Phytochrome et germination des semences de Rumex

The Rumex alpinus L. achenes show a special type of positive photosensitivity: several red light irradiations are necessary to induce optimal germination. The achenes deprived of their pericarp lose their photosensitivity and germinate readily in the dark. By spectrophotometry in vivo, the presence of the Pr form of phytochrome is revealed in these seeds. Short periods at extreme temperatures (30°C or 5°C) induce a good proportion of achenes to germinate. The gibberellins are inefficient on achene germination contrary to benzyladenine which exhibits some activity. A slight improvement of GA₄ effects has been detected on scarified fruits.     

Effects of low temperatures on the loss of innate dormancy and the development of induced dormancy in seeds of Rumex obtusifolius L. and Rumex crispus L.

Abstract It is possible to remove the innate dormancy of seeds of Rumex crispus L and Rumex obtusifolius L. by an initial period of low-temperature stratification, providing the seeds are then transferred to a higher temperature. The lower the initial temperature within the range 1.5°-15°C, the greater the germination; there is no stratification effect at 20°C. Although 10°C and 15°C were shown to be suitable for both stratification and for the process of germination itself, neither temperature results in any germination if given constantly: a change from a lower to a higher temperature is essential. The optimum period for stratification depends on two separate processes which occur during the treatment–a rapid loss of innate or primary dormancy and a slower development of induced or secondary dormancy. Within the range 1.5°-15°C the rate of loss of innate dormancy appears to be independent of light and is probably independent of temperature. In contrast, the rate of induction of secondary dormancy increases with increase in temperature, and is more rapid in the dark than the light. The rate of induction of secondary dormancy during stratification is greater in R. crispus than in R. obtusifolius. As a consequence, maximum germination was obtained in both species after stratification at 1.5°C in the light, the optimum period of treatment being about 4 weeks in R. Obtusifolius and 6 weeks in R. crispus, while the maximum germination obtained and the optimal period of stratification decrease in both species with increase in stratification temperature.     

Dormancy of<Emphasis Type="Italic"> Arabidopsis</Emphasis> seeds and barley grains can be broken by nitric oxide

Seeds of Arabidopsis thaliana (L.) Heynh. and grains of barley (Hordeum vulgare L.) were used to characterize the affects of nitric oxide (NO) on seed dormancy. Seeds of the C24 and Col-1 ecotypes of Arabidopsis are almost completely dormant when freshly harvested, but dormancy was broken by stratification for 3 days at 4°C or by imbibition of seeds with the NO donor sodium nitroprusside (SNP). This effect of SNP on dormancy of Arabidopsis seeds was concentration dependent. SNP concentrations as low as 25 M reduced dormancy and stimulated germination, but SNP at 250 M or more impaired seedling development, including root growth, and inhibited germination. Dormancy was also reduced when Arabidopsis seeds were exposed to gasses that are generated by solutions of SNP. Nitrate and nitrite, two other oxides of nitrogen, reduced the dormancy of Arabidopsis seeds, but much higher concentrations of these were required compared to SNP. Furthermore, the kinetics of germination were slower for seeds imbibed with either nitrate or nitrite than for seeds imbibed with SNP. Although seeds imbibed with SNP had reduced dormancy, seeds imbibed with SNP and abscisic acid (ABA) remained strongly dormant. This may indicate that the effects of ABA action on germination are downstream of NO action. The NO scavenger 2-(4-carboxyphenyl)-4,4,5,5-tetramethylimidazoline-1-oxyl-3 oxide (cPTIO) strengthened dormancy of unstratified and briefly stratified Arabidopsis seeds. Dormancy of three cultivars of barley was also reduced by SNP. Furthermore, dormancy in barley grain was strengthened by imbibition of grain with cPTIO. The data presented here support the conclusion that NO is a potent dormancy breaking agent for seeds and grains. Experiments with the NO scavenger suggest that NO is an endogenous regulator of seed dormancy.Abbreviations ABA Abscisic acid - cPTIO 2-(4-Carboxyphenyl)-4,4,5,5-tetramethylimidazoline-1-oxyl-3 oxide - GA Gibberellin - SNP Sodium nitroprusside - NO_x Gaseous oxides of nitrogen     

