Action of Respiratory Inhibitors on Seed Germination and Oxygen Uptake in Avena fatua L.

The effects of sodium azide, potassium cyanide (cytochrome oxidaseinhibitors), and salicylhydroxamic acid (SHAM; an alternativerespiration inhibitor) on germination and respiration of Avenafatua L. seeds were studied. Azide and cyanide released seeddormancy at similar concentrations and treatment durations.Cyanide, however, stimulated germination of seeds with littleafter-ripening, whereas azide had no effect under similar conditionsunless the seeds were after-ripened for several months; theduration of after-ripening required for seeds to respond toazide varied with seed batch. There was also a greater lag priorto germination in the case of azide, compared to cyanide treatedseeds. SHAM inhibited the stimulation of germination and respirationby azide, but not by cyanide. Furthermore, respiration induced by azide or cyanide could notbe inhibited by the subsequent application of SHAM. These findingssuggest that the respiration stimulated by azide and cyanideis not alternative (SHAM-sensitive) and, therefore, this respiratorypathway cannot be involved in the stimulation of germinationby cytochrome oxidase inhibitors. While embryos excised fromcontrol, azide or cyanide pretreated seeds had the capacityto perform alternative respiration, the actual contributionof this pathway was negligible. A large proportion of respirationof embryos excised from azide or cyanide pretreated seeds wasresidual, i.e. insensitive to both SHAM and cyanide. Alternative respiration, azide, cyanide, dormancy, salicylhydroxamic acid, wild oats     

The Germination of Avena fatua under Different Gaseous Environments

The atmosphere in which seeds germinate can profoundly affect the level of germination and dormancy. Seeds were germinated in atmospheres containing various concentrations of carbon dioxide and oxygen. At the same time the effect of light on these systems was examined. The germination of partially dormant populations of wild oat seed is inhibited by white light. This response to light is most apparent when the curyopsis is enclosed in the pales. Investigations into the effect of the ambient atmosphere on germination have indicated that, while oxygen is a necessary factor in the germination of tliis species, carbon dioxide also has an effect. A lack of carbon dioxide increases the degree of light inhibition of germination; 3 per cent carbon dioxide (by volume) allows germination in light; 20 per cent carbon dioxide inhibits germination in light and darkness at all tested oxygen concentrations.     

Germination stimulation in wild oats (Avena fatua L.) by synthetic strigol analogs and gibberellic acid

At concentrations of 0.01–1 mM, five synthetic multiring analogs of strigol were effective germination stimulants of intact and dehulled wild oat (Avena fatua L.) seeds. The effect was concentration-dependent and equaled or exceeded that produced by equimolar gibberellic acid (GA₃). The most effective strigol analog treatments induced 55–80% germination within 7 days in intact wild oat seeds and resulted in 63–86% germination and normal seedling growth over 14 days. Intact wild oat controls germinated 14% after 14 days. The stimulation of wild oat germination by these synthetic strigol analogs demonstrates that these compounds, initially developed as germination stimulants for the seeds of the parasitic weed, witchweed (Striga asiatica L. Kuntz.), have bioregulatory activity in dormant seeds of monocots, as well as dicots. None of the compounds tested significantly affected the germination of nondormant cultivated oat seeds (Avena sativa L.). The commonly used dispersal agent, Tween 20 (0.1%), was found to inhibit germination of cultivated oats, alone and in the presence of 2% acetone.     

Developmental Differences Between Germinating After-ripened and Dormant Excised Avena fatua L. Embryos

Based on physiological and molecular differences associatedwith the germination of after-ripened and dormant caryopsesand excised embryos, it has been hypothesized that various methodsof after-ripening are the only treatments that facilitate thetransition of dormant wild oat embryos to a non-dormant state.To further investigate this hypothesis, analytical methods wereused to evaluate physical and temporal changes associated withgermination and subsequent growth of after-ripened and dormantexcised embryos (AR-embryos and D-embryos, respectively) inducedto germinate with fructose (Fru) and/or gibberellic acid (GA).While chemical treatments of Fru, GA, and Fru+GA have littleeffect on the germination and short-term growth of AR-embryos,they do induce germination of D-embryos. Growth following germinationof D-embryos varied according to treatment with the combinationof Fru+GA inducing the greatest growth over the duration ofthe experiment. Even considering differences in the time tocomplete germination, growth of D-embryos was not comparablewith that of AR-embryos. This provides physical evidence thatchemical treatments induce germination without fulfilling therequirements for normal after-ripening-enhanced germination/growth,and indicates that fructose and/or gibberellic acid do not removethe dormancy-block or rate limiting step in the same manneras after-ripening. Avena fatua ; after-ripening; dormancy; fructose; germination; gibberellic acid; wild oats     

