Striga asiatica seed conditioning and 1-aminocyclopropane-1-carboxylate oxidase activity

Conditioned seeds of Striga asiatica (L.) Kuntze release ethylene, which elicits germination. We investigated the activity of 1-aminocyclopropane-1-carboxylate (ACC) oxidase and respiration during conditioning. Seeds incubated in vivo with ACC, the substrate for ACC oxidase, produced negligible ethylene at the beginning of conditioning or if they were dormant (i.e. would not germinate after conditioning and treatment with stimulant). Non-dormant seeds produced 3000 ηL of ethylene/600 seeds/24 h after 12 days of conditioning. In vitro ACC oxidase activity at day 0 of conditioning produced 500 ηL of ethylene/μg protein/h and 8000 ηL of ethylene/μg protein/h after 12 days of conditioning. Incubation of seeds in strigol before protein extraction did not enhance enzyme activity. Seeds released 4000 μL/L CO₂ in the first 24 h of conditioning, with the rate increasing to 15 000 μL/L/24 h on day 4 and then remaining roughly unchanged. Maximum in vitro activity of ACC oxidase required ACC, catalase, O₂, Fe²⁺, ascorbate and CO₂. In vivo activity of ACC oxidase required ACC and/or germination stimulant(s), suggesting that stimulants may be involved in providing substrates for the ACC oxidase. No difference was observed in the separation of extracted proteins, which suggests that ACC oxidase is activated during conditioning, perhaps as a result of changes in co-factor concentration. Application of these findings to Striga control is discussed.     

A qualitative quick-test for detection of herbicide resistance to tribenuron-methyl in Papaver rhoeas

A qualitative seed-based method useful for the detection of resistance to the herbicide tribenuron-methyl in Papaver rhoeas L. is described. Seeds were germinated on 35 mL of a 1.3% agar medium containing 2 g KNO₃ L^–1 in 8.5 cm Petri dishes in a growth chamber under 20 μmol s^–1 m^–2 of fluorescent light. When 0.24 μM tribenuron-methyl or more was added, growth in susceptible plants stopped after the cotyledon stage and they turned chlorotic. The resistant plants continued developing new leaves. The same effect was achieved when 0.2 g gibberellin (GA₃) L^–1 and 7.68 μM tribenuron-methyl or 0.5 g GA₃ L^–1 and 61.44 μM tribenuron-methyl were added. Germination percentage rose with gibberellin in the presence or absence of the herbicide. Plants developed rapidly, with only about 14 d needed to finish the test but sometimes root growth was reduced because of the addition of gibberellin. In the absence of gibberellin but in the presence of the herbicide, plants grew more slowly and developed smaller leaves with a 17-d evaluation period requirement. The test was validated with pot experiments in a greenhouse and also with field trials. The best combination was found to be 0.2 g GA₃ L^–1 and 7.68 μM tribenuron-methyl, assuring homogenous germination and testing of dormant seeds but avoiding root inhibition associated with too much gibberellin.     

Evaluation of variability in Striga aspera, Striga hermonthica and their hybrids using morphological characters and random amplified polymorphic DNA markers

Striga aspera and Striga hermonthica are recognized as separate species, but their close morphological similarity causes difficulty in distinguishing between them in areas where they coexist in Africa. In this study, crosses between the species were made using randomly selected morphologically typical parental plants collected from different locations in Nigeria. Genetic analysis of both species and their reciprocal F₁ hybrids were determined using cluster analysis of DNA profiles derived from genetic polymorphism (RAPD)-polymerase chain reaction (PCR) markers. Principal component and hierarchical cluster analyses were used to separate parental and hybrid populations based on 13 morphological characteristics. Morphological data from wild samples of both species were compared with the hand-pollinated parental, F₁ and F₂ hybrids, and back-crosses. Results showed that S. aspera and S. hermonthica were genetically and morphologically distinct. Morphological and genetic analyses revealed two major clusters: a S. aspera cluster and a S. hermonthica cluster. Genetically, the F₁ hybrids showed closer affinity to their maternal parents, while morphologically, the F₁ hybrids formed distinct clusters intermediate to the parents. Most F₂ plants and back-crosses were morphologically similar to S. hermonthica . Comparative morphological analysis of wild and hand-pollinated populations showed some samples from the wild clustered with the hybrids, suggesting that hybrids may exist in nature.     

