Dependence of phytochrome action in seeds on membrane organization

Germination of Amaranthus retroflexus L. seeds imbibed at 40 C is enhanced by establishing the active form of phytochrome before a reduction in temperature to <32 C. The half-time for effectiveness of the lower temperature is about 8 min at 15 C. Isolated membrane fragments of A. retroflexus seeds associated with the fluorescent probe 1,8-anilino-naphthalene sulfonate (ANS) increase in structural order as the temperature is lowered through the 32 C region. The germination response is decreased by the membrane-disruptive substances tris, octonoate, and ethanol. The results show that phytochrome activity is associated with an organized membrane. By using ANS with membrane fragments from Setaria faberi Herrm. seeds, leakage of amino acid was found to be enhanced at temperatures >32 C by a transition in the plasmalemma.     

Seed germination ecology of the annual grass Leptochloa panicea ssp. mucronata and a comparison with L. panicoides and L. fusca

Leptochloa panicea ssp. mucronata is an annual grass that grows in relatively dry habitats. Requirements for dormancy loss and germination were determined for seeds of this species and compared to those of two species from wet habitats. Seeds of L. panicea were dormant at maturity in autumn, but when exposed to actual or simulated autumn temperatures (e.g. 20/10, 15/6 °C), they entered conditional dormancy and thus germinated to high percentages in light at 35/20 °C. Seeds buried in non-flooded soil exposed to natural seasonal temperature changes in Kentucky (USA) were non-dormant by the following summer and germinated to 80–100 % in light at 25/15, 30/15 and 35/20 °C. Seeds buried in non-flooded soil exhibited an annual conditional dormancy/non-dormancy cycle, with seeds mostly germinating to 80–100 % in light at 30/15 and 35/20 °C throughout the year but to 80–100 % in light at 25/15 °C only in summer. Results for L. panicea were compared to published data for L. panicoides and L. fusca. Whereas seeds of L. panicea buried in flooded soil failed to come out of dormancy, those of L. panicoides, an annual of moist habitats such as mudflats, exhibited an annual conditional dormancy/non-dormancy cycle, and those of L. fusca, a semi-aquatic, required flooding for both dormancy loss and germination. Differences in dormancy breaking and germination responses of seeds of Leptochloa species may help to explain why this genus occupies a wide range of habitats with regard to soil moisture conditions.     

Breaking Setaria parviflora seed dormancy by nitrates and light is part of a mechanism that detects a drawdown period after flooding

We explored the hypothesis that, in flood-prone habitats, nitrates can signal to seeds that a drawdown period has begun. To investigate this issue, Setaria parviflora (Poir.) Kerguélen seeds were buried in a never-flooded upland and a nearby, flood-prone lowland grassland. Seeds were exhumed during the flooding period. Additionally, grassland mesocosms with buried S. parviflora seeds were flooded during 20 d (controls drained). After both field and mesocosm pretreatments germination was assayed in laboratory at 25 °C in a medium with or without nitrates, under red light pulses or in darkness. Seeds exhumed from the never-flooded upland showed no specific requirements to germinate. In contrast, seeds exhumed from the flooded lowland germinated ca. 65% when nitrates were combined with red light pulses, significantly higher than in the rest of the treatments. Seeds exhumed from drained mesocosms germinated equally in all treatments. However, in the seeds exhumed from the flooded mesocosms, nitrates increased germination by more than 20% compared with seeds imbibed in water. Seeds germinated ca. 85% when nitrates were combined with red light pulses, significantly higher than in the other treatments. We can conclude that after flooding, S. parviflora seeds require nitrate and light to germinate. Therefore, a large fraction of seeds do not germinate unless nitrates are combined with light, indicating a drawdown period after floods and vegetation gaps.     

