Allelochemicals in wheat (Triticum aestivum L.): production and exudation of 2,4-dihydroxy-7-methoxy-1,4-benzoxazin-3-one

An analytical technique employing gas chromatography and tandem mass spectrometry (GC/MS/MS) was employed to systematically screen fifty-eight wheat accessions for their differential production of 2,4-dihydroxy-7-methoxy-1,4-benzoxazin-3-one (DIMBOA) from three consecutive sources, i.e., the shoots, roots, and in the associated agar growth medium (collected as root exudates) of 17-day-old wheat seedlings. DIMBOA content differed significantly in the shoots, roots, or in the agar growth medium between accessions. DIMBOA accumulated differentially within the plant, with roots containing more DIMBOA than the shoots. Only 19% of accessions were able to exude DIMBOA from living roots into their growth medium, indicating the exudation of DIMBOA is accession-specific. DIMBOA level in root tissues is expected to be high when a high level of DIMBOA content is detected in the shoots. Wheat seedlings did not release detectable amounts of DIMBOA when the DIMBOA level was low in the root tissues. The valuable genetic material with high levels of DIMBOA in the shoots or roots identified in the present research could be used to breed for wheat cultivars with elevated allelopathic activity.     

Correlation Between Phytotoxicity on Annual Ryegrass (Lolium rigidum) and Production Dynamics of Allelochemicals Within Root Exudates of an Allelopathic Wheat

An improved allelopathic correlation between phytotoxicity measured in root growth bioassay upon annual ryegrass (Lolium rigidum Gaud.) and the concentrations of a selection of dynamically produced allelochemicals quantified in the root exudates of cv. Khapli wheat (Triticum turgidum ssp. durum (Desf.) Husn.) monitored during the first 15 days of wheat seedling growth in a sterile, agar–water medium, has been established. Changes over the 15-day growth period in the quantities of five exuded benzoxazinones and seven phenolic acids were measured simultaneously using GC/MS/MS. Substantiating pure compound dose–response measurements were conducted over a range of concentrations for the putative allelochemicals within the wheat exudates. One synergism-based proposal using the monitored compounds to explain the observed low-exudate-concentration phytotoxicity was explored, but was found to be experimentally inadequate.     

Allelochemicals fromPolygonum sachalinense Fr. Schm. (Polygonaceae)

The root exudates fromPolygonum sachalinense in a recirculating system significantly inhibited lettuce seedling growth. The rhizomes and roots ofP. sachalinense were extracted with 80% acetone. Bioassay of the neutral-acidic fraction on the TLC agar plate showed the inhibitory activity corresponded to the two yellow pigment bands. Two orange needles were isolated and identified as anthraquinone compounds: emodin and physcion. Both compounds exhibited inhibitory activities against the seedling growth of several testing plant species. Glucosides were isolated fromP. sachalinense and were identified as emodin-1-O-β-D-glucoside and physcion-1-O-β-D-glucoside, respectively. On plant growth bioassay, these glucosides showed no phytotoxic activity against lettuce seedlings. The concentrations of emodin, physcion, and their glucosides from rhizome with roots, aerial parts, fallen leaves, and soil were determined. The rhizome with roots and fallen leaves contained emodin and physcion at relatively high concentrations. Emodin also occurs in the soil of this plant community with effective concentrations in the fall. The results indicate that these anthraquinones are responsible for the observed interference and are potent allelopathic substances.     

Allelopathy of oats. I. Assessment of allelopathic potential of extract of oat shoots and identification of an allelochemical

The allelopathic potential of oat (Avena sativa L.) extracts was investigated under laboratory conditions. The ethyl ether-, acetone-, and water-soluble fractions obtained from the extract of oat shoots inhibited the germination and growth of roots and hypocotyls of lettuce (Lactuca sativa L.). The inhibitory activity of the water-soluble fraction was maximum, followed by that of ethyl ether-soluble and acetone-soluble fraction. An active principle of the water-soluble fraction was isolated and its structure was determined by spectral data asl-tryptophan.l-Tryptophan inhibited the growth of hypocotyls and roots of lettuce seedlings at concentrations greater than 0.03 and 0.1 mM, respectively. These results suggested thatl-tryptophan may be an allelochemical which affects the growth or germination of different plant species.     