Effect of fungal toxins on germination of Striga hermonthica seeds

Fourteen fungal toxins were assayed in vitro to evaluate their effect on seed germination of the parasitic weed Striga hermonthica . Among them, T-2 toxin proved to be the most active, being able to inhibit 100% seed germination at 10⁻⁵ M, and being still active when tested at a concentration of 10⁻⁷ M (19% inhibition). Deoxinivalenol was also very active, causing 100% and 69% reduction in germination when assayed at 10⁻⁴ and 10⁻⁵ M respectively. Cytochalasin E, tenuazonic acid, fumonisin B₁, enniatin and nivalenol were shown to have an inhibitory effect of around 50% at 10⁻⁴ M, whereas other toxins had lower or no activity. The high activity shown by some fungal toxins suggests that they may have potential for use as more natural and safe herbicides to suppress parasite seed germination.     

Comparison of commercial glyphosate formulations for control of prickly sida, purple nutsedge, morningglory and sicklepod

The efficacy of the commercial glyphosate [( N -phosphonomethyl) glycine] formulations Roundup Ultra, Touchdown and Engame were compared for the control of prickly sida ( Sida spinosa L.), morningglory ( Ipomeae hederacea var. integriuscula Gray), sicklepod ( Senna obtusifolia L.) and purple nutsedge ( Cyperus rotundus L.). Engame is a new formulation of glyphosate that contains glyphosate acid and 1-aminomethanamide dihydrogen tetraoxosulfate (AMADS), a proprietary mixture of sulfuric acid and urea, other than glyphosate salt and surfactants. Injury by Engame differed from Roundup Ultra and Touchdown in that necrotic lesions formed on leaves several hours after treatment. Leaves of very susceptible species, such as prickly sida, were rapidly, although incompletely, desiccated and then became chlorotic and died in a manner typical of other glyphosate formulations. Engame was 2–3 times more active to growth inhibition than either the Roundup Ultra or Touchdown formulations, based on GR₅₀ comparisons expressed on an acid equivalent basis. The GR₅₀ estimates did not change over the 3 week evaluation period for prickly sida and purple nutsedge, and after 2 weeks after treatment for morningglory. The GR₅₀ estimates for sicklepod decreased over the 3 week evaluation period indicating a slower response to glyphosate. The application of AMADS alone caused minute necrotic lesions on sicklepod and purple nutsedge, and lesions up to 3 mm in diameter on prickly sida and morningglory. Further injury from AMADS was not noted and plants resumed growth without apparent delay. At glyphosate rates above 1120 g ha⁻¹, greater than 80% control was achieved at 7 days after treatment. These results demonstrate that glyphosate efficacy can be further enhanced by formulations that apparently improve uptake and translocation.     

Disinfection of pepper seed infected with different strains of capsicum mosaic virus by trisodium phosphate and dry heat treatment

Disinfection of pepper seed infected with capsicum mosaic virus (CaMV) by immersion in 100 g/1 Na₃ P0₄ solution was compared with dry heat treatment at 76°C. The virus content of the seed varied with the CaMV strains used to infect the pepper cultivars and the time of harvest of seeds from infected plants. Immersion times in Na₃PO₄ had to be increased from 15 min to 2 h to obtain near-complete virus inactivation; these treatments had no effects on germination.
Heating seed in an oven at 76°C for 3 days following a waiting period of 3 months after harvest always eliminated all the virus present, but adversely affected germination. This resulted in delayed emergence and a reduction in the number of seedlings suitable for further raising. The viability of heat-treated seeds also decreased with continued storage after treatment.
There were inconsistent differences in germination of seed from healthy plants and plants infected with the CaMV strains P8 or P11. The possibility of internal seed infection and practical consequences are discussed.     