Effects of seed maturation time and dry storage on light and temperature requirements during germination in invasive Prosopis juliflora

The effects of time of seed maturation and dry seed storage and of light and temperature requirements during seed incubation on final germination percentage and germination rate were assessed for the invasive shrub Prosopis juliflora (Sw.) D.C., grown under desert environmental conditions of the United Arab Emirates (UAE). Seeds were collected from Fujira on the northern coast of the UAE at different times during the growing seasons (autumn, winter and spring) and were germinated immediately and after 8 months of dry storage under room temperature (20±3 °C). Seeds were germinated at three temperatures (15, 25 and 40 °C) in both continuous light and darkness. The results showed significant effects for time of seed collection, seed storage, light and temperature of seed incubation and many of their interactions on both germination percentage and rate. Fresh seeds matured during autumn and winter germinated significantly greater at 40 °C and in light than at lower temperatures and in dark. Storage significantly increased germination percentage and rate; the increase was greater for seeds matured during winter than for seeds matured during spring. This indicates that dormancy breakage was greater in seeds of winter than seeds of spring. The need for high temperature to achieve greater germination was significantly reduced after seed storage, especially for seeds matured in autumn and winter.     

Interactions of light and a temperature shift on seed germination

Germination of Rumex obtusifolius L. seeds is potentiated to an observable degree in 2 minutes by a single shift in temperature from 20 to 35 C. Half-maximal potentiation requires less than 32 minutes at the higher temperature. Similar sensitivities to shifts in temperature were observed for seeds of Barbarea vulgaris, R.Br. B. verna (Mill.) Asch., and Lepidium virginicum L. A shift in temperature interacts strongly with change in form of phytochrome induced by light on germination of the four kinds of seeds. The potentiated effects for R. obtusifolius are only moderately affected by 40 μm cycloheximide. Both the temperature shift and light actions are apparently independent of processes of synthesis necessary for growth.     

Tolerance and recovery responses of playa halophytes to light,salinity and temperature stresses during seed germination

Halogeton glomeratus (M. Bieb.) C.A. Mey., Lepidium latifolium Linn. and Peganum harmala Linn. are distributed in temperate salt playa habitats of Upper Hunza, Pakistan. Seeds were germinated under various salinity (0–500 mM NaCl), light (12 h-light:12 h-dark and 24 h-dark) and temperature (5/15, 10/20, 15/25, 20/30, and 25/35 °C, dark/light) regimes for 20 days to determine the optimal conditions for germination and recovery of seeds from these factors when exposed to less than optimal conditions. Seeds that failed to germinate in dark were transferred successively to 12 h-photoperiod, salinity to distilled water and from various temperature regimes to 20/30 °C, to determine the effect of these stresses and the ability of these seeds to recover respectively. Highest seed germination (H. glomeratus and L. latifolium: 100%; P. harmala: 80%) was obtained in non-saline control at 20/30 °C in 12 h-photoperiod, however, increase in salinity progressively inhibited seed germination. Seed germination of H. glomeratus and P. harmala was substantially inhibited and that of L. latifolium was prevented in dark. Salinity and dark treatments have a synergistic effect in inhibiting seed germination of all species. No seed of any species germinated at 5/15 °C; germination was substantially inhibited at 25/35 °C both for H. glomeratus and P. harmala while L. latifolium failed to germinate at 25/35 °C. Rate of germination also decreased with an increase in salinity at all temperature regimes but this effect was minimal at optimal temperature regime of 20/30 °C. After successive elimination of light, salinity and temperature stresses, final seed germination was identical to respective controls. The results indicate that seeds of these temperate halophytes could endure environmental stresses without losing viability and germinate readily when these stresses are removed. Under the extremely variable conditions of the playa habitat these species are highly opportunistic exploiting the windows of opportunity available during spring or early summer.     

