatures are presentedfor two cultivars. One with relatively high tolerance to high-temperatureinducedinhibition, the other with low tolerance. The results are discussed in relation to the original distributionof the species as a wild plant and its history of cultivationby man. Lactuca sativa L., Lettuce, achene germination, temperature response     

Analysis of temperature effect on the germination of New York lettuce seeds

When New York lettuce seeds were imbibed below 20°C in thedark, full germination was observed, whereas none occurred above35°C even under red-light irradiation. Partial treatmentwith high temperature during imbibition revealed the involvementof a thermo-labile process in the germination. This processwas completely inhibited above 30°C in the dark, but re-activatedby the following incubation below 20°C. The longer the seedswere preincubated above 30°C, the longer the period below20°C required for re-activation. The results suggest thatthe thermo-labile process is controlled by a thermo-labile factor.The factor was inactivated slowly at 25°C and rapidly above30°C to a form which could be re-activated below 20°Cbut was irreversibly inactivated when the seeds were imbibedat temperatures above 45°C for more than 40 hr. The escapereaction of the phytochrome system proceeded even at 35°Cwhereas no germination was observed at that temperature. Thus,die germination of lettuce seeds is regulated not only by thephytochrome system but also by the thermolabile factor. (Received August 11, 1975; )     

Allelopathy in Heracleum laciniatum: Inhibition of Lettuce Seed Germination and Root Growth

Both dark and red light germination of lettuce seeds (cv. “Maikönig”) as well as their root and hypocotol elongation were inhibited when the seeds were sown in petri dishes together with a few seeds of Heracleum laciniatum Horn. This inhibition was not significantly counteracted by the presence of gibberellic acid (GA₃) or/and 6-benzylaminopurine (BA). However, a large proportion of the lettuce seeds germinated abnormally (only cotyledons emerged) when treated with BA in the presence of Heracleum seeds. GA₃ had alone no significant effect on abnormal germination, but it counteracted the effect of BA to some extent. The inhibitory effect of Heracleum seeds gradually disappeared during a moist incubation period of one to seven days in darkness at 25°C. When lettuce seeds were pre-incubated together with Heracleum seeds for one to five days the remaining, non-germinated lettuce seeds had lost their ability for subsequent germination in darkness in distilled water. This induced dark dormancy was to a great extent broken by red light, but not by GA₃ or/and BA. H. laciniatum seeds inhibited the germination of Salix pentandra seeds and to some extent also the germination of radish but had no effect on the germination of spruce.     

PRECONDITIONING OF GERMINATION IN LETTUCE AT TIME OF FRUIT RIPENING

Koller , D. (The Hebrew U., Jerusalem. ² ) Preconditioning of germination in lettuce at time of fruit ripening. Amer. Jour. Bot. 49(8): 841–844. 1962.—The effects of controlled environments during maturation on subsequent germination response of the seed were investigated with the light-sensitive lettuce ‘Grand Rapids.‘ The plants matured under a variety of temperature/photoperiod combinations, after completing vegetative growth and flower initiation under identical conditions. Good seed set was obtained only at night temperatures between 17 and 23 C, under short-day (8-hr) conditions. Seed germination was tested at 20, 23 and 26 C, in continuous dark and after a light-break, respectively. The conditions under which the seed had ripened brought about quantitative changes in the germination response. The stimulating effects of a light-break on germination at high temperatures did not change, but maturation under high temperatures or in continuous light increased the high-temperature tolerance of seed germination both in continuous dark and after a short light-break.     

Promotive effects of organic solvents and kinetin on dark germination of lettuce seeds

Significant promotion in dark germination was observed when Grand Rapids lettuce (Lactuca sativa L.) seeds were soaked in acetone or dichloromethane, vacuum-dried, and imbibed at 25 C. Permeation of kinetin via these organic solvents further enhanced the dark germination. Those seeds that were affected by acetone and acetone-kinetin treatments and germinated in the dark escaped red-far red photocontrol of germination. Although abscisic acid was not detected in the organic solvent leachates, they did contain other inhibitory substances affecting lettuce seedling growth. In the light, acetone and acetone-kinetin treatments also enhanced the rate of germination and the increased germination by acetone-kinetin treatment was correlated with increased polyribosome formation. The possible mechanisms involved in promotion of lettuce seed germination by organic solvents and kinetin are discussed.     

Nitric oxide stimulates seed germination and de-etiolation, and inhibits hypocotyl elongation, three light-inducible responses in plants

Seed germination, greening of etiolated plants and inhibition of hypocotyl elongation are stimulated by light, which is sensed by various types of photoreceptor. Nitric oxide (NO) has proven to be a bioactive molecule, especially in mammalian cells and, most recently, in plants. Like some phytochrome-dependent processes, many NO-mediated ones are accomplished through increases in cGMP levels. Given these similarities, we proposed that NO could take part in light-mediated events in plants. Here we show that NO promotes seed germination and de-etiolation, and inhibits hypocotyl and internode elongation, processes mediated by light. Two NO donors, sodium nitroprusside (SNP) and S-nitroso-N-acetylpenicillamine induced germination of lettuce (Lactuca sativa L. cv. Grand Rapids) seeds in conditions in which this process is dependent on light (e.g. 26 °C). This was a dose-dependent response and was arrested by addition of an NO scavenger, carboxy-PTIO. In addition, nitrite and nitrate, two NO-decomposition products were ineffective in stimulating germination. Wheat seedlings sprayed with SNP and grown in darkness contained 30–40% more chlorophyll than control seedlings. Nitric-oxide-mediated partial greening was increased by light pulses, wounding and biotic stress. Arabidopsis thaliana (L.) Heynh. (ecotype Columbia) and lettuce seedlings grown in the dark had 20%-shorter hypocotyls in NO treatments than in control ones. On the other hand, internode lengths of potato plants growing under low light intensity and sprayed with 100 μM SNP were also 20% shorter than control ones. These results implicate NO as a stimulator molecule in plant photomorphogenesis, either dependent on or independent of plant photoreceptors. Received: 27 April 1999 / Accepted: 16 June 1999     