A possible involvement of gibberellin in the mechanism of Avenafatua resistance to triallate and cross-resistance to difenzoquat

Avena fatua L. populations from numerous fields in Alberta have simultaneously developed resistance to recommended rates of two dissimilar herbicides, triallate and difenzoquat. We used exogenous applications of gibberellic acid (GA₃) to investigate the possibility that endogenous gibberellins were involved in the A. fatua 's resistance mechanism. For control plants, without applied GA₃, shoots of the most resistant (R) populations emerged more rapidly and elongated faster than shoots of the susceptible (S) populations. Increases in shoot elongation in response to exogenous GA₃ were significantly lower in R populations compared with S populations. This suggests that R populations may have elevated endogenous gibberellin levels, relative to S populations. Additionally, inhibition of S population shoot elongation and shoot anatomical abnormalities caused by relatively low concentrations of triallate and difenzoquat could be prevented by exogenous application of GA₃. These results suggest that there may be a phytohormonal involvement in the mechanism of triallate/difenzoquat resistance in A. fatua . That is, higher endogenous gibberellin levels in R populations may result in meristematic growth that is rapid enough to preclude phytotoxic levels of these herbicides from reaching the shoot meristem, which is the likely site of action.     

Fungitoxicity of plant growth regulators (PGRs) and PGR/fungicide mixtures in soak treatments to Septoria apiicola pycnidiospores

Mixtures of the plant growth regulators (PGRs) gibberellins A4 and A7 (GA 4/7) and ethephon (2-chloroethylphosphonic acid), applied as soak treatments at 20° C for 24 h, were toxic to pycnidiospores of Septoria apiicola on agar, apparently as a result of low pH of their solutions. Ultraviolet spectroscopy showed that the aqueous solubility of the fungicides benomyl and carbendazim increased significantly when buffered at low pH or mixed with aqueous solutions of GA 4/7 and ethephon. This resulted in greater quantities of ¹⁴C-labelled fungicides entering celery seeds imbibed in PGR/fungicide mixtures in comparison with fungicides alone.
A 24-h seed soak at 20° C in a mixture of aqueous benomyl, GA 4/7 and ethephon eliminated S. apiicola in infected celery seeds. A thiram soak was also effective but PGR mixtures alone did not completely eliminate the pathogen.     

Paclobutrazol and other plant growth-retarding chemicals increase resistance of melon seedlings to fusarium wilt

Studies were made of plant growth retardation and effects on resistance of melon to fusarium wilt by soil application of paclobutrazol, other ergosterol biosynthesis inhibitors, the fungicide benomyl and the herbicide dinitramine. Paclobutrazol and ancymidol delayed the onset of wilting and were the most effective in reducing wilt incidence. A relationship between effectiveness in retarding seedling elongation and increasing resistance to fusarium wilt was observed. No chemical had a significant effect on pathogen population level in the plant, as determined by stem colonization and direct assessment tests in seedlings grown in soil treated with the chemicals. Excluding benomyl, and to a lesser extent dinitramine, leaf and stem extracts had no inhibitory effect on conidial germination. Gibberellins GA₄₊₇, when applied to seedlings inoculated after germination in paclobutrazol-treated soil, nullified growth retardation and increased disease incidence. We suggest that disease incidence reduction by paclobutrazol is due to an effect on plant metabolic processes and not to direct fungitoxicity of the compound.     

A study of assimilation and translocation in Cuscuta hyalina Heyne ex Roth., Orobanche ramosa L. and Striga hermonthica Benth.

Quantitative determinations of chlorophyll showed that Cuscuta hyalina and Striga hermonthica possess chlorophyll. No trace of chlorophyll was found in Orobanche ramosa. Autoradiographic evidence showed that both radiocarbon and radiophosphorous moved from host to parasite in alt three species. Photosynthesis occurs in S. hermonthica when the parasite is isolated from its host. The adventitious roots of S. hermonthica and O. ramosa appear to be functional in absorbing a proportion of the required inorganic substances and water. S. hermonthica is a hemiparasite, O. ramosa is a holoparasite but C. hyalina is best described as a partial parasite.     