A simulation model of Avena fatua L. (wild-oat) growth and development

An eco-physiological simulation model of the growth and development of Avena fatua was parameterised and tested. The model simulates growth ofA. fatua, in kg dry matter ha^-1 day^-1 from sowing to maturity as a function of irradiance, temperature and various species characteristics. Parameter values were derived from the literature and from field experiments, including both autumn and spring sowings of A. fatua over three years at two sites in southern England. With two exceptions, a single set of parameter values was sufficient to accurately simulate the emergence, growth and development of both autumn and spring cohorts over all years and sites. The two exceptions were the result of differences between autumn and spring cohorts of A. fatua in the rate of early leaf area growth and in the relationship between specific leaf area and developmental stage.     

Genome structure and evolution in the allohexaploid weed Avena fatua L. (Poaceae).

Allohexaploid wild oat, Avena fatua L. (Poaceae; 2n = 6x = 42), is one of the world's worst weeds, yet unlike some of the other Avena hexaploids, its genomic structure has been relatively little researched. Consequently, in situ hybridisation was carried out on one accession of A. fatua using an 18S-25S ribosomal DNA (rDNA) sequence and genomic DNA from A. strigosa (AA-genome diploid) and A. clauda (CC-genome diploid) as probes. Comparing these results with those for other hexaploids studied previously: (i) confirmed that the genomic composition of A. fatua was similar to the other hexaploid Avena taxa (i.e., AACCDD), (ii) identified major sites of rDNA on three pairs of A/D-genome chromosomes, in common with other Avena hexaploids, and (iii) revealed eight chromosome pairs carrying intergenomic translocations between the A/D- and C-genomes in the accession studied. Based on karyotype structure, the identity of some of these recombinant chromosomes was proposed, and this showed that some of these could be divided into two types, (i) those common to all hexaploid Avena species analysed (3 translocations) and (ii) one translocation in this A. fatua accession not previously observed in reports on other hexaploid Avena species. If this translocation is found to be unique to A. fatua, then this information, combined with more traditional morphological data, will add support to the view that A. fatua is genetically distinct from other hexaploid Avena species and thus should retain its full specific status.     

ATP Synthesis during Water Imbibition in Caryopses of Genetically Dormant and Non-dormant Lines of Wild Oat (Avena fatua L.)

Levels of ATP in dry caryopses of wild oats (Avena fatua L.)were much lower than in imbibed seeds of the seven geneticallypure lines surveyed. The ATP content of the lines with highgenetic dormancy was consistently lower than the ATP contentof genetically non-dormant lines, but no significant correlationwith depth of dormancy was found apart from this. Massive increasesin ATP content occurred within 30 min of water uptake by caryopsesof both dormant and non-dormant lines. The synthetic pathwaystudied utilized inorganic phosphate with great avidity to formATP. The ability to form ATP upon imbibition was present inboth embryo and de-embryonated caryopsis. The ATP levels attainedin imbibing caryopses appeared sufficient to support considerablesynthetic activity, and this reduced the possibility that adeficiency in ATP was responsible for the maintenance of dormancyin such imbibed seeds. The low levels of inorganic phosphatein the embryos of genetically dormant lines of wild oat couldrepresent a limiting factor, if the active formation of ATPupon water imbibition resulted in a scarcity of phosphate forother reactions essential to germination. Key words: Avena fatua, ATP synthesis, Inorganic phosphorus, Seed dormancy, Germination, Water uptake     

Inhibition of Gibberellic Acid-induced Starch Mobilization by Salicylhydroxamic acid in Avena fatua L. Seed

