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Sweet corn (Zea mays L.) varieties carrying the sh2 gene are in high demand, but such varieties have poor stress tolerance, especially during plant establishment. Trichoderma harzianum Rifai strain 1295-22 is a biocontrol fungus developed to provide season-long colonization of crop roots. It has the potential to reduce root rot and increase root growth. In the absence of detectable disease, colonization by Trichoderma increased root and shoot growth by an average of 66%. The enhancement was not uniform among the plants. Low- and intermediate-vigor plants were larger in the presence of Trichoderma, but high-vigor plants were not further enhanced by the fungus. Seeds that were subjected to oxidative stress with 0.05% NaOCI had much-reduced vigor; subsequent treatment with Trichoderma fully restored vigor. This result indicates that the damage caused by hypochlorite is specifically repaired by Trichoderma. Treatment of imbibed but unemerged seeds with cold (5/10 °C night/day) for varying periods reduced subsequent growth. Plants with Trichoderma-colonized roots were 70% larger at all durations of cold treatment. The absence of interation indicates the growth reduction due to cold and the growth enhancement due to Trichoderma are by different mechanisms. Allelopathic reduction in root growth by rye was mimicked by applying benzoxazolinone to the soil. Trichoderma-colonized roots grew faster, but the characteristic shortening of the radicle still occurred. There was no interaction between Trichoderma and allelopathy, indicating that these two treatments affect growth by independent mechanisms. The different ways that growth was enhanced by Trichoderma lead us to propose that this fungus acts, in part, by reversing injurious oxidation of lipids and membrane proteins. Root growth is markedly enhanced by colonization with Trichoderma harzianum. This enhancement can restore some stress-induced growth reduction and may directly reverse oxidative injury.

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seedling growth of various crops. Despite this progress, several aspects of allelopathy by sunn hemp are not well understood. Sunn hemp has considerable genetic diversity ( Wang et al., 2006 ), but possible differences in allelopathic potential of different

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Crude aqueous extracts from dead stems, crowns, and roots from both field-grown and tissue-cultured asparagus plants delayed, but did not prevent, germination of asparagus seed. Root extract inhibited root and shoot development of asparagus seedlings grown in growth pouches. Stem and crown extracts reduced root growth but not shoot growth. The extracts of all 3 tissues caused more secondary root formation and root branching. The highest concentration of extract from crown-plus-root tissues, 5 g of tissue/100 ml water, inhibited radicle growth and killed seedlings. Toxicity of the crown-root extract was not reduced by adding activated charcoal to the extract or by autoclaving the extract. These results suggest that toxic substances in dead asparagus tissue are water-soluble and stable and may persist in old asparagus fields.

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Germination bioassays were conducted to assess if water-soluble extracts of broccoli (Brassica oleracea L. var. italica L.) affect germination of broccoli, cabbage (Brassica oleracea L. var. capitata L.), and cauliflower (Brassica oleracea L. var. botrytis L.). Greenhouse experiments also examined the phytotoxic potential of soil previously cropped with broccoli and broccoli plant parts on seedling growth of those species. The first bioassay used nonsterile extracts (NSEs) and filter-sterilized extracts (FSEs) of broccoli leaves. The second bioassay used nonsterile and filter-sterilized leaf extracts (LEs), stem and root extracts (SREs), and whole broccoli plant (leaves, stems, and roots) extracts (WPEs). Broccoli and cabbage germination were not affected by NSEs or FSEs, but the latter reduced cauliflower germination by 22%. LEs and SREs decreased germination speed for broccoli, cabbage and cauliflower. Greenhouse seedlings were grown in soil previously cropped with broccoli or fallow soil at three fertilizer levels. Broccoli soil was phytotoxic to cauliflower but enhanced broccoli and cabbage seedling growth. The differential sensitivity to broccoli plant residue was in the order of cauliflower > broccoli = cabbage, with SR residue having the highest phytotoxic potential.

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Several cinnamic acids have been identified as principal toxic components of asparagus (Asparagus officinalis L.) root autotoxin and have been shown to synergize Fusarium infection of asparagus. The basis for this synergism was studied by exposing asparagus seeds and radicles from pregerminated seeds to ferulic (FA), caffeic (CA), or methylenedioxycinnamic (MDA) acids alone and in combinations of two or three of these acids. After treatment, seeds were placed in pots of peat-lite mix, and, depending on the experiment, all or half were inoculated with F. oxysporum (Schlecht) f. sp. asparagi (Cohen). Seedling emergence from each pot was used as a measure of toxicity. All cinnamic acids at 1% suppressed emergence compared with the control. Solutions combining FA and CA (0.5%/0.5%, v/v) were substantially more toxic than 1% solutions of either alone. Exposure of radicles (early postgermination) for 10 minutes to combined FA/CA before planting decreased emergence from pots, whereas emergence following a 10-minute exposure to 1% CA or FA alone did not differ from the controls. The 2-hour exposure to FA or to FA/CA and the 24-hour exposure to CA, FA, or FA/CA decreased emergence, with toxicity progressing as follows: CA < FA < FA/CA. Root tip squashes showed fewer mitotic figures in treated than in untreated radicles, and scanning electron microscopic (SEM) examination of the radicle epidermis revealed damage to the surface of epidermal cells and precocious root hair development, the extent of which paralleled treatment toxicity.