Effects of Ca2+ and Mg2+ on Botrytis cinerea and Penicillium expansum in vitro and on the biocontrol activity of Candida oleophila

Calcium salts have been reported to play an important role in the inhibition of postharvest decay of apples and in enhancing the efficacy of postharvest biocontrol agents. Therefore, the present study was conducted in order to examine and compare the effects of calcium and magnesium salts on the germination and metabolism of the postharvest pathogens Botrytis cinerea and Penicillium expansum , and to determine the effects of these salts on the biocontrol activity of two isolates (182 and 247) of the yeast Candida oleophila. Increasing concentrations of CaCl₂ (25–175 mM) resulted in decreased spore germination and germ-tube growth of both pathogens. The greatest effect was observed in the case of B. cinerea. The inhibitory effect could be overcome by the addition of glucose to the germination medium. MgCl₂ (25–175 mM) had no effect on germination or germ-tube growth of either pathogen, indicating that the calcium cation rather than the chloride anion was responsible for the inhibition. The pectinolytic activity of crude enzyme obtained from the culture medium of both pathogens was also inhibited by 25–175 mM CaCl₂, with the greatest effect on the crude enzyme from P. expansum. Biocontrol activity of isolate 182 was enhanced by the addition of 90 or 180 MM CaCl₂, whereas there was no effect on the biocontrol activity of isolate 247. This was apparently due to the inability of isolate 247 to proliferate in apple wounds. It is postulated that enhanced biocontrol activity of isolate 182 of the yeast C. oleophila in the presence of Ca²⁺ ions is directly due to the inhibitory effects of calcium ions on pathogen spore germination and metabolism, and indirectly due to the ability of isolate 182 to maintain normal metabolism in the presence of"toxic" levels of calcium.     

Prohexadione-calcium induces sunflower (Helianthus annuus) resistance against the root parasitic weed Orobanche cumana

Prohexadione-calcium (PHDC) is a plant growth retardant that inhibits gibberellin biosynthesis and transiently alters the spectrum of flavonoids and their phenolic precursors. In this study, root chamber and pot experiments were conducted to evaluate the effects of PHDC on Orobanche cumana seed germination and induction of sunflower resistance to this root parasitic weed. Root chamber experiments indicated that PHDC did not interfere with the progress of Orobanche seed germination, but retarded O. cumana tubercle formation and development. In pot experiments, control of O. cumana by PHDC depended on its dose, method of application and sunflower cultivar. In sunflower cv. HA89, PHDC at 10 mg LS⁻¹ (litre soil) as two drenches or at 1 m m as two foliar sprays significantly reduced the total number and dry matter of O. cumana without influencing flower, leaf and root development. In sunflower cv. Albena, PHDC at 20 and 30 mg LS⁻¹, but not at 10 mg LS⁻¹, as two drenches significantly reduced O. cumana infection. Sunflower cv. HA89 was less susceptible to O. cumana and more inducible by PHDC than sunflower cv. Albena. The biochemical analysis of root extracts revealed a significantly higher accumulation of free phenolics in sunflower cv. HA89 after PHDC soil drench, but not in Albena. No significant lignification was observed after PHDC treatments. The results suggest that PHDC reduces O. cumana infection in sunflower by inducing host resistance, not by directly interfering with the parasite. Free phenolics play an important role in sunflower response to the parasite.     

A note on the chemical composition, intake and digestion of Striga hermonthica herbage by sheep

Striga hermonthica causes serious crop yield losses in West Africa. Hand pulling, an effective method for the reduction of light infestations, might be encouraged if farmers could use this weed as livestock feed. This study evaluated the chemical composition and the voluntary intake and digestion of S. hermonthica herbage by sheep. Crude protein (g kg⁻¹ dry matter (d.m.)) was 184 in the whole plant, 230 in the leaf and 87 in the stem. Ash content varied from 183 to 253 g kg d.m.⁻¹. The concentration of neutral and acid detergent fibre and lignin in whole pot-grown plants was 364, 278 and 127 g kg d.m.⁻¹ respectively. The digestibility of dry and organic (o.m.) matter was 493 and 657 g kg⁻¹, respectively, and intake of digestible o.m. was 27.1 g kg W^−0.75. The relatively high N and P levels in S. hermonthica warrant further evaluation in terms of its potential use as a source of protein or for compost. Its use as a feed appears to be limited by the high ash content and possibly by anti-nutritional effects on animals. These effects should be further investigated before recommending its use for this purpose.     