Seed dormancy-breaking and germination requirements ofDrosera anglica,an insectivorous species of the Northern Hemisphere

Seeds of Drosera anglica collected in Sweden were dormant at maturity in late summer, and dormancy break occurred during cold stratification. Stratified seeds required light for germination, but light had to be given after temperatures were high enough to be favorable for germination. Seeds stratified in darkness at 5/1 °C and incubated in light at 12/12 h daily temperature regimes of 15/6, 20/10 and 25/15 °C germinated slower and to a significantly lower percentage at each temperature regime than those stratified in light and incubated in light. Length of the stratification period required before seeds would germinate to high percentages depended on (1) whether seeds were in light or in darkness during stratification and during the subsequent incubation period, and (2) the temperature regime during incubation. Seeds collected in 1999 germinated to 4, 24 and 92 % in light at 15/6, 20/10 and 25/15 °C, respectively, after 2 weeks of stratification in light. Seeds stratified in light for 18 weeks and incubated in light at 15/6, 20/10 and 25/15 °C germinated to 87, 95 and 100 %, respectively, while those stratified in darkness for 18 weeks and incubated in light germinated to 6, 82 and 91 %, respectively. Seeds collected from the same site in 1998 and 1999, stratified in light at 5/1 °C and incubated in light at 15/6 °C germinated to 22 and 87 %, respectively, indicating year-to-year variation in degree of dormancy. As dormancy break occurred, the minimum temperature for germination decreased. Thus, seed dormancy is broken in nature by cold stratification during winter, and by spring, seeds are capable of germinating at low habitat temperatures, if they are exposed to light.     

Promotion of seed germination by nitrate, nitrite, hydroxylamine, and ammonium salts

Action and uptake of azides, nitrates, nitrites, hydroxylamines, and ammonium salts were measured on germination of Amaranthus albus, Lactuca sativa, Phleum pratense, Barbarea vulgaris, B. verna, and Setaria glauca seeds. Nitrate and nitrite reductase activities were measured in vivo for each of these kinds of seeds. Activities were measured in vitro for catalase, peroxidase, glycolate oxidase, and pyridine nucleotide quinone reductase on extracts of A. albus and L. sativa seeds before and after germination. The enzymic activities measured and the responsiveness of the haemproteins to inhibition by the several compounds indicate that nitrites, azides, and hydroxylamines promote seed germination by inhibition of H₂O₂ decomposition by catalase. Ammonium salts showed pronounced promotive activity only for B. verna and B. vulgaris seeds, for which they served as metabolic substrates.     

Osmoconditioning and Ageing of Pepper Seeds During Storage

The effects of osmoconditioning on the germination at 15 and25 °C of pepper (Capsicum annuum L.) seeds were studiedover a 3-year period with respect to temperature of storage.Untreated seeds stored at 5 °C showed high germinabilitythroughout the entire storage period, whereas untreated seedsstored at 25 °C showed a progressive decline in germinability,especially when assayed at 15 °C. Seeds that had been osmoconditionedprior to storage retained a high level of germinability irrespectiveof either storage or germination temperatures. When seeds thathad been stored at 25 °C were osmoconditioned after storage,there was a significantly higher germinability (assayed at 15 °C) in comparison with the corresponding untreated seeds.Seeds that were osmoconditioned twice (prior to and after storage)germinated in a similar way to those that had been osmoconditionedonce only Lactuca saliva L., lettuce, Hordeum oulgare L., barley, seed storage, moisture content, relative humidity, water potential, temperature, oxygen     

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.     

Sterile Germination Requirements of Seeds of Some Water Plants

A sterile germination study with seeds of some common phanerogamic water plants showed almost 100 per cent germination for seeds of Alisma plantago–aquatica, Baldellia ranunculoides and Nymphaea alba. Seeds of Potamogeton lucens could be germinated to about 40 per cent, seeds of Polygonum amphibium germinated sporadically while those of Cladium mariscus could not be germinated at all. Freshly harvested seeds of Alisma and Baldellia showed an ability to germinate at both 20°C and 35°C. A stratification period of one month at +4°C gave germination of all species tested, with the exception of Cladium. Potamogeton germinated in light only, the other species both in light and darkness. Treatment times for surface sterilization in disinfectants are given.     