Hydroxamic Acid Content and Toxicity of Rye at Selected Growth Stages

Rye (Secale cereale L.) is an important cover crop that provides many benefits to cropping systems including weed and pest suppression resulting from allelopathic substances. Hydroxamic acids have been identified as allelopathic compounds in rye. This research was conducted to improve the methodology for quantifying hydroxamic acids and to determine the relationship between hydroxamic acid content and phytotoxicity of extracts of rye root and shoot tissue harvested at selected growth stages. Detection limits for an LC/MS-MS method for analysis of hydroxamic acids from crude aqueous extracts were better than have been reported previously. (2R)-2-β-d-Glucopyranosyloxy-4-hydroxy-(2H)-1,4-benzoxazin-3(4H)-one (DIBOA-G), 2,4-dihydroxy-(2H)-1,4-benzoxazin-3(4H)-one (DIBOA), benzoxazolin-2(3H)-one (BOA), and the methoxy-substituted form of these compounds, (2R)-2-β-d-glucopyranosyloxy-4-hydroxy-7-methoxy-(2H)-1,4-benzoxazin-3(4H)-one (DIMBOA glucose), 2,4-hydroxy-7-methoxy-(2H)-1,4-benzoxazin-3(4H)-one (DIMBOA), and 6-methoxy-benzoxazolin-2(3H)-one (MBOA), were all detected in rye tissue. DIBOA and BOA were prevalent in shoot tissue, whereas the methoxy-substituted compounds, DIMBOA glucose and MBOA, were prevalent in root tissue. Total hydroxamic acid concentration in rye tissue generally declined with age. Aqueous crude extracts of rye shoot tissue were more toxic than extracts of root tissue to lettuce (Lactuca sativa L.) and tomato (Lycopersicon esculentum Mill.) root length. Extracts of rye seedlings (Feekes growth stage 2) were most phytotoxic, but there was no pattern to the phytotoxicity of extracts of rye sampled at growth stages 4 to 10.5.4, and no correlation of hydroxamic acid content and phytotoxicity (I₅₀ values). Analysis of dose–response model slope coefficients indicated a lack of parallelism among models for rye extracts from different growth stages, suggesting that phytotoxicity may be attributed to compounds with different modes of action at different stages. Hydroxamic acids may account for the phytoxicity of extracts derived from rye at early growth stages, but other compounds are probably responsible in later growth stages.     

Screening for Effects of Phytochemical Variability on Cytoplasmic Protein Synthesis Pattern of Crop Plants

Crop plants have to cope with phytochemical variability along with other environmental stresses. Allelochemicals affect several cellular processes. We tested the effect of toxic aqueous leachates from Sicyos deppei, Acacia sedillense, Sebastiania adenophora, and Lantana camara on the radicle growth and cytoplasmic protein synthesis patterns of Zea mays (maize), Phaseolus vulgaris (bean), Cucurbita pepo (squash), and Lycopersicon esculentum (tomato). 2D-PAGE and gel scan densitometry analysis were used to detect differences in cytoplasmic root protein pattern expression. High-, medium-, and low-molecular-weight cytoplasmic proteins were affected by the different aqueous leachates. Crop plant responses were diverse, but in general, an increase in protein synthesis was observed in the treated roots. Maize was the least affected, but both the radicle growth and also the protein pattern of tomato were severely inhibited by all allelopathic plants. The changes observed in protein expression may indicate a biochemical alteration at the cellular level of the tested crop plants.     