Inhibition of GA₃-induced endosperm mobilization in Avena fatuaL. by salicylhydroxamic acid (SHAM), a widely used alternativerespiration inhibitor, was studied. SHAM strongly inhibitedthe GA₃-induced release of reducing sugars in the incubationmedium by 3 mm de-embryonated endosperm segments; at 4 mM SHAM,GA₃-induced sugar release was inhibited by 66–79 per cent.Extracts prepared from segments incubated in 0.05 mM GA₃ with2, 5 and 10 mM SHAM showed 30, 53 and 71 per cent lower -amylaseactivity, respectively, compared to the GA₃-alone treatment.Addition of SHAM (0.5–5 mM) during the enzyme assay hadno effect on the activity of -amylase. Thus, the inhibitionof starch mobilization in endosperm by SHAM is due to inhibitionof the production and not the activity of -amylase. The inhibitionof Avena fatua seedling growth by SHAM reported earlier may,in part, be due to its effect on endosperm mobilization. Since (1) Avena fatua seeds have been shown to have little orno SHAM-sensitive respiration, and (2) concentrations of SHAMnecessary for inhibiting endosperm mobilization were significantlyhigher than those generally necessary for inhibiting alternativerespiration, the inhibition of endosperm mobilization by thiscompound does not appear to involve its effect on alternativerespiration. Avena fatua L., wild oat, -amylase, endosperm, gibberellic acid, salicylhydroxamic acid, seed     

Gibberellin-photoaffinity labeling of wild oat (Avena fatua L.) aleurone protoplasts

Aleurone protoplasts of wild oat (Avena fatua L.), and subcellular fractions isolated from them, were photoaffinity labeled using the synthetic gibberellin (GA) derivative GA₄-17-yl-1-(1-thia)propan-3-ol-4-azido-5-[¹²⁵I]iodosalicylate. Labeled polypeptides were identified by electrophoresis under denaturing conditions followed by autoradiography. GA-photoaffinity labeling of both intact protoplasts and isolated subcellular fractions led to the covalent attachment of the reagent to many polypeptides. A 50 kD polypeptide in the soluble fraction of homogenates of aleurone protoplasts GA-photoaffinity labeled in vivo showed specific binding. The biologically active GA₁, GA₄ and GA₄-17-yl-1(1-thia)propan-3-ol-4-azidosalicylate completed for binding whereas the biologically inactive GA₈ and GA₃₄ did not. The GA-photoaffinity labeling characteristics of this polypeptide suggested that it might interact specifically with biologically active GAs in vivo. Attempts to detect specific GA-binding in in vitro GA-photoaffinity labeling experiments met with only limited success perhaps indicating the labile nature of specific binding observed in vivo. The potential of GA-photoaffinity labeling for identifying GA-binding proteins in aleurone and other GA-responsive tissues is discussed.Abbreviations azido IAA = 5-azido-7-[³H]indole-3-acetic acid - azido NPA = 5-azido-[3,6-³H]1-N-napthylpthalamic acid - BTP = 1,3-bis(Tris(hydroxymethyl)methylamino)-propane - GA₄-O-ASA = GA₄-17-yl-1-(1-thia)propane-3-ol-4-azidosalicylate - [¹²⁵I]GA₄-O-ASA = GA₄-17-yl-1-(1-thia)propan-3-ol-4-azido-5-[¹²⁵I]iodosalicylate - NPA = 1-Naphthylphthalmic acid - PAGE = Polyacrylamide gel electrophoresis - PMSF = phenylmethylsulfonyl fluoride - SDS = Sodium dodecyl sulphate - TLCK = L-1-Chloro-3-(4-tosylamido)-7-amino-2-heptanone-HCl     

Gibberellic acid controls specific acid-phosphatase isozymes in aleurone cells and protoplasts of Avena fatua L.

In the presence of gibberellic acid (GA₃) aleurone layers and isolated aleurone protoplasts of Avena fatua accumulate specific isozymes of acid phosphatase (EC 3.1.3.2). Some of these may be involved in mobilizing aleurone-grain phosphate reserves during germination. The hormone also controls secretion of other specific molecular forms of the enzyme that probably assist in endosperm hydrolysis. The accumulation and secretion of putative cell-wall-associated isozymes are stimulated by the action of GA₃ in isolated protoplasts. This effect however, is apparently over-ridden in the intact tissue, possibly by a cell-wall-based feedback mechanism.Abbreviations GA₃ gibberellic acid - pI isoelectric point(s)     