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Asparagus (Asparagus officinalis L.) root tissue and root extracts were used to investigate the previously reported release of toxic chemicals from senescing root tissue. Greenhouse studies showed that the severity of crown or root rot of asparagus seedlings increased in direct proportion to increased amounts of dried root tissue incorporated into soil with either F. oxysporum f. sp. asparagi, F. moniliforme, or a combination of these two pathogens. When excised asparagus roots were treated with increasing concentrations of a water extract of dried asparagus root tissues, electrolyte efflux increased, peroxidase activity decreased linearly, and respiration decreased. Active components in the extracts were heat-stable. Our data suggest allelochemicals of asparagus may have direct physiological and biochemical effects on asparagus plants that predisposes them to fusarium diseases.

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Poor emergence of commercially grown lettuce has been observed when planted immediately after the removal of a celery crop. Greenhouse experiments were conducted to evaluate the possible allelopathic effects of celery residue on the emergence and growth of lettuce. The influence of amount and type of celery tissue, growth medium and fertility, incubation time in soil, and amendment of growth medium containing celery residue with activated charcoal was evaluated with respect to the allelopathic potential of celery. Celery root tissue was 1.8 and 1.6 times more toxic to lettuce seedling growth than was celery petiole or lamina tissue, respectively. Lettuce shoot growth was inhibited to a greater extent when grown in sand amended with celery residue rather than either amended vermiculite or potting soil. Incubation of celery root residue in soil for 4 weeks increased phytotoxicity at 1% (v/v) and decreased it at 4% (v/v). Increasing the fertility of pure sand with varying amounts of Hoagland's solution did not reverse the allelopathic effects of celery residue. The addition of activated carbon to the medium increased the growth of lettuce exposed to celery residues. Celery residues possess allelopathic potential to developing lettuce seedlings. Celery tissue type and concentration, soil type, incubation of celery root residue in soil, and addition of activated carbon to the growing medium influenced the magnitude of the observed phytotoxicity.

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Abstract

Asparagus (Asparagus officinalis L.) root filtrate (RF) depressed asparagus seedling emergence in a sterile peat–vermiculite medium. In a medium inoculated with Fusarium oxysporum f. sp. asparagi (FO), the effect was magnified. The response to RF dose, regardless of level of FO infestation, was quadratic. Comparisons with a sucrose solution of the same percentage of soluble solids as the RF suggested that reduced emergence may have been due in part to enhancement of FO growth by the energy source added to the medium. However, after germinating seeds in RF until radicle emergence, then rinsing and transferring them to FO-inoculated medium, emergence was reduced relative to controls. Therefore, depression of emergence apparently related both to an autotoxin somehow predisposing young radicles and/or hypocotyls to increased FO infection and to stimulation of FO in the rhizosphere by the soluble solids content of the root exudate. Infection was confirmed to be the only role of FO: sterilization of the spore suspension by Millipore (0.2 µm) filtration eliminated pathogen toxicity.

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Abstract

To determine whether grape (Vitis sp.) replant problem might be an example of autotoxicity, soil samples from fields that had or had not been replanted with grape cuttings were extracted with a neutral solution of ethylenediaminetetraacetic acid (EDTA) and by leaching with half-strength Hoagland's solution. Rooted grape cuttings growing in sand also were leached. Fractions of the leachates and extracts were assayed for toxicity using lettuce seedlings. Grape roots and replant soil yielded a toxic substance not present, or possibly present only at low levels, in non-replant soil. The replant soil extract inhibited phenylalanine ammonia-lyase activity in grape roots more than did the non-replant soil extract. The toxic substance in the replant soil extract was partially purified and a 1H nuclear magnetic resonance (NMR) spectrum taken, but the substance has not been identified. Thus, grape roots appear to be the source of at least one compound that is toxic to plants and accumulates in the soil in which grapes are grown.

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Abstract

Aqueous extracts of asparagus (Asparagus officinalis L.) roots inhibited seed germination in tomato and lettuce, but not in cucumber. The extracts reduced hypocotyl growth in lettuce, shoot growth in asparagus, and inhibited radicle elongation in barley, lettuce, and asparagus. Seedling growth in tomato and two cultivars of wheat were not affected. Inhibition was concentration-dependent. Radicle growth in ‘Grand Rapids’ lettuce was sensitive to an extract concentration as low as 0.05 g dry root tissue/100 ml H2O. Asparagus radicles were more sensitive than asparagus shoots. In one experiment, phytotoxicity of crude extract was not altered by autoclaving. Aqueous root extracts of A. racemosis Willd. also inhibited germination and radicle growth in ‘Grand Rapids’ lettuce. A crude extract was purified by solvent partitioning, and charcoal adsorption, cation exchange, and thin-layer chromatography (TLC). A band from the TLC was found to fluoresce under ultraviolet light, react with phenolic-sensitive localization reagents, and inhibit the growth of lettuce and asparagus radicles.

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