Consumption and utilization of experimentally altered corn by southern armyworm: Iron,nitrogen, and cyclic hydroxamates

The effects of differential leaf water, leaf nitrogen and cyclic hydroxamate (DIMBOA) concentrations in corn seedlings were analyzed for a polyphagous insect, the southern armyworm (Spodoptera eridania Cram.). Six different combinations of nutrients and allelochemicals [DIMBOA = 2,4-dihydroxy-7-methoxy(2H)-benzoxazin-3(4H)-one] were generated using two corn genotypes (WF9 and CI3IA) and three fertility regimes (complete nutrient, Fe-deficient, and N-deficient solutions) in the University Biotron. Poorest larval growth was observed in the low-nitrogen treatments (1.2% and 1.7% leaf N) and was the result of both low consumption rates and high metabolic costs (low efficiency of conversion of digested food, ECD). Fastest growth rates were observed forthe larvae fed leaves from the high-nitrogen treatments (4.6% and 4.4% leaf N). It is noteworthy that these treatments also contained the highest concentration of cyclic hydroxamates, which are generally believed to be the primary defensive chemicals mediating resistance against the European corn borer,Ostrinia nubilalis (Hubner). If these hydroxamates do have any deleterious or costly effects (perhaps accounting for a large portion of metabolic expenditures), the high digestibility of the leaf tissue and the increased consumption rates more than compensate, resulting in rapid growth (growth rate = consumption rate × approximate digestibility × efficiency of conversion of the digested food). These studies illustrate that variation in key nutrients and allelochemicals within a single plant species (Zea mays L.) may have significantly different effects upon various potential leaf-chewing caterpillars, such as these armyworms versus corn borers (which cannot handle the cyclic hydroxamates, even if provided with young nutritious leaf tissues).     

Allelochemicals in Wheat (Triticum aestivum L.): Variation of Phenolic Acids in Shoot Tissues

Seven known phenolic acids implicated in wheat allelopathy were analyzed in a worldwide collection of 58 wheat accessions by gas chromatography and tandem mass spectrometry (GC-MS-MS). Chemical analysis showed that accessions differed significantly in the production of p-hydroxybenzoic, vanillic, syringic, trans-p-coumaric, cis-p-coumaric, trans-ferulic, and cis-ferulic acids in the shoots of 17-day-old wheat seedlings. The concentrations of p-hydroxybenzoic, vanillic, cis-p-coumaric, and cis-ferulic acids were normally distributed in the 58 accessions. A binormal distribution was found for syringic and trans-ferulic acids and a skewed normal distribution for trans-p-coumaric acid. The concentration of each compound also varied with phenolic acids. The relative abundance of each phenolic acid was ordered decreasingly as trans-ferulic, vanillic, trans-p-coumaric, p-hydroxybenzoic, syringic, cis-ferulic, and cis-p-coumaric acids. The concentration of total identified phenolic acids varied from 93.2 to 453.8 mg/kg in the shoots of 58 accessions. The content of each phenolic acid or group was highly associated with others in the shoots of wheat seedlings. Wheat accessions with high levels of total identified phenolic acids in the shoots are generally strongly allelopathic to the growth of annual ryegrass.     

A chemical basis for differential allelopathic potential of sorghum hybrids on wheat

The basis for differential allelopathic potentials among sorghum (Sorghum bicolor L. Moench) hybrids was investigated by conducting quantitative and qualitative studies of their phenolic contents. Total phenolic content in sorghum plant parts varied within hybrids, among hybrids, and between growing seasons. Inhibition of wheat (Triticum aestivum L.) radicle growth was positively associated (r=0.66) with concentrations of total phenolics contained in plant parts. Extracts from culms contributed the higherst proportion of toxicity from sorghum plants, inhibiting radicle growth up to 74.7%. Concentrations of five phenolic acids,p-hydroxybenzoic (POH), vanillic (VAN), syringic (SYR),p-coumaric (PCO), and ferulic (FER), differed in all plant parts of the three sorghum hybrids. Concentrations of POH, VAN, and SYR were consistently higher than PCO and FER. PCO and FER wer absent from some plant parts, with FER being the most frequently missing. Inhibition of wheat radicle growth was found to be positively associated with the concentration of each phenolic acid. Vanillic acid was most highly associated (r=0.44) with inhition. Thus, above-ground sorghum tissues contained phenolic acids that contributed to allelopathic potential. Additionally, sorghum roots exuded POH, VAN, and SYR that may enhance the overall allelopathic potential of sorghum during growth and after harvest when residues remain on the soil surface or are incorporated prior to planting a subsquent crop.Journal article No. 12245 of the Missouri Agricultural Experiment Station. Product names are necessary to report factually on available data; however, the USDA neither guarantees nor warrants the standard of the products, and the use of names by USDA implies no approval of the products to the exclusion of others that may be suitable.     