Physical and physiological dormancy in black henbane (<Emphasis Type="Italic">Hyoscyamus niger</Emphasis> L.) seeds

Aim of this study was to investigate the nature of dormancy in black henbane (Hyoscyamus niger) seeds which have low germination rate under normal laboratory conditions. To do this, before placing the seeds in Petri dishes, they were soaked in 5,10 and 15 mg/L GA; 1,2 and 3% H₂SO₄, 15 mg/L GA + 1% H₂SO₄, 0.01 M KNO₃ solutions, tap water, 40, 50 and 60°C hot water for 30 min. The study was performed under both continuous illumination and darkness in growth chambers to evaluate the effect of light on germination rate. The results showed that H₂SO₄ and GA treatments were the most important factors affecting seed germination and their germination enhancing effects were more evident in darkness. The results also suggested that black henbane seeds exhibit double dormancy involving a hard seed coat and a partially dormant embryo and have a partial dark requirement to germinate.     

Changes in endogenous abscisic acid levels during dormancy release and maintenance of mature seeds: studies with the Cape Verde Islands ecotype,the dormant model of<Emphasis Type="Italic"> Arabidopsis thaliana</Emphasis>

Mature seeds of the Cape Verde Islands (Cvi) ecotype of Arabidopsis thaliana (L.) Heynh. show a very marked dormancy. Dormant (D) seeds completely fail to germinate in conditions that are favourable for germination whereas non-dormant (ND) seeds germinate easily. Cvi seed dormancy is alleviated by after-ripening, stratification, and also by nitrate or fluridone treatment. Addition of gibberellins to D seeds does not suppress dormancy efficiently, suggesting that gibberellins are not directly involved in the breaking of dormancy. Dormancy expression of Cvi seeds is strongly dependent on temperature: D seeds do not germinate at warm temperatures (20–27°C) but do so easily at a low temperature (13°C) or when a fluridone treatment is given to D seeds sown at high temperature. To investigate the role of abscisic acid (ABA) in dormancy release and maintenance, we measured the ABA content in both ND and D seeds imbibed using various dormancy-breaking conditions. It was found that dry D seeds contained higher amounts of ABA than dry ND after-ripened seeds. During early imbibition in standard conditions, there was a decrease in ABA content in both seeds, the rate of which was slower in D seeds. Three days after sowing, the ABA content in D seeds increased specifically and then remained at a high level. When imbibed with fluridone, nitrate or stratified, the ABA content of D seeds decreased and reached a level very near to that of ND seeds. In contrast, gibberellic acid (GA₃) treatment caused a transient increase in ABA content. When D seeds were sown at low optimal temperature their ABA content also decreased to the level observed in ND seeds. The present study indicates that Cvi D and ND seeds can be easily distinguished by their ability to synthesize ABA following imbibition. Treatments used here to break dormancy reduced the ABA level in imbibed D seeds to the level observed in ND seeds, with the exception of GA₃ treatment, which was active in promoting germination only when ABA synthesis was inhibited.Abbreviations ABA Abscisic acid - Cvi Cape Verde Islands - D Dormant - GA Gibberellin - GA₃ Gibberellic acid - ND Non dormant     

Seed germination behavior of five Resedaceae species from arid Arabian desert: gibberellic acid and light effects

Abstract

Light and growth-promoting compounds, such as gibberellic acids (GA₃), are among the most important factors that can break physiological seed dormancy. Here, we investigate the effects of GA₃ and light on germination of five species of Resedaceae that are known to have different levels of physiological dormancy. Seeds were incubated at 20/30?°C in both 12-hr photoperiod and complete darkness. To study the effect of growth hormone on germination, seeds were soaked for 24?h in different concentrations of GA₃ before sowing. The annuals (Reseda aucheri and Oligomeris linifolia) and the perennial Ochradenus arabicus had deep physiological dormancy, and exogenous application of GA₃ enhanced their germination in the light, but not in darkness; few or no seeds germinated in the dark in these species. Ochradenus aucheri and O. baccatus had intermediate and non-deep dormancy, respectively, and application of GA₃ enhanced their germination in both light and darkness. Germination of the annual species was much slower than that of the shrubby perennials. Overall, these results indicate that conditions under which seed developed, matured and stored on maternal plants as well as incubation conditions should be taken into consideration when assessing germination behavior of the perennial species of Resedaceae.     

