Soybean seedlings were used as the test species to investigate the effect of phydroxybenzoic acid (PHBA) on growth and plant-water relationships. Plants were grown in nutrient solution under greenhouse conditions and were subjected to pHBA through amendments to the growth medium. Treatments were initiated 10 days after germination and continued for either 14 or 28 days with stomatal conductance, water potential, and water use monitored periodically. At harvest, effects on growth were determined and, in the 28-day study, the carbon isotope ratio (C-13 : C-12) of leaf tissue was analyzed as an indicator of integrated effects on plant-water status. Soybean growth was significantly reduced by 0.5 mM pHBA, or higher concentrations, with the degree of inhibition being concentration dependent. Plants treated with 0.75 mM pHBA had significantly lower stomatal conductance, lower water potential, and less discrimination against C-13. Similar trends were apparent in 0.5 mM pHBA-treated plants. Interference with plant-water balance appears to be one mechanism of action of pHBA causing a reduction in plant growth.

Allelopathic chemicals alter plant growth and development by a multiplicity of actions on physiological processes because there are hundreds of different structures and many of the compounds have several phytotoxic effects. Whole-plant bioassays and physiological tests designed to use a small quantity of compound are keys to strategies for elucidating mechanisms of action. DNA appears to be a primary molecular target for a number of alkaloids, but others do fit this mode. Two quinones established in allelopathy, juglone and sorgoleone, block PSII but a conclusion of primary-site activity is complicated since they also inhibit mitochondrial function and plasma membrane H+-ATPase activity. Terpenoid diversity precludes any common molecular targets and illustrates the importance of structural configuration to activity. The monoterpene 1,4-cineole is a potent inhibitory of asparagine synthetase and the sesquiterpene lactone artemisinin reduces mitotic activity, chlorophyll, and PSII efficiency. Coumarins and the cinnamic and benzoic acids exhibit seedling growth-inhibition thresholds between 0.1 and 1.0 mM. In this range they alter phytohormone activity, ion uptake, plant-water balance, respiratory and photosynthetic activity, and a variety of enzyme functions. Tissue of soybean inhibited by several phenolics has a higher fraction of C-13, indicating sustained interference with water relations. I suggest membrane perturbations are a common starting point for effects of the phenolic acids. The current evidence does not allow narrowing to a primary site of action for most plant allelochemicals. Future expansion of biotechnology and functional genomics will be utilized to enhance plants in the production of secondary metabolites and these tools should provide physiological information relevant to determining the mechanisms of action of many allelochemicals.

Allelopathic inhibition typically results from the combined action of a group of allelochemicals which, collectively, interfere with several physiological processes. The objectives of this paper are to summarize research that illustrates the joint action of allelochemicals, and to provide evidence that both the amount and detrimental action of these compounds depends on the extent of associated abiotic and biotic stresses. Allelopathy is strongly coupled with other stresses of the crop environment, including insects and disease, temperature extremes, nutrient and moisture variables, radiation, and herbicides. These stress conditions often enhance allelochemical production, thus increasing the potential for allelopathic interference. In the paradigm of interactions, the data indicate that crops are more sensitive to allelopathy when moisture, temperature, or nutrient conditions are less than optimal. For example, the inhibition threshold concentration for ferulic acid to affect seedling growth was reduced with even minor moisture stress (psi = -0.15 MPa) or a growth temperature at the higher end of the normal range for a species. Under greenhouse conditions, additive inhibition resulted from the joint action of ferulic acid with low levels of alachlor [2-chloro-N-(2,6-diethylphenyl)-N-(methoxymethyl)acetamide], atrazine [6-chloro-N-ethyl-N'-(1-methylethyl)-1,3,5-triazine-2,4-diamine], or trifluralin [2,6-dinitro-N,N-dipropyl-4-(trifluoromethyl)benzenamine]. Interactions of multiple stresses in crop environments will determine the relative impact of allelopathy. Allelopathy-stress interactions also have implications for herbicide and residue management strategies, crop rotations, biological control measures, and tillage practices that can contribute to a more sustainable agriculture.

The primary mode of action has not been established for any allelopathic compound, albeit some physiological actions are known. The array of compounds cuts across many chemical classes, and it is unlikely they have a common mechanism of action. Allelochemicals active against higher plants are typically characterized as suppressing seed germination, causing injury to root growth and other meristems, or inhibiting seedling growth. A primary action on ATP production is indicated for the two quinones, juglone and sorgoleone, since they inhibit chloroplast oxygen evolution (I-50 = 0.2 and 2.0 mu M respectively) and strongly affect mitochondrial functions. The chloroplast block by sorgoleone is in the photosystem II complex. Cinnamic and benzoic acid derivatives alter membrane potential and have several physiological effects that suggest membrane perturbations are their initial site of action. Their thresholds (100 to 1000 mu M) for inhibition of seedling growth, singly or in combinations, correlate with impairment of plant-water relationships. These phenolic compounds also alter mineral uptake, chlorophyll content, photosynthesis, carbon flow, and phytohormone activity. Phytotoxicity of many allelopathic chemicals may be from a generalized cellular disruption rather than a specific mechanism. A case study of Sorghum allelopathy suggests that inhibition of a. receiving species results from the joint action of a number of allelochemicals with different cellular sites of action.