Inhibition of linolenic Acid synthesis and modification of chilling resistance in cotton seedlings

The temperature at which cotton seeds (Gossypium hirsutum L.) germinated influenced the fatty acid composition of the polar lipids of developing root tips. Seeds were germinated at 15, 20, 25, and 30 C. As the temperature decreased the linolemic acid content of the polar lipid fraction increased. Sandoz 9785[4-chloro-5-(dimethylamino)-2-phenyl-3(2H)-pyridazinone] reduced the low temperature-induced increase in linolenic acid content of the polar lipids and reduced seedling ability to withstand 8 C chilling. The results are consistent with the conclusion that chilling resistance in cotton seedlings is related to the level of linolenic acid in the polar lipids in the developing root tips.     

DORMANCY AND GERMINATION IN SEEDS OF COMMON RAGWEED WITH REFERENCE TO BEAL'S BURIED SEED EXPERIMENT

Common ragweed (Ambrosia artemisiifolia L.) was one of 19 herbaceous weedy species used by Beal in his buried viable seed experiment started in 1879. No seeds germinated during the first 35 years of the experiment when germination tests were performed in late spring, summer or early autumn. Germination did occur in seeds buried for 40 years when seeds were exhumed and tested for germination in early spring. Data obtained in more recent research provide the probable explanation for these results. Seeds of common ragweed that do not germinate in spring enter secondary dormancy by mid to late spring and will not germinate until dormancy is broken the following late autumn and winter. Thus, during the first 35 years of the experiment seeds were dormant when tested for germination, whereas seeds buried for 40 years were nondormant. Seeds buried 50 years or longer did not germinate when tested in spring, probably because they had lost viability and/or seeds germinated during burial and seedlings died.     

Role of temperature in the germination ecology of three summer annual weeds

Summary Ambrosia artemisiifolia L., Chenopodium album L., and Amaranthus retroflexus L. are three summer annual weeds that occur in disturbed habitats. In nature, the peak germination season for A. artemisiifolia and C. album is in early to mid-spring, while in A. retroflexus the peak germination season is late spring to early summer. Furthermore, seeds of A. artemisiifolia germinate only in spring, while seeds of C. album and A. retroflexus germinate throughout the summer. In an attempt to explain the differential germination behavior of these three species in nature, changes in their germination responses to temperature during burial in a non-heated greenhouse from October 1974 to October 1975 were monitored. A high percentage of the seeds of all three species after-ripened during winter. Seeds of A. artemisiifolia and C. album germinated at temperatures characteristic of those in the field in early and mid-spring, but seeds of A. retroflexus required the higher temperatures of late spring and early summer for germination. Seeds of all three species germinated to higher percentages in light than in darkness. Non-dormant seeds of A. artemisiifolia that did not germinate in spring entered secondary dormancy. On the other hand, seeds of C. album and A. retroflexus that did not germinate when temperatures first became favorable for germination, did not enter secondary dormancy and, thus, retained the ability to germinate at summer field temperatures during summer. Thus, temporal differences in the germination behavior of these three species are caused by the differential reaction of the seeds to temperature during the annual temperature cycle.     

Strict requirement of fluctuating temperatures as a reliable gap signal in <Emphasis Type="Italic">Picris hieracioides</Emphasis> var. <Emphasis Type="Italic">japonica</Emphasis> seed germination