Effects of benzoic and cinnamic acids on growth,mineral composition,and chlorophyll content of soybean

Organic acids are major water-soluble allelochemicals found in soil infested with quackgrass and are involved in several processes that are important in plant growth and development. This study was carried out to gain more information on the effects of benzoic acid (BEN) andtrans-cinnamic acid (CIN) on growth, mineral composition, and chlorophyll content of soybean [Glycine max (L.) Merr. cv. Maple Bell] grown in nutrient solution. The two allelochemicals reduced root and shoot dry biomass of soybean. Treated plants had fewer lateral roots and tended to grow more horizontally compared to the untreated plants. Lateral roots were stunted and less flexible. The amounts of P, K, Mg, Mn, Cl^–, and SO ₄ ^2– were lower, and Zn and Fe contents were higher in roots of plants grown with BEN or CIN as compared to untreated plants. Shoots of plants grown with the allelochemical showed greater accumulation of Ca, Mg, and Zn, whereas P and Fe contents were reduced. The BEN and CIN also caused reductions in leaf chlorophyll content. The BEN and CIN may be responsible for negative allelopathic effects of quackgrass on soybean by inhibiting root growth, by altering ion uptake and transport, and by reducing chlorophyll content.Contribution 493 of the Soils and Crop Research Center.     

Hydroxamic acid glucosides in honeydew of aphids feeding on wheat

DIMBOA glucoside (2-O-/gb-D-glucopyranosyl-4-hydroxy-7-meth-oxy-1,4-benzoxazin-3-one), the main hydroxamic acid (Hx) in intact wheat plants, was detected in the honey dew ofRhopalosiphum padi feeding on seedlings of six wheat cultivars that differed in their concentration of Hx, suggesting that the chemical circulates in the phloem. Neither the aglucone (DIMBOA) nor its main breakdown product were found in any of the honeydew samples. Honey dew production by aphids caged on seedlings of the wheat cultivars and DIMBOA glucoside concentrations in the honeydew followed biphasic curves when plotted against Hx concentration, suggesting passive ingestion of the chemical from the phloem at low Hx concentrations and limited ingestion due to feeding deterrency by Hx in mesophyll cells at high Hx concentrations. The presence of plant toxins such as Hx glucosides in the phloem sap, the main ingesta of aphids, and in the mesophyll cells, has major implications for plant defense, through a feeding deterrent effect during stylet penetration, and deterrency (antixenosis) along with antibiosis during feeding.     

Effect of DIMBOA,an aphid resistance factor in wheat,on the aphid predatorEriopis connexa Germar (Coleoptera: Coccinellidae)

DIMBOA (2,4-dihydroxy-7-methoxy-1,4-benzoxazin-3-one), a secondary metabolite found in cereal extracts, confers resistance in wheat to aphids. Its effect on beneficial organisms was tested on larvae of the aphid predatorEriopis connexa Germar. Larvae were fed until pupation on artificial diets to which different concentrations of DIMBOA (2–200g/g diet) were added, as well as on aphids that had been feeding on wheat seedlings with different DIMBOA levels (140–440 g/g fresh tissue). In diets, the effect of DIMBOA was greatest on survival of third-instar larvae and on the duration of the second and fourth instars. When aphids were provided as food, those that had fed on a wheat cultivar with an intermediate DIMBOA level led to a significantly longer larval duration in the predator than did those that fed on either low or high DIMBOA cultivars. Shortest predator development times were obtained with aphid prey that had fed on high DIMBOA seedlings. Higher DIMBOA levels in the plant appear to reduce aphid feeding rates (and rates of DIMBOA ingestion), decreasing aphid survival and minimizing the effect of the toxin on the predator.On leave from Departamento de Entomologia, Universidad Nacional Agraria La Molina, Apartado aéreo 456, Lima, Perú.     