Nitrite,sodium nitroprusside,potassium ferricyanide and hydrogen peroxide release dormancy of <Emphasis Type="Italic">Amaranthus retroflexus</Emphasis> seeds in a nitric oxide-dependent manner

Nitric oxide (NO) and reactive oxygen species (ROS) are important regulators involving various processes of plant growth and development. Amaranthus retroflexus L. seeds possess a relative dormancy property that means freshly collected seeds can only germinate over a limited, high temperature range. Here, we show that the relative dormancy of A. retroflexus seeds could be significantly released following treatments with exogenous NO/cyanide (CN) donors such as nitrite, gases evolved from acidified nitrite, sodium nitroprusside (SNP), potassium ferricyanide (Fe(III)CN) and gases evolved from SNP or Fe(III)CN solutions, as well as exogenously supplied ROS, hydrogen peroxide (H₂O₂). However, the effectiveness varied among these chemicals. Gases evolved from acidified nitrite displayed maximum effect while H₂O₂ had minimum effect. We also show that the effects of these compounds could be significantly inhibited by NO specific scavenger 2-phenyl-4,4,5,5-tetramethylimidazoline-1-oxyl 3-oxide (PTIO), indicating that NO signaling pathway might play a central role in the dormancy release and germination of A. retroflexus seeds, while both ROS and CN might act through NO-dependent signaling cascades.     

Tolerance of kikuyu grass to long term salt stress is associated with induction of antioxidant defences

Primary dormancy in A. retroflexus seeds wascompletely broken by dry storage or ethylene treatment and partially removedwith GA₃. Norbornadiene counteracted the dormancy breaking action ofethylene and GA₃. The GA₃ effect was lowered bycobaltous ions. ABA increased the ethylene requirement in primary dormant seeds.Dormant seeds had a similar or different ability to produce ethylene and ACCoxidase in vivo activity than did non-dormant seeds,depending on the period of incubation. Dormant seeds contained less endogenousACC than non-dormant seeds. Thus, ethylene seems to play an essential role inthe release of primary dormancy in A. retroflexus seeds.Ethylene also participates in the release of dormancy achieved by GA₃treatment. The results indicate that both ethylene biosynthesis and action isinvolved in the control of primary dormancy in Amaranthusretroflexus seeds.     

Dormancy of <Emphasis Type="Italic">Nicotiana benthamiana</Emphasis> seeds can be broken by different compounds

The influence of after-ripening, sodium nitroprusside, potassium ferricyanide, cyanide, paclobutrazol and nitrite on germination of seeds of Nicotiana benthamiana was investigated as well as the influence of plant hormones such as gibberellins and abscisic acid. Dormancy of N. benthamiana seeds was broken by all treatments except treatments with abscisic acid, paclobutrazol and gibberellic acid (GA₃). Gibberellins had an interesting effect on dormancy breakage of studied seeds which was dependent on use of particular gibberellin: GA₃ or GA₄₊₇. Unlike GA₃, GA₄₊₇ had broken seed dormancy.     

Nitrate, a signal relieving seed dormancy in Arabidopsis

Nitrate is an important nitrogen source for plants, but also a signal molecule that controls various aspects of plant development. In the present study the role of nitrate on seed dormancy in Arabidopsis was investigated. The effects of either mutations affecting the Arabidopsis nitrate reductase genes or of different nitrate regimes of mother plants on the dormancy of the seeds produced were analysed. Altogether, data show that conditions favouring nitrate accumulation in mother plants and in seeds lead to a lower dormancy of seeds with little other morphological or biochemical differences. Analysis of germination during seed development indicated that nitrate does not prevent the onset of dormancy but rather its maintenance. The effect of an exogenous supply of nitrate on seed germination was tested: nitrate in contrast to glutamine or potassium chloride clearly stimulated the germination of dormant seeds. Data show, moreover, that the Arabidopsis dual affinity nitrate transporter NRT1.1 (CHL1) may be involved in conveying the nitrate signal into seeds. Thus, nitrate provided exogenously or by mother plants to the produced seeds, acts as a signal molecule favouring germination in Arabidopsis. This signalling may involve interaction with the abscisic acid or gibberellin pathway.     