The responsiveness of Avena fatua aleurone protoplasts to gibberellic acid

The sensitivity to gibberellic acid (GA₃) of aleurone protoplasts isolated from a single harvest of an inbred line of Avena fatua seed that had been after-ripened over anhydrous CaCl₂ at 25±2°C and 4±2°C for three years was assessed. Protoplasts isolated from aleurones of seed stored at 25°C produced substantially more -amylase in response to 10^–7 M GA₃ than those isolated from aleurones of seed stored at 4°C. The apparent difference in responsiveness does not appear to be due to a change in the duration of the lag phase between addition of GA₃ and the production of -amylase. The dose response of aleurone protoplasts to GA₃, measured as -amylase production, is complex and appears to have three phases. Protoplasts from seed stored at both temperatures respond appreciably to 10^–14 M GA₃. With increasing concentrations of GA₃, up to 10^–9 M, -amylase production increases similarly in protoplasts from both lots of seed, reaching a level approximately 2.7–3.8 times greater than when no GA₃ is applied. GA₃-induced -amylase production increases markedly as the concentration is raised from 10^–9 M to 10^–6 M, and the response then appears to be saturated. Over this part of the response curve protoplasts from the two seed lots differ markedly in their responsiveness to GA₃. Those from seed stored at 25°C produce considerably more -amylase, >130-fold higher than the minus GA₃ control, than those from seed stored at 4°C, <35-fold higher than the minus GA₃ control. This apparent difference in the responsiveness of aleurone protoplasts to GA₃ could be correlated with the loss of embryo dormancy in seed stored at 25°C. Seed stored at 4°C retained the dormancy characteristics present immediately after harvesting.     

The Influence of Light and Potassium Nitrate on the Dormancy and Germination of Avena fatua L. (wild oat) Seed Stored Buried Under Natural Conditions

The influence of light and potassium nitrate on the dormancyand germination of Avena fatua L. seed stored buried under naturalconditions was studied over a 13 month period. Natural emergenceof seedlings from buried seed in relation to rainfall and temperatureregimes and fluctuation was also monitored. The present findingsshow that potassium nitrate is stimulatory to germination onlyin the light and, further, that seeds from two positions inthe spikelet respond differently. Key words: Dormancy, Light, Nitrate     

Secondary dormancy in Avena fatua: Effect of temperature and after-ripening

To evaluate the effect of after-ripening on secondary dormancy induction in pure genetic lines of Avena fatua L., seed samples were periodically removed from standard conditions of storage and the caryopses then subjected to anoxia. Anoxia did not induce secondary dormancy in SH430, a line characterized by no primary dormancy at harvest maturity; secondary dormancy was induced in caryopses of other lines that had been after-ripened to over-come primary dormancy ranging in duration from a few days (CS40, CS166) to several months (AN51, AN127). Germination response to low GA₃ concentrations indicated that secondary dormancy in CS40 and CS166 was less intense than in AN51 and AN127. The longer the period of dry after-ripening prior to anoxia treatment, the lower the intensity of secondary dormancy induced. After a period of dry after-ripening, which was characteristic for each line, anoxia became an ineffective dormancy-inducing treatment. Caryopses selected for their response to dormancy induction by anoxia were subjected to temperatures from 5 to 35°C to investigate the effect of low (5 to 18°C) and high (20 to 35°C) temperatures on both thermo- and secondary dormancy induction. SH430 was not responsive to any treatment, while CS40, CS166 and AN51 were induced into a thermo-dormancy at temperatures above 20°C and CS166 and AN51 were induced into secondary dormancy by anoxia at temperatures from 5 to 35°C. The effect of anoxia on secondary dormancy induction in a range of pure genetic lines is discussed with reference to primary dormancy, after-ripening and temperature.     