Picris hieracioides var. japonica (Asteraceae), which grows in occasionally disturbed habitats such as riverbanks, is rarely observed under dense vegetation. We examined the effect of the experience and timing of receiving leaf-transmitted light in gap-detecting seed germination in this plant. Seeds under unfiltered light, which simulated the light conditions of seeds on the soil surface in a canopy gap, germinated at a constant temperature of 20°C. However, most seeds in darkness, which simulated the light conditions of seeds buried in the soil without receiving leaf-transmitted light, germinated under temperature fluctuations of over 4°C. Seeds in darkness after receiving leaf-transmitted light for 1 week, which simulated the light conditions of seeds buried in the soil after receiving leaf-transmitted light, germinated under temperature fluctuations of over 8°C. Finally, seeds under continuous leaf-transmitted light, which simulated the light conditions of seeds on the soil surface below preexisting vegetation, germinated under temperature fluctuations of over 12°C. Seeds that experience unfiltered light, which suggests that they are in a gap, should not delay germination. In contrast, seeds that have received leaf-transmitted light should delay germination until the vegetation above is removed. Seeds exposed to leaf-transmitted light required larger temperature fluctuations in darkness than did untreated seeds, and seeds under continuous leaf-transmitted light required the largest temperature fluctuations. The various germination reactions to each gap signal in P. hieracioides var. japonica seeds allow the more reliable detection of gaps for subsequent seedling establishment. The requirement for gap signals that created high precision of timing in the germination process results in the germination of this species only in gaps. Therefore, P. hieracioides var. japonica is rarely found under dense vegetation.     

Desert species adapted for dispersal and germination during floods: Experimental evidence in two Astrophytum species (Cactaceae)

Dispersal of organisms by water is known as hydrochory and is an important means of seed transportation. Here we present experimental evidence of possible dispersal and germination of seeds in water for arid land cacti. Seeds from Astrophytum spp. have unusual traits for plants from arid environments, traits that are typical of water dispersed seeds. Astrophytum capricorne and A. ornatum seeds disperse by floating in the Chihuahuan Desert. Seeds of other native species (Acacia farnesiana, Opuntia streptacantha and Prosopis laevigata (mesquite) were used as control, as well as commercial beans (Phaseolus vulgaris). Seed structures of A. capricorne and A. ornatum are consistent with water dispersal (hat shape, a funicular envelope covering a prominent hilum, and air chambers throughout the tegument). Seeds of A. capricorne showed the greatest water permeability, higher than those of A. ornatum which had similar permeability to common beans and mesquite. Acacia farnesiana and Opuntia streptacantha had the lowest permeability. Seeds of A. capricorne without a funicular layer floated more than those with a funicular layer. All seeds of A. ornatum floated initially, but 10% of those with funicular layer sank after a 30 days floating period. P. laevigata seeds floated also, but less than those of Astrophytum spp., and seeds of A. farnesiana and Phaseolus vulgaris did not float. Germination of A. capricorne was high under all treatments, both when immersed in a water layer or only moistened. Germination of A. ornatum was generally low (<30%), with <5% germination for seeds immersed in water and for those placed on a water film. Seeds of A. capricorne germinated slower in a water layer than in other treatments. No difference in germination rate was found across all treatments in A. ornatum.     

Changes in Lipids of Bean Seeds (Phaseolus vulgaris) and Corn Caryopses (Zea mays) Aged in Contrasting Environments

Seeds of French bean (Phaseolus vulgaris L. cv. The Prince)and caryopses of sweet corn (maize; Zea mays L.F1 hybrid Firstof All) were stored in environments of 79% relative humidityand 25 °C, 80% r.h. and 40 °C or 100% r.h. and 45 °C,giving ageing (loss of gcrminability and vigour) over periodsof months, weeks or days, respectively. The relationship betweenchanges in lipids and changes in germinability or vigour wasunaffected in general by the speed of ageing. In the corn caryopsesthere was no evidence of hydrolysis of phospholipids or peroxidationof fatty acids during ageing. In the bean seeds phosphatidicacid increased during the ageing period and phosphatyl cholinedeclined. The percentage of fatty acids as hnolenic acid initiallyfell during bean ageing, but in the slower ageing conditionsit rose again as germination reached zero. In bean, the presenceof phosphatidic acid could be a sensitive indictator of lossof vigour, but relative proportions of the different fatty acidswould be a misleading indicator of quality. Rapid artificialageing may be an adequate model, in some species, of ageingat moderate speeds and of ageing under some ambient conditions. French bean (Phaseolus vulgaris L. cv. The Prince), sweet corn (maize, Zea mays L., Fl hybrid First of All), seed ageing, phospholipids, fatty acids     