Phytochrome Transformation and Action in Seeds of Rumex crispus L. during Secondary Dormancy

Promotion of germination by red light fails after prolonged dark imbibition of Rumex crispus L. seeds, indicative of a secondary dormancy. The degree and rate of inception of the dormancy increases with increasing temperature. Following establishment of the dormancy, germination response to red light can be restored by either prolonged cold treatment or brief high temperature shifts. Loss of phytochrome was not a factor in the initial establishment of the dormancy. When the seeds are in secondary dormancy, the chromophore of phytochrome can be transformed to the far red-absorbing form, but the far red-absorbing form cannot induce germination. The responses to changes in temperature suggested dependence of germination on order disorder transitions in components of the seeds.     

Variation within species and inter-species comparison of seed dormancy and germination of four annual Lamium species

In an ecological context, knowledge of intra-species variation in dormancy and germination is necessary both for practical and theoretical reasons. We used four or five seed batches (replicates) of four closely related annuals co-occurring in arable fields in Sweden: Lamium amplexicaule, L. confertum, L. hybridum and L. purpureum. Seeds used for experiments stemmed from plants cultivated on two sites, each site harbouring one population of each species, thereby ensuring similar environmental history of seeds. Seeds were tested for germination when fresh and after three different pre-treatments (cold or warm stratification, or dry storage) for up to 24 weeks. Seeds were also sown outdoors. Despite substantial intra-species variation, there were clear differences between species. The general seed dormancy pattern, i.e. which environmental circumstances that affect dormancy, was similar for all species; dormancy reduction occurred during warm stratification or dry storage. Even though the response to warm stratification indicates a winter annual pattern, successful plants in Sweden were mostly spring emerged. Germination in autumn occurred, but plants survived winters poorly. Consequently, as cold stratification did not reduce dormancy, strong dormancy in combination with dormancy reduction during dry periods might explain spring germination. It is hypothesised that local adaptations occur through changes mainly in dormancy strength, i.e. how much effort is needed to reduce dormancy. Strong dormancy restricts the part of each seed batch that germinate during autumn, and thus reduces the risk of winter mortality, in Sweden.     

Geographic variation in seed dormancy among populations of Echinochloa crus-galli

In 1991–1993, we investigated the incidence of seed dormancy in 25 local populations of barnyard grass, Echinochloa crus-galli (L.) P.Beauv., in the western Czech Republic. The percentage of germination after 4 months afterripening of dry seeds at 25°C varied between 0.0 and 83.6%. Although there were significant annual differences in the percentage of germination at some localities, typical proportions of dormant seeds persisted over 3 years at field sites where the seed bank was not disturbed. One-way ANOVA (using data from 14 cultivated or abandoned fields) revealed that 73.0% of variance in seed dormancy incidence could be attributed to the effect of locality (P<0.001). Incidence of dormancy was not correlated with mother plant stature (dry above-ground biomass, number of tillers, maximal stem height) nor seed mass. There was a significant correlation (r ²=0.403, P<0.005) between dormancy incidence at natural localities in 1991 and in F₁ offspring sown at experimental grounds at Praha-Ruzyn in 1992. The results indicate that heredity is important in maintaining local variation in seed dormancy, probably favoured by the self-pollinating reproduction of barnyard grass.     

ENVIRONMENTAL CONTROL OF SEED GERMINATION IN THLASPI ARVENSE (CRUCIFERAE)