Resistance of cereals to aphids: The interaction between hydroxamic acids and UDP-glucose transferases in the aphidSitobion avenue (Homoptera: Aphididae)

UPD-glucose transferases are found in the cytosolic and microsomal fractions of the grain aphidSitobion avenae F. Gel filtration and SDSPAGE revealed that the microsomal fraction contained several forms of the enzyme. The molecular weights of the three most active fractions might be 68,000, 66,000, and 36,500. There was a negative correlation between the enzymes' activity in extracts of aphids and the concentration of DIMBOAaglucone in the winter wheat variety fed on by the aphid. A strong inhibition of the activity of the UPD-glucose transferases was observedin vitro at a concentration of DIMBOA as low as 0.01 mM. There was a greater activity of the enzymes in aphids fed on seedlings of susceptible than on moderately resistant wheat cultivars. Prolonged feeding on resistant cultivars resulted in a further reduction in the activity of the aphid's enzymes. The significance for cereal aphids of the role of their UDP-glucose tranferases in the detoxification of plant allelochemicals and adaptation to resistant varieties of cereals is discussed.     

Sorghum Allelopathy—From Ecosystem to Molecule

Sorghum allelopathy has been reported in a series of field experiments following sorghum establishment. In recent years, sorghum phytotoxicity and allelopathic interference also have been well-described in greenhouse and laboratory settings. Observations of allelopathy have occurred in diverse locations and with various sorghum plant parts. Phytotoxicity has been reported when sorghum was incorporated into the soil as a green manure, when residues remained on the soil surface in reduced tillage settings, or when sorghum was cultivated as a crop in managed fields. Allelochemicals present in sorghum tissues have varied with plant part, age, and cultivar evaluated. A diverse group of sorghum allelochemicals, including numerous phenolics, a cyanogenic glycoside (dhurrin), and a hydrophobic p-benzoquinone (sorgoleone) have been isolated and identified in recent years from sorghum shoots, roots, and root exudates, as our capacity to analyze and identify complex secondary products in trace quantities in the plant and in the soil rhizosphere has improved. These allelochemicals, particularly sorgoleone, have been widely investigated in terms of their mode(s) of action, specific activity and selectivity, release into the rhizosphere, and uptake and translocation into sensitive indicator species. Both genetics and environment have been shown to influence sorgoleone production and expression of genes involved in sorgoleone biosynthesis. In the soil rhizosphere, sorgoleone is released continuously by living root hairs where it accumulates in significant concentrations around its roots. Further experimentation designed to study the regulation of sorgoleone production by living sorghum root hairs may result in increased capacity to utilize sorghum cover crops more effectively for suppression of germinating weed seedlings, in a manner similar to that of soil-applied preemergent herbicides like trifluralin.     

Allelopathic potential ofnuphar lutea (L.) Sibth. & Sm. (Nymphaeaceae)

Aqueous extracts ofNuphar lutea (L.) Sibth. & Sm. leaves (blades plus petioles) and roots plus rhizomes were tested for allelopathic activity using lettuce seedling andLemna minor L. assay systems. The 12.5, 25, 125, and 250 parts per thousand (ppt) treatments of both extracts killed the lettuce seedlings. At 2.5 ppt of extract, radicle growth of lettuce was 29% of the control for leaves and 31% of the control for roots plus rhizomes.Lemna minor frond number was reduced to 34% of the control by the 25 ppt leaf extract and to 43% of the control by the 25 ppt roots plus rhizomes extract.L. minor was killed by concentrations of 125 ppt and above of both plant part extracts. As expected, the frond number and total chlorophyll content measured by theL. minor assay were highly correlated. Osmotic potentials below 143 mOsmol/kg had no influence onL. minor growth. Neither the osmotic potential nor the pH of the undiluted extracts ofN. lutea were in the range known to influence the growth of either lettuce seedlings orL. minor. Nuphar lutea extracts were many times more inhibitory than 16 other hydrophytes we previously examined.     

