No abstract

Fine fescue grasses displace neighboring plants by depositing large quantities of an aqueous phytotoxic root exudate in the soil rhizosphere. Via activity-guided fractionation, we have isolated and identified the nonprotein amino acid m-tyrosine as the major active component. m-Tyrosine is significantly more phytotoxic than its structural isomers o-and p-tyrosine. We show that mtyrosine exposure results in growth inhibition for a wide range of plant species and propose that the release of this nonprotein amino acid interferes with root development of competing plants. Acid hydrolysis of total root protein from Arabidopsis thaliana showed incorporation of m-tyrosine, suggesting this as a possible mechanism of phytotoxicity. m-Tyrosine inhibition of A. thaliana root growth is counteracted by exogenous addition of protein amino acids, with phenylalanine having the most significant effect. The discovery of m-tyrosine, as well as a further understanding of its mode(s) of action, could lead to the development of biorational approaches to weed control.allelopathy ͉ festuca ͉ rhizosphere ͉ root ecology ͉ Arabidopsis R oot exudation of small molecules plays a major role in plant ecosystems and is often associated with the development of competitive advantage through allelopathy (1, 2). Juglone, a highly phytotoxic naphthoquinone produced by black walnut (Juglans nigra L.), and sorgoleone, a substituted quinone from sorghum (Sorghum spp.), are two classic examples of potently active allelochemicals deposited via the plant's living root system ( Fig. 1) (3). Elucidation of the structures and mode of action of previously unknown root-derived phytotoxins could lead to new biorational approaches to weed control.Because of their stress tolerance and disease resistance, fescue (Festuca spp.) grasses are commonly used in landscape, roadside, and pasture settings, as well as for conservation purposes (4, 5). The unusual ability of many fine leaf fescue species to outcompete other plants is well known, and previous investigations suggested that fescue root exudates have phytotoxic properties (4). Here, we report the isolation, identification, and biological activity of m-tyrosine, a potent, structurally unusual broadspectrum phytotoxin exuded by the roots of some fine leaf fescue grasses. Results and DiscussionIn an initial field evaluation of 80 fine fescue cultivars, 8 cultivars with strong weed suppressive potential were identified and their allelopathic potential in laboratory settings was confirmed (4). Based on both field and laboratory results, we selected ''Intrigue,'' a common Chewing's fescue cultivar (Festuca rubra L. ssp. commutata), for further studies.To identify the allelopathic compound(s) contained in Intrigue root exudates, we developed an activity-guided separation scheme based on the inhibition of lettuce (Lactuca sativa L.) radicle elongation in a filter paper-based assay. By using this assay, we compared the phytotoxicity of root surface washes (hexanes, dichloromethane, methanol, and water) prepared from 2-w...

A series of field studies were conducted from 1999 to 2005 in Ithaca, NY, at the Cornell Turfgrass Research Center as part of the National Turfgrass Evaluation Program (NTEP) to evaluate a collection of 78 fine-leaf fescue cultivars (Festuca spp.) for turfgrass quality, seedling vigor, and ability to inhibit the establishment of common annual and perennial weeds. Using these criteria, we evaluated the overall suitability of the cultivars for use in turfgrass settings, as well as their potential weed suppressive or allelopathic ability. The ability of fine-leaf fescue to displace weeds was visually evaluated by density-wise comparison, and several cultivars of the 78 studied consistently established well and provided good to very good suppression (greater than 70%) of common turf weeds when established at the same planting density. Other cultivars provided moderate (between 35% and 70%) to (< 30%) little weed suppression. Greater weed suppressivity is likely associated with the differential ability of fescue cultivars to establish rapidly and to form a dense canopy, as well as potential allelopathic interference. This study was conducted in conjunction with laboratory experiments that revealed that certain fine-leaf fescue cultivars produced phytotoxic root exudates that were released into the rhizosphere over time. Additional field studies conducted in Ithaca showed that cultivars Intrigue, Columbra, and Sandpiper were consistently more weed suppressive than the other fine-leaf fescues evaluated. Although our understanding of the dynamics of production and degradation of fine-leaf fescue root exudates in the rhizosphere is limited, recent field studies also suggest that allelopathic interference as well as the ability to rapidly establish influence subsequent weed infestation in fine-leaf fescue stands. From a more practical standpoint, certain fine-leaf fescue cultivars, including Intrigue, Columbra, Sandpiper, and Reliant II, could be recommended for use in low-maintenance turf settings in the northeastern United States due to their aesthetic appeal and their limited weed infestation in circumstances where herbicides are not applied.

The significance of soil-allelochemical interactions was addressed in this paper through studies conducted with m-tyrosine, an amino acid analogue and a potent plant growth inhibitor, in a series of laboratory assays performed in field soil or growth media. The studies were performed as a basis for further evaluation of m-tyrosine activity in field soils containing living plant roots. Here, we examined the role of common soil amendments, including ammonium nitrate fertilizer and activated carbon, in overcoming plant growth inhibition in soils in a laboratory setting by using lettuce as a sensitive indicator of plant toxicity. The phytotoxicity of m-tyrosine was not influenced significantly by soil N amendment; however, when significant amounts of activated carbon were added to the soil medium, growth inhibition in treated lettuce seedlings was strongly reduced. Soil texture did not influence the bioavailability or activity of m-tyrosine, as activity in high organic growth media was similar to that of sand and soil mixtures. Similar to other purported allelochemicals, soil persistence of m-tyrosine was limited, with a predicted half life of less than 1 day in soil in a controlled laboratory setting. Rapid degradation of this molecule likely was due to microbial activity but degradation did not appear to be influenced significantly by soil N amendment. Given the observed activity of m-tyrosine in soil and growth media on seedling growth, potential may exist for development of m-tyrosine as a soil applied herbicide if formulations can be stabilized under soil conditions.

Laboratory screening studies were conducted to evaluate the allelopathic potential of fine leaf fescues. Of the seven accessions selected from prior field evaluations for weed-suppressive ability, all inhibited root growth of large crabgrass and curly cress in laboratory assays. Grown in agar as a growth medium and in the presence of living fescue seedlings for 14 or 21 days, test species were sensitive depending on the fescue cultivars. Growth inhibition increased when fescue was grown for increasing periods of time in agar. Seedling fescues produced significant quantities of bioactive root exudates, which were released into the agar medium. Bioactive root exudates were extracted from living fescue roots by using methylene chloride. Shoot tissue was extracted in water and the aqueous extract was partitioned against hexane, ethyl acetate, and methylene chloride. Extracts were tested for inhibitory activity on seedling growth as measured by inhibition of curly cress germination and radicle elongation. Root exudates were more toxic (70% inhibition) than shoot extracts (up 40% inhibition), when formulated at 0.25 mg/ml concentration. Light microscopy and transmission electron microscopy were utilized in an attempt to identify the cellular location of production of secondary products contained in bioactive root exudates. Ultrastructural analysis indicated that the exudate is produced in actively dividing tips of fibrous root cells. The mode of release of these exudates into the environment remains unknown.