Effect of temperature on glycerol metabolism in membranes and on phospholipases C and D of germinating pea embryos

The effect of temperature of imbibition on the synthesis and turnover of membrane phosphatidyl choline was studied. Pea seeds (Pisum sativum cv. Alaska) were imbibed in [U-¹⁴C]glycerol and then germinated. Seeds were kept constantly either at 5° or 25°, or were imbibed at one temperature and then germinated at the other one. Glycerol incorporation into phosphatidyl choline in the ER and the plasma membrane, obtained from the embryonic axes after germination, and the glycerol pool were measured. Embryos from seeds kept constantly at 25° showed a rapid incorporation of glycerol into membranes followed by a loss of label; in embryos from seeds kept at 5° incorporation was much lower. Embryos from seeds transferred from 25° to 5° behaved as if continuously kept at 25°, while the behaviour of the embryos from seeds transferred from 5° to 25° resembled embryos from seeds maintained at 5°. The glycerol content of the axes rose during imbibition and fell thereafter. The activities of phospholipases C and D also responded to the initial temperature of imbibition, but the two activities changed differently. The results are discussed in relation to the effect of transient exposure to temperature changes in the seed membranes and the possible way in which such changes are sensed.     

Seed Development in Phaseolus vulgaris L. cv Seminole: II. Precocious Germination in Late Maturation

Seeds of Phaseolus vulgaris L. cv Seminole in late maturation phase germinated precociously in vitro. Germination occurred in the absence of free water after 5 days but within 24 to 48 hours in contact with water. Excised axes germinated within 12 hours and embryos by 48 hours only if supplied with water. Ethylene accelerated the germination of seeds and embryos irrespective of water availability. There was no effect of ethylene on the rate of axis germination. Ethylene was equally effective within the range 0.5 to 1000 parts per million and 1 hour exposure was fully effective. Induction of precocious germination in vivo was observed by manipulating water content inside pods or by ethylene injection, whether pods were attached to the parent plant or not. These results demonstrate the importance of endogenous regulation of water supply in suppressing precocious germination. Ethylene is identified as a powerful antagonist to the natural control.     

Seed maturation time influences the germination requirements of perennial grasses in desert climate of Arabian Gulf

Qatar has a dry, subtropical desert climate, with minimum annual rainfall and intensely hot and humid summers. Using indigenous grass, those adapted to local conditions have the potential to be used for fodder and can also be used for restoration or rehabilitation of degraded rangelands. Chloris virgata, Coelachyrum brevifolium and Cenchrus ciliaris bloom twice a year from April to May (summer) and September to October (winter) under the nursery condition. Therefore, it is important to understand, how seeds produced in different seasons affect the dormancy as well as germination of these species. Seeds of C. virgata, C. brevifolium and C. ciliaris, three desert grasses, were collected from the plants growing on Shahniya nursery in two different seasons, summer (May) and winter (October). The seeds collected in May (summer) were stored up to winter. However seeds collected in October (winter) were immediately used for experiment. We compared the germination potential of seeds that matured in different season at different alternating temperatures at 15/25, 20/30 and 25/35 °C. Lower temperatures correspond to the dark period, while higher temperatures reflect the light period. Seeds collected in summer season (old seeds) were heavier as compared to seeds collected in winter season (new seeds). Winter seeds of C. virgata seem to be dormant, while summer seeds, germinated well in all the tested temperature regimes. However, C. ciliaris seeds showed opposite trends.