Variation in stomatal aperture in leaves of Avena fatua L. observed by low-temperature scanning electron microscopy

Abstract. A novel technique to record the variability of stomatal aperture over the leaf surface is described. This combines observations of leaf surfaces using low-temperature scanning electron microscopy (LTSEM), with digital image analysis to produce the most accurate aperture measurements obtained to date. Leaf samples are rapidly immobilized by cryo-fixation in liquid nitrogen and stored in a purpose-built cryo-storage system. Specimens can be collected in the field, remote from the cryopreparation system, and stored for up to several weeks before being examined on the LTSEM. The advantages of this method are that the time frame of the measurements is accurately known, and is identical for all stomatal apertures in a sample, and the precision of the measurements is not limited by the resolving power of the microscope. Measurements of stomatal aperture were obtained from leaves of field grown Avena fatua using the above procedure. Leaf surface conductance (g_sur) was determined by porometry immediately before cryo-fixation of the same region of the leaf. Measurements of aperture size showed a high degree of variability within each specimen, with coefficients of variation similar to those found in previous studies. Stomatal conductance (g_s) was calculated from stomatal dimensions using formulae derived elsewhere. A linear regression between the computed values of g_s and porometric estimates of g_sur showed good agreement with the regression line passing through the origin with a slope of 1.0 (R²=0.96). Applications of the experimental system are discussed.     

Avena fatua L. seed and seedling nutrient dynamics as influenced by mycorrhizal infection of the maternal generation

Abstract. The objective of this study was to determine how mycorrhizal infection of one generation of plants influences the nutrient dynamics of seeds and seedlings comprising the subsequent generation. We showed that, for Avena fatua L., seeds produced by mycorrhizal (M) plants consistently contained significantly more phosphorus (particularly the phytate P and residual P fractions) than seeds produced by non-mycorrhizal (NM) plants. We also followed the development of spikelets produced by M and NM plants. The rates of increase in spikelet dry weight and nitrogen content were largely unaffected by mycorrhizal infection. However, the rate of P accumulation into spikelets was significantly increased by mycorrhizal infection. Greater endosperm P reserves in seeds produced by M plants were associated with greater rates of P accumulation in resultant seedlings. Moreover, offspring plants (all NM) produced by M mother plants had significantly higher root and rhizosphere phosphatase, ATPase and phytase activities than offspring plants produced by NM mother plants. This persistent maternal effect has never before been described. Our results suggest that mycorrhizal infection of one generation of plants may have substantial positive effects on the offspring generation, and thus, may influence plant population dynamics.     

Herbicide resistance and gene flow in wild-oats (Avena fatua and Avena sterilis ssp. ludoviciana)

The process of resistance evolution to fenoxaprop-P-ethyl was investigated in the cereal weeds wild-oats (Avena fatua L. and Avena sterilis ssp. ludoviciana Malzew) at a number of locations in England, including one farm where distinct patches occur within fields. Genetic fingerprints produced using PCR-based techniques provided evidence for hybridisation between the species and that resistance had spread from one patch to others. The proportion of total variation due to differences between populations (G_st) was estimated at 33^2%, and herbicide-resistant patches contained on average less genetic diversity than herbicide-sensitive counterparts: both findings were consistent with a high degree of self-pollination. It was however concluded that cross-pollination occurs both within and possibly between species, and that this can result in the spread of herbicide resistance.     

Necessity of gibberellin for stimulatory effect of KAR1 on germination of dormant Avena fatua L. caryopses

Avena fatua L. florets (caryopses enclosed by lemma and palea) were partially dormant at 10–20 °C and did not germinate at temperatures outside this range. After-ripening florets at 25 °C for 12 weeks completely removed dormancy. Caryopses (florets without lemma and palea) were able to germinate totally at 20 °C. Karrikinolide (KAR₁) and gibberellic acid (GA₃) applied at 10–25 °C partially or markedly induced germination of dormant florets and caryopses, respectively. Both florets and caryopses were more sensitive to KAR₁ than to GA₃. To obtain similar effects, 1,000 to 10,000 times lower concentrations of KAR₁ than GA₃ were required. After-ripening with time gradually increased sensitivity of caryopses to these regulators. Likewise, after-ripened, non-dormant caryopses were sensitive to KAR₁ and GA₃. Inhibitors of gibberellin biosynthesis, ancymidol, paclobutrazol and flurprimidol inhibited the effect of KAR₁. This inhibition was reversed by GA₃. Caryopses pre-incubated in water with ancymidol or paclobutrazol in the presence or absence of KAR₁ germinated completely but with different rates after transfer to GA₃. KAR₁ probably requires gibberellin biosynthesis to stimulate germination of dormant Avena fatua L. caryopses. Both KAR₁ and GA₃ increased α-amylase, β-amylase and dehydrogenases activities during imbibition before visible germination occurred.     