生物碱参与阿魏种子休眠的调控

为揭示濒危植物阜康阿魏（Ferula fukanensis Shen.）种子休眠的成因,对阜康阿魏种子分别采用流水漂洗处理后的发芽试验、蒸馏水和NaHCO₃溶液浸提液以及化学法提取生物碱中的小麦种子萌发实验,并观察蒸馏水和NaHCO₃浸泡阿魏种子对介质pH的影响等,综合分析阿魏种子休眠的成因。流水浸洗处理5 d和20 d的发芽率分别为77%和85%以上,较对照（47%）有显著提高(P<0.01);NaHCO₃浸提液对小麦种子萌发的抑制作用显著高于蒸馏水浸提液;用pH较高的NaHCO3溶液和蒸馏水浸泡阿魏种子,该介质的pH向8.0～8.3收敛;化学法提取生物碱对小麦发芽有显著抑制作用,当10 g·（100mL）^-1时抑制率达80%。实验结果显示,除脱落酸（ABA）外,还有生物碱在阿魏种子休眠的调控中起重要作用。     

Factors affecting the activity of ethephon in stimulating seed germination of Striga hermonthica (Del.) Benth.

Ethephon (2-chloroethylphosphonic acid), which generates ethylene, stimulated the germination of pre-conditioned seeds of Striga hermonthica when it was added to the alkaline Gezira clay soil at concentrations of 2.5 to 30 mg kg^?1. As little as 5 min contact with treated soil was enough to stimulate germination. Ethephon in soil did not cause germination of unconditioned seeds for periods of up to 12 days and also had an adverse effect on seed germination when such seeds were given a second ethephon exposure after a storage period which was adequate, in untreated soil, to give the necessary pre-conditioning. Ethephon activity persisted in air-dry soil but declined over a 14 day period in moist soil. In the field ethephon at 0.6 to 4.8 kg ha^?1 decreased the number of Striga shoots and increased sorghum height and flowering.     

Differential gene expression of the α-chain of mitochondrial H+-transporting ATP synthase between dormant and non-dormant seeds of paddy Echinochloa weeds

Gene expression was compared under favorable germination conditions between dormant and non-dormant seeds of rice paddy Echinochloa weeds and a domesticated Echinochloa species lacking dormancy. Two dormancy-specific cDNAs, Ecd1 and Ecd2 , were identified by differential display. Northern blot analysis revealed that these genes were more strongly expressed in dormant seeds than in non-dormant seeds. A database search for the Ecd1 sequence revealed no significant homology with any known proteins, but the Ecd2 sequence was highly homologous with the α-chain of mitochondrial H⁺-transporting adenosine triphosphate (ATP) synthase in rice ( Oryza sativa L.). These findings indicate that the gene encoding the enzyme associated with conventional aerobic respiration is more abundantly transcribed in dormant seeds. The results reported in the present study suggest that dormant seeds of paddy Echinochloa weeds, which appear during the period when paddy soil becomes aerobic by drainage, may maintain viability primarily by efficient conventional aerobic respiration, including ATP synthesis catalyzed by the mitochondrial H⁺-transporting ATP synthase.     

Reproductive Ability of Hybrids of Striga aspera and Striga hermonthica

ABSTRACT Striga aspera and S. hermonthica are sympatric in Africa. Each may serve as virulent gene reservoirs for the other if they hybridize and their hybrids are virulent and fertile. Intraspecific and interspecific crosses were made within and between the species, and reproductive success was determined. Freshly harvested seeds from the parental and F(1) crosses were tested over time for germination. Chromosome counts from shoot-tip squashes of seedlings of S. aspera were determined as n = 18, and as n = 19 for S. hermonthica. Hybridization results indicated that S. aspera and S. hermonthica could be intercrossed and their hybrids successfully backcrossed to either parent. Reproductive success in all crosses ranged from 68 to 95% and seeds of all the crosses were viable, germinated on exposure to a synthetic germination stimulant, and were pathogenic on maize. Seeds from the hybrids and backcrosses were less viable and germinable than either parent, suggesting that the two species were not 100% compatible. Results suggest that the two species are closely related, but are separate taxa, perhaps at the subspecies level. Seed dormancy for both species and the F(1) hybrids was less than 84 days after pollination. Germination of S. hermonthica seeds reached 31% at 28 days after pollination. Frequently occurring seed germination peaks were observed for all the seeds tested.     