Why Phenolic Acids Are Unlikely Primary Allelochemicals in Rice

Allelopathy in rice (Oryza sativa, L.) effective against weeds has been found in about 3.5% of tested rice germplasm in both laboratory and field experimentation. However, the allelochemicals responsible for growth inhibition of rice-associated weeds have not yet been identified. In the literature, phenolic acids are often mentioned as putative allelochemicals. If phenolic acids commonly reach growth inhibitory concentrations in rice ecosystems, it must be expected that the degree of tolerance to phenolic acids will vary among traditional rice cultivars or plant species adapted to rice environments having inherently different phenolic acid concentrations. Phenolic acids concentrations are normally greater in submerged than in aerobic soils. A dose–response study, however, showed that seedlings of rice cultivars adapted to submerged anaerobic soils did not have higher level of tolerance against p-hydroxybenzoic acid than did seedlings of varieties adapted to aerobic upland soils. Moreover, traditional rice cultivars had no greater tolerance than did improved cultivars that were recently bred for traits other than tolerance of phenolic acids. Similarly, there were no differences in tolerance of p-hydroxybenzoic acid between two Echinochloa weed species adapted to either anaerobic or aerobic growth conditions. Thus, neither the rice cultivars nor weed species had evolved different tolerance levels against the phenolic acid. However, all rice cultivars had significantly greater tolerance of p-hydroxybenzoic acid than did either weed species. In a second experiment, the rates at which rice plants released phenolic acids into solution cultures were measured for at least one month, the time period of greatest allelopathic activity following planting under field conditions. The maximum release rate of phenolic acids during the first month of growth was approximately 10 g/plant/day. At a conventional plant density, the release rate of phenolic acids would be approximately 1 mg/m²day. This order of release rate cannot provide concentrations remotely close to phytotoxic levels determined for these rice cultivars and weed species. The results presented in this paper do not preclude the possibility that phenolic acids might be one component in a mixture of chemicals that, when present simultaneously, are allelopathic.     

<Emphasis Type="Italic">Zea Mays</Emphasis>: Benzoxazolinone Detoxification Under Sulfur Deficiency Conditions—A Complex Allelopathic Alliance Including Endophytic <Emphasis Type="Italic">Fusarium Verticillioides</Emphasis>

Benzoxazolinone detoxification is similar in plants grown under sulfur deficiency conditions and in control plants grown with an optimal S supply. However, when S-deficient plants were treated with metolachlor before benzoxazolin-2(3H)-one (BOA) incubation, detoxification was reduced, as indicated by a lower accumulation of the detoxification products BOA-6-O-glucoside and glucoside carbamate and by a loss of BOA-6-OH glucosyltransfease activity. Root colonizing microorganisms and the endophytic fungus Fusarium verticillioides participated in benzoxazolinone detoxification by converting the compound to 2-acetamidophenol (AAP) or O-hydroxyphenyl malonamic acid (OHPMA), a process accompanied by the appearance of phenoxazinone. Maize roots, however, absorbed AAP and OHPMA only in traces. Absorbed traces of OHPMA stimulated maize radicle growth, and traces of AAP stimulated cress. Phenoxazinone inhibited the growth of cress radicles at concentrations higher than 500 μM, whereas maize radicles were hardly affected. F. verticillioides did not convert benzoxazolinone to any known microbial degradation product when the medium of maize seedlings was inoculated with the fungus under sterile condition. Plant–fungus interactions seem to be important in plant survival of allelopathic attacks. This study points to a complicated network of allelopathic interactions that are influenced by biotic and abiotic factors, including herbicides.     