The effect of environmental conditions during storage and imbibition on germination was investigated in field pennycress (Thlaspi arvense L.), a weed species that can behave as a winter or a summer annual. Freshly harvested seeds of an inbred line with a cold requirement for flowering exhibited primary dormancy that was rapidly lost following 1 month of afterripening in a dry state. Nondormant seeds were positively photoblastic. The light effect was mediated through phytochrome since germination was promoted by red light and inhibited by far red light. Seedling emergence was also inhibited by light filtered through a canopy of wheat leaves. Germination of field pennycress seeds was considerably more sensitive to moisture stress than two sympatric species, wild oat (Avena fatua L.) and wheat (Triticum aestivum L., cv. ERA). Seeds of the latter two species were chosen in order to compare the effect of water potential on germination in field pennycress with that in sympatric species. It was concluded that the major environmental factor limiting nondormant field pennycress seeds on the soil surface was water availability. Imbibition of fully afterripened seeds at low temperatures (6 C) induced a deep secondary dormancy. In contrast to primary dormancy, cold-induced dormancy was not alleviated by red light, alternating temperatures (21/5 C), or 2 months of dry storage at 6, 15, or 35 C. However, exogenous gibberellin A₃ or 24 weeks of dry storage resulted in germination in cold-induced dormant seeds. Secondary dormancy was not observed in fully afterripened seeds that were preincubated at 21 C for 1 or 2 days prior to the cold treatment. These results may explain the failure in field experiments to observe the cold-induced secondary dormancy that limits spring emergence in other winter annuals (J. Baskin, C. Baskin, Weed Res. 1979 19: 285–292).     

Mycorrhizal infection of wild oats: maternal effects on offspring growth and reproduction

Summary The objective of this study was to determine whether infection of Avena fatua L. plants by the mycorrhizal fungus Glomus intraradices Schenck & Smith could influence the vigor of the offspring generation. Two experiments demonstrated that mycorrhizal infection of the maternal generation had slight but persistent positive effects on offspring leaf expansion in the early stages of growth. In two other experiments, mycorrhizal infection of mother plants had several long lasting effects on their offspring. Offspring produced by mycorrhizal mother plants had greater leaf areas, shoot and root nutrient contents and root:shoot ratios compared to those produced by non-mycorrhizal mother plants. Moreover, mycorrhizal infection of mother plants significantly reduced the weight of individual seeds produced by offspring plants while it increased the P concentrations of the seeds and the number of seeds per spikelet produced by offspring plants. The effects of mycorrhizal infections of maternal plants on the vigor and performance of offspring plants were associated with higher seed phosphorus contents but generally lighter seeds. The results suggest that mycorrhizal infection may influence plant fitness by increasing offspring vigor and offspring reproductive success in addition to previously reported increases in maternal fecundity.     

Effects of salinity and nitrate on production and germination of dimorphic seeds applied both through the mother plant and exogenously during germination in Suaeda salsa

Salinity and nitrogen are two important environmental factors that affect the distribution of halophytes in their natural saline habitats. Seeds of the euhalophyte Suaeda salsa L. were harvested from plants that had been treated with 1 or 500 mm NaCl combined with 0.5 or 5 mm NO₃^?‐N (nitrate) for 115 days in a glasshouse. Germination was evaluated under different concentrations of NaCl and nitrate. Plants exposed to high salinity (500 mm ) and low nitrate (0.5 mm ) tended to produce heavy seeds. Either high salinity (500 mm ) or high nitrate (5 mm ) increased the brown/black seed ratio. The concentrations of Na⁺, K⁺, and Cl^? were higher in brown than in black seeds, and NO₃^? concentrations were higher in black than in brown seeds, regardless of NaCl and nitrate treatments during plant culture. Regardless of NaCl and nitrate concentrations during germination, seeds from plants grown with 0.5 mm nitrate generally germinated more rapidly than seeds from plants grown with 5 mm nitrate, and the difference was greater for black than for brown seeds. Exogenous nitrate during germination enhanced the germination of brown seeds less than that of black seeds. Producing more brown seeds and heavy black or brown seeds appears to be an adaptation of S. suaeda to saline environments. Producing more black seeds, which tend to remain dormant, should reduce competition for nitrogen and appears to be an adaptation to nitrogen‐limited environments. In conclusion, nitrate provided exogenously or by mother plants to black seeds may act as a signal molecule that enhances the germination of black S. suaeda seeds.     