Photophysiology of turion formation and germination inSpirodela polyrhiza

Standardized laboratory techniques for the vegetative growth of the duckweedSpirodela polyrhiza (Lemnaceaé), and for formation as well as germination of their turions were described. Increasing photon fluence rates of blue or red light increased the yield of turions. A specific stimulating effect of blue light was demonstrated under autotrophic but not under mixotrophic conditions. Therefore the spectral composition of light is not important in mixotrophic formation of turions whereas in autotrophic formation light sources with a higher portion of blue light are recommended. Dark-grown (etiolated) turions showed accelerated germination and higher germination percentage in comparison with light-grown turions after induction by a single red light pulse. This difference was overcome in continuous red light by speeding up the germination response of light-grown turions. Use of Petri dishes (8 cm³ nutrient solution) instead of Erlenmeyer flasks (50 cm³ nutrient solution) retarded germination response. Especially for long term experiments the use of Erlenmeyer flasks is recommended. Storage of turions for 72 h at 25 ‡C following at 5 ‡C in darkness after-ripening resulted in a decreased lag phase of the light-induced germination both after induction by a single light pulse and in continuous light. We thank Dr. Halina Gabrys, University of Crakow, Poland for critical discussion.     

Respiration of Grand Rapids lettuce (Lactuca sativa L.) seeds in relation to chemical and photocontrol of germination

A 10-min irradiation with red light (R) of ‘Grand Rapids’lettuce (Lactuca sativa L.) seeds increased respiratory rates(QO₂ and QCO₂) over those of far red (FR)-irradiated seeds within2 to 3 hr. Differences in respiration between R- and FR-irradiatedseeds became more pronounced with time. The respiration, indarkness at 25?C, of seeds air dried after germination promotingimpregnation treatments with 1.0 mM GA₃+0.5 mM kinetin in acetonewas stimulated slightly by the 6th hour after planting and markedlyby the 12th and 24th hour. Results with seeds in GA₃+kinetindissolved in water were similar. ABA, which inhibits germinationand seedling growth, prevented stimulation in respiration oflight-induced seeds. ABA also repressed respiration when usedwith GA₃+kinetin. The germination and growth retardant, isopropylN-(3-chlorophenyl) carbamate (CIPC), did not affect the respirationof either R- or FR-treated seeds. A comparison of the effectsof chemical and light treatments on respiration indicate thatchemical and photocontrol mechanisms are not identical. (Received May 17, 1976; )     

Dark Reversion of Phytochrome in Lettuce Seeds Stored in a Water-saturated Atmosphere

Dark reversion of the far red-absorbing form of phytochrome, which does not occur in dry lettuce (Lactuca sativa var. Grand Rapids) seeds, appears to take place in seeds stored in a water-saturated atmosphere. The water content (approximately 70% after 10 days) of such seeds is insufficient to support germination; however the treatment enhances germination in seeds stored for 1 to 5 days, but this enhancement subsequently disappears, and the effect of extended storage (up to 28 days) is inhibiting. The half-time for dark far red-absorbing phytochrome reversion is 7 to 8 days, and at this time it can be completely reversed by exposing the seeds to a flash of red light. Storage of more than 7 to 8 days decreases red light enhancement of germination.     

THE EFFECTS OF TEMPERATURE ON THE GERMINATION AND RADICLE GROWTH OF PHOTOSENSITIVE LETTUCE SEED

1. The time course of germination of Grand Rapids lettuce seedshas heen followed with different combinations of temperature(3°–35°) and irradiation (red or far-red light).For each set of conditions the following three parameters weredetermined: (i) the time required for half maximum germination,(ii) the rate of germination during the actively germinatingphase, and (iii) the maximum germination attained. In general,as the temperature was lowered, with dark-imbibed seeds, (i)became longer, (ii) became lower, but (iii) became progressivelyhigher. The effect of red light at any temperature was to shorten(i) and increase (ii) and (iii) over the values dark controls.Far-red light exerted an effect opposite to that of red light.Temperatures higher than 25° inhibited (ii) and (iii) underany light conditions. The optimum temperature to the actionof red and far-red light is 25°, at which the stimulatoryeffect of red light and the inhibition of this effect by far-redlight are both maximal. 2. The growth of the radicles of de-coated seeds of Grand Rapidslettuce shows two phases at all temperatures studied. PhaseI is characterized by slow but linear growth which continuesuntil shortly after visible differentiation of the radicle intothe hypocotyl and the root. Phase II is a phase of active growthin which the total length reflects mainly the length of theroot. The optimum temperature for Phase I is 25°-35°,and that, for Phase II is 25°. In neither phase, and atnone of the temperatures studied, is there any effect of redor far-red radiation on the growth of the radicle. The firstvisible sign of radicle elongation in red light induced seeds,however, takes place at exactly the same time as that of germination. 3. Similarities and dissimilarities between the germinationand the growth are pointed out, and it is concluded that thetwo phenomena are different, but proceed at sites closely associatedin the embryo. ¹Present address: Johnson Foundation for Medical Physics, Universityof Pennsylvania, Philadelphia, Pa., U.S.A.