The phenomenon of allelopathy encompasses all types of chemical interactions among plants and microorganisms. Several hundred different organic compounds (allelochemicals) released from plants and microbes are known to affect the growth or aspects of function of the receiving species. Many new allelochemicals have been identified in recent years and it has become clear that the actions of allelochemicals are important features characterizing the interrelationships among organisms. These compounds influence patterns in vegetational communities, plant succession, seed preservation, germination of fungal spores, the nitrogen cycle, mutualistic associations, crop productivity, and plant defense. Allelopathy is tightly coupled with competition for resources and stress from disease, temperature extremes, moisture deficit, and herbicides. Such stresses often increase allelochemical production and accentuate their action. Allelopathic inhibition typically results from a combination of allelochemicals which interfere with several physiological processes in the receiving plant or microorganism. Other than the autecological study of specific species, there are persistent challenges in allelopathy to determine the mechanism of action of compounds, isolate new compounds, evaluate environmental interactions, and understand activity in the soil. New frontiers will focus on ways to capitalize on allelopathy to enhance crap production and develop a more sustainable agriculture, including weed and pest control through crop rotations, residue management, and a variety of approaches in biocontrol. Other goals are to adapt allelochemicals as herbicides, pesticides, and growth stimulants, modify crop genomes to manipulate allelochemical production, and better elucidate chemical communications that generate associations between microorganisms and higher plants.

Bioassays with grain sorghum [Sorghum bicolor (L.) Moench.]seedlings grown under summer glasshouse conditions demonstrated that leaf diffusive resistance increased and water potential decreased following treatments with ferulic acid, p-coumaric acid, and extracts from several allelopathic weeds. During the week following treatment with 0.5 mM ferulic or p-coumaric acid, sorghum leaf resistances indicated that stomates were almost closed. Effects on water potential were found at a lower treatment level, with 0.25 mM ferulic or p-coumaric acid-treated sorghum having midday leaf water potentials of approximately -10 bars, compared to -5 bars for controls. Water potential changes resulted from reductions in both osmotic potential and turgor pressure. Aqueous extracts from Kochia scoparia, Helianthus tuberosus, and Xanthium pensylvanicum caused growth reductions in sorghum that correlated with high diffusive resistances and low water potentials, with these effects found using extracts from 1 g fresh-leaf material in 60 ml of nutrient medium. Growth of sorghum in soil containing dried residue from these weeds was also reduced. The data suggest one mechanism of allelopathic action is a disruption of plant water balance.

The allelopathic potential of Kochia scoparia (L.) Schrad., a common weed in cultivated fields, was demonstrated on grain sorghum and soybeans. Growth of sorghum seedlings was reduced when the nutrient medium contained an extract equivalent of 1 g fresh weight of Kochia in 60 mL of nutrient solution, or higher quantities. Soybean seedling growth was depressed by as little as 1 g of Kochia in 240 mL of nutrient solution. Treatments with Kochia extracts that reduced growth also caused seedlings to have either an increase in leaf diffusive resistance, a decrease in water potential, or both. The addition of dried Kochia to soil pots in which seedlings were germinated and grown showed that 0.5 g of debris per 80 g of soil resulted in a significant reduction in sorghum growth, and seedlings grown with 2.0-g additions also had an increase in leaf resistance and decrease in water potential. Effects on water metabolism are indicated as one mechanism of action of allelochemicals from Kochia. It is suggested that biochemical interference from Kochia weeds in sorghum and soybean fields can contribute to production losses.

Sampling in a soybean field established that presence of velvetleaf (A. theophrasti) weeds interfered with soybean production. Number of soybean pods and number of pods/stem were significantly lower in transect segments adjacent to velvetleaf plants. In bioassays for phytotoxicity of velvetleaf, several dilutions of aqueous extracts from fresh field‐collected leaves depressed germination of radish seeds and inhibited growth of soybean seedlings. Seed germination bioassays from eluates of chromatograms developed in one dimension showed that two of three bands containing phenolic compounds were inhibitory to radish seed germination. Soybeans inhibited by aqueous velvetleaf extracts had increased diffusive resistance, suggesting partial stomatal closure. Inhibited plants also gave evidence of water stress, with leaf water potentials often as low as –20 bars and reduced water content, when compared with controls. Quantification of chlorophyll on a leaf area basis showed that chlorophyll of inhibited plants was below controls. These data demonstrate the allelopathic potential of velvetleaf and suggest that interference with water balance and chlorophyll content may be two mechanisms of inhibitory action of toxins present in the leaves of velvetleaf.

Ferulic acid (FA), p-coumaric acid (pCA) and vanillic acid (VA) were tested to establish threshold levels required for inhibition of sorghum seed germination. An equimolar combination of 3.3 × 10−3 M of each of the 3 phenolics showed synergistic inhibition of germination. This combination of the three also depressed germination more than any combination of two of these phytotoxins.