Echinochloa crus‐galli seed physiological dormancy and germination responses to hypoxic floodwaters

• Hypoxic floodwaters can seriously damage seedlings. Seed dormancy could be an effective trait to avoid lethal underwater germination. This research aimed to discover novel adaptive dormancy responses to hypoxic floodwaters in seeds of Echinochloa crus‐galli, a noxious weed from rice fields and lowland croplands.
• Echinochloa crus‐galli dormant seeds were subjected to a series of sequential treatments. Seeds were: (i) submerged under hypoxic floodwater (simulated with hypoxic flasks) at different temperatures for 15 or 30 days, and germination tested under drained conditions while exposing seeds to dormancy‐breaking signals (alternating temperatures, nitrate (KNO₃), light); or (ii) exposed to dormancy‐breaking signals during hypoxic submergence, and germination monitored during incubation and after transfer to drained conditions.
• Echinochloa crus‐galli seed primary dormancy was attenuated under hypoxic submergence but to a lesser extent than under drained conditions. Hypoxic floodwater did not reinforced dormancy but hindered secondary dormancy induction in warm temperatures. Seeds did not germinate under hypoxic submergence even when subjected to dormancy‐breaking signals; however, these signals broke dormancy in seeds submerged under normoxic water. Seeds submerged in hypoxic water could sense light through phytochrome signals and germinated when normoxic conditions were regained.
• Hypoxic floodwaters interfere with E. crus‐galli seed seasonal dormancy changes. Dormancy‐breaking signals are overridden during hypoxic floods, drastically decreasing underwater germination. In addition, results indicate that a fraction of E. crus‐galli seeds perceive dormancy‐breaking signals under hypoxic water and germinate immediately after aerobic conditions are regained, a hazardous yet less competitive environment for establishment.

     

Construction and characteristics of transgenic tobacco <Emphasis Type="Italic">Nicotiana tabacum</Emphasis> L. plants expressing <Emphasis Type="Italic">CYP11A1</Emphasis> cDNA encoding cytochrome P450<Subscript>SCC</Subscript>

In steroidogenic animal tissues cytochrome P450_SCC catalizes the conversion of cholesterol into pregnenolone, a common metabolic precursor of all steroid hormones. To study the possibility of functioning of mammalian cytochrome P450_SCC in plants and the mechanism of its integration in the plant steroidogenic system, transgenic plants of tobacco Nicotiana tabacum L. were developed carrying cDNA of CYP11A1 encoding cytochrome P450_SCC of bovine adrenal cortex. Pregnenolone, a product of the reaction catalyzed by cytochrome P450_SCC, was discovered in the steroid-containing fraction of transgenic plants. Transgenic plants are characterized by a reduced period of vegetative development (early flowering and maturation of bolls) and increased productivity. The contents of soluble protein and carbohydrates in leaves and seeds of transgenic plants are essentially higher than the contents of these components in leaves and seeds of control plants.     

<Emphasis Type="Italic">In vitro</Emphasis> germination and micropropagation of <Emphasis Type="Italic">Givotia rottleriformis</Emphasis> Griff.

The propagation of Givotia rottleriformis Griff. is difficult as a result of long seed dormancy associated with poor seed germination. The present study was undertaken to develop a protocol to overcome seed dormancy by culture of zygotic embryo axes and then develop an efficient method for micropropagation of Givotia. Best germination frequency (78.3%) was achieved from mature zygotic embryo axes isolated from acid-scarified fresh seeds when cultured on Murashige and Skoog (MS) medium (half-strength major salts) with 28.9 μM gibberellic acid (GA₃). Efficient plant conversion was achieved by transfer of 10-d-old germinated embryos to MS medium (half-strength major salts) supplemented with 1.2 μM kinetin (KN) and 0.5 μM indole-3-butyric acid (IBA). However, acid scarification of 1-yr-old seeds decreased the germination frequency of zygotic embryo axes in comparison to those obtained from non-acid-scarified seeds which germinated (96.2%) and converted into plants (80.3%) on MS basal (half-strength major salts) medium. Multiple shoot bud induction was achieved by culture of shoot tips derived from in vitro germinated seedlings on MS medium with 0.5 μM thidiazuron for 4 wk, and the shoots elongated after transfer to a secondary medium with 1.2 μM KN. A maximum number of 7.8 shoots per explant with an average shoot length of 3.2 cm was achieved after two subcultures on this medium. The in vitro regenerated shoots rooted (41.5%) on half-strength MS medium with 0.5 μM IBA. The in vitro generated seedlings and micropropagated plants were established in soil with a survival frequency of 70% and 60%, respectively.