Germination, emergence, and dormancy of Mimosa pudica

Mimosa pudica (common sensitive plant) is a problematic weed in many crops in tropical countries. Eight experiments were conducted to determine the effects of light, seed scarification, temperature, salt and osmotic stress, pH, burial depth, and rice residue on the germination, seedling emergence, and dormancy of M. pudica seeds. Scarification released the seeds from dormancy and stimulated germination, though the germination of the scarified seeds was not influenced by light. The scarification results indicate that a hard seed coat is the primary mechanism that restricts germination. The germination increased markedly with the exposure to high temperature "pretreatment" (e.g. 150°C), which was achieved by placing non-scarified seeds in an oven for 5 min followed by incubation at 35/25°C day/night temperatures for 14 days. The germination of the scarified seeds was tolerant of salt and osmotic stress, as some seeds germinated even at 250 mmol L⁻¹ NaCl (23%) and at an osmotic potential of −0.8 MPa (5%). The germination of the scarified seeds was >74% over a pH range of 5–10. The seedling emergence of the scarified seeds was 73–88% at depths of 0–2 cm and it gradually decreased with an increasing depth, with no seedling emergence at the 8 cm depth. The rice residue applied to the soil surface at rates of ≤6 t ha⁻¹ did not influence the seedling emergence and dry weight. The information gained from this study identifies some of the factors that facilitate M. pudica becoming a widespread weed in the humid tropics and might help in developing components of integrated weed management practises to control this weed.     

Virulence for faba bean and production of ascochitine by Ascochyta fabae

The production of ascochitine by seven isolates of Ascochyta fabae accounted for the toxicity of culture filtrates of the fungus to cells isolated from leaves of Vicia faba. The LD₅₀ value for cells from cultivars that were susceptible, tolerant or resistant to the fungus was similar i.e. 3·0 × 10⁻⁵ m , 3·8 × 10⁻⁵ m and 2·9 × 10⁻⁵ M, respectively. Ascochitine affected neither the germination of seeds nor the growth of mature plants at 5·17 × 10⁻⁴ m but caused necrosis and wilting of plant cuttings at 2·5 × 10⁻⁴ m and 5·10⁻⁴ m . There was no association between virulence of 16 isolates of A. fabae for three cultivars of V.faba and the production of ascochitine in vitro. One isolate produced no ascochitine in vitro and yet was the most virulent for two of three cultivars. The toxin could not be extracted from infected plants.     

Inhibitory effects of aqueous extracts of black mustard on germination and growth of alfalfa

The present study investigated the allelopathic effects of various black mustard ( Brassica nigra L.) plant parts on alfalfa germination and seedling growth. Aqueous extracts of black mustard ( Brassica nigra L.) leaf, stem, flower and root plant parts were made to determine their effects on germination and dry weights of hypocotyl, and radicle length of 5-day-old alfalfa seedlings over a range of extract concentrations. All aqueous extracts significantly inhibited alfalfa seed germination and seedling growth when compared with the distilled water control. Increasing the aqueous extract concentrations of separated Brassica nigra plant parts significantly inhibited alfalfa germination, seedling length and weight. Based on significant radicle length reactions to aqueous extracts, the toxicity may be classified in the following order of decreasing inhibition: flower, leaf, mixture of all plant parts, root, and stem. Increasing the concentration of aqueous flower extracts significantly inhibited the water uptake by germinated alfalfa seeds. The greatest inhibition in water uptake occurred at a 12 g kg⁻¹ extract concentration for seeds soaked for 8 h.