Characterization of cDNA clones for differentially expressed genes in embryos of dormant and nondormant Avena fatua L. caryopses

The molecular regulation of seed dormancy was investigated using differential display to visualize and isolate cDNAs representing differentially expressed genes during early imbibition of dormant and nondormant Avena fatua L. embryos. Of about 3000 cDNA bands examined, 5 cDNAs hybridized with mRNAs exhibiting dormancy-associated expression patterns during the first 48 h of imbibition, while many more nondormancy-associated cDNAs were observed. Dormancy-associated clone AFD1 hybridized with a 1.5 kb mRNA barely detectable in dry dormant and nondormant embryos that became more abundant in dormant embryos after 24 h of imbibition. Clone AFD2 hybridized with two mRNAs, a 1.3 kb message constitutively expressed in dormant and nondormant embryos and a 0.9 kb message present at higher levels in dormant embryos after 3 h of imbibition. Nondormancy-associated clones AFN1, AFN2 and AFN3 hybridized with 1.5 kb, 1.7 kb and 1.1 kb mRNAs, respectively, that were more abundant in nondormant embryos during imbibition. Expression patterns of some mRNAs in dormant embryos induced to germinate by GA₃ treatment were different than water controls, but were not identical to those observed in nondormant embryos. DNA sequence analysis revealed 76% sequence identity between clone AFN3 and a Citrus sinensis glutathione peroxidase-like cDNA, while significant sequence similarities with known genes were not found for other clones. Southern hybridization analyses showed that all clones represent low (1 to 4) copy number genes.     

Membrene turnover in imbibed and dormant embryos of the wild oat (Avena fatua L.)

A comparative study of protein synthesis has been carried out with embryos excised from dormant (D) and non-dormant (ND) caryopses of the wild oat. Although D embryos imbibed in water or ND embryos imbibed in abscisic acid do not germinate, they incorporate [¹⁴C]leucine into TCA-insoluble material for the first 48 h as readily as embryos that do germinate (ND embryos imbibed in water, or D embryos imbibed in gibberellic acid). Pulsechase experiments with [¹⁴]leucine show that in both D and ND embryos the proteins associated with the membranes undergo turnover. The rates of decay of incorporated radioactivity are similar in both dormant and germinating embryos up to 98 h following embryo excision. Fractionation of the membrane proteins in SDS-polyacrylamide gels indicates that the different polypeptides have different rates of turnover. It is concluded that membrane proteins in imbibed D embryos are in a state of constant turnover, and that this is a part of the replacement processes necessary to maintain the integrity of hydrated cells. The continuation of such synthetic events could account for long term survival of dormant Avena fatua in the imbibed state.Abbreviations CCRSE cytochrome relative stain equivalents - D dormant - ND nondormant - ABA abscisic acid - GA gibberellic acid GA₃     

Membrane turnover in imbibed dormant embryos of the wild oat (Avena fatua L.)

Germinating non-dormant (ND) embryos of wild oat incorporate [³H]glycerol into phospholipid, and a 250% increase in total extractable phospholipid occurs within 72 h. During germination, leveles of phosphatidyl inositol showed the greatest change, increasing approximately 5-fold.Imbibed dormant (D) embryos of the wild oat also incorporate [³H]gycerol into phospholipids, but there is no net synthesis. A continuous turnover of membrane phospholipids could be demonstrated in pulse chase experiments, and although the proportions of most phospholipids does not change, there was a decrease of 50% in phosphatidyl serine.The half-life of [³H]glycerol in the extracted phospholipids of D and ND embryos varies between 35 and 57 h, and in membrane fractions separated on sucrose density gradients the half-lives vary between 26 and 56 h.D embryos induced to germinate with GA and ND embryos in which germination is repressed by ABA show similar phospholipid changes to ND and D embryos respectively, with the exception that the proportion of phosphatidyl serine remained unchanged in the ND-ABA embryos.It is concluded that the continual turnover of membranes of imbibed dormant embryos is consistent with the maintenance of cellular integrity determining the longevity of the seed under natural conditions.Abbreviations D dormant - ND nondormant - ABA abscisic acid - GA gibberellic acid (GA₃)