Phytotoxicity of Vulpia Residues: IV. Dynamics of Allelochemicals During Decomposition of Vulpia Residues and Their Corresponding Phytotoxicity

The behavior and dynamics of 20 identified allelochemicals in Vulpia residues were collectively and individually monitored, and their kinetic phytotoxicity was assessed. The total content of the identified allelochemicals in decaying Vulpia residues increased from 0.31 mg/g to 1.24 mg/g dry residue over a 21–day decomposition period, while the total phenolic content increased from 1.86 mg/g to 2.16 mg/g dry residue. This corresponded to a phytotoxicity increase from 42% to 82% of radical inhibition. Allelochemicals changed in composition and quantity over the duration of the residue decomposition. Addition of soil to the residues reduced the total allelochemical contents extracted and altered the dynamic pattern. In the same period, the total content of allelochemicals declined from 0.061 mg/g to 0.046 mg/g residue + soil, with the total phenolics dropped from 0.20 mg/g to 0.11 mg/g residue + soil, corresponding to a radical length increase from 53% to 109% of control. Only 14 of the identified allelochemicals were detected in the mix of soil and residues, in contrast to 20 present in the residues alone. The implications of these findings are discussed.     

Defoliation Affects Chemical Defenses in All Plant Parts of Rye Seedlings

The effect of defoliation and consecutive defoliation (condefoliation) of rye seedlings on the allocation patterns of biomass and hydroxamic acids (Hx) was evaluated five days after treatment. Growth of condefoliated seedlings was lower than that of defoliated and nondefoliated ones. Concentration of Hx decreased in shoots of condefoliated seedlings compared to nondefoliated ones, while concentration of Hx in root exudates increased. Allocation of Hx to roots and root exudates increased at the expense of allocation to the shoots in condefoliated seedlings. The ratio of Hx-aglucone to Hx-glucoside was higher in shoots of defoliated and condefoliated seedlings. The decrease in quantity of defense in shoots was accompanied by an increase in its quality, given that aglucones are more toxic than glucosides. The increase in concentration of Hx—an allelopathic compound also involved in mineral uptake by roots—in root exudates of condefoliated seedlings was suggested to lead to an advantage in the acquisition of resources for the regeneration of lost biomass.     

Inhibition of seedling growth of crop species by recirculating root exudates ofBidens pilosa L.

The allelopathic effects of root exudates ofBidens pilosa L. on seedling growth ofLactuca sativa L.,Phaseolus vulgaris L.,Zea mays L., andSorghum bicolor (L.) Moench were studied using a root exudate recirculating system that allows continuous exposure of crop plants to allelopathic chemicals. This system maintains an undisturbed rhizosphere and eliminates competition and physical contact between the donor and acceptor plants. Comparison of responses to hydrophobic and hydrophilic root exudates is made possible by removal of hydrophobic compounds using XAD-4. Treatments consisted ofB. pilosa, B. pilosa with an Amberlite XAD-4 resin column attached to the donor pot to remove hydrophobic allelochemicals, and a donor pot without weeds.B. pilosa significantly inhibited seedling growth of all crop species tested. The crop species varied in response to the root exudates, withL. sativa being most sensitive. Larger and olderB. pilosa plants caused greater inhibition of seedling growth ofL. sativa andP. vulgaris than did smaller (younger)B. pilosa plants.B. pilosa with XAD-4 caused significantly less inhibition to all crop species, exceptZ. mays, thanB. pilosa without XAD-4, indicating that the hydrophobic exudates played an important role in the allelopathic growth inhibition. Variability in species response toB. pilosa with and without XAD-4 was probably due to differences in sensitivity to hydophobic and hydrophilic allelochemicals.Supported by a grant under USDA Agreement No. 83-CRSR-2-2293. Journal Series No. 2887 of the Hawaii Institute of Tropical Agriculture and Human Resources.