| Summary: | Allelopathy, secondary metabolite–mediated plant-to-plant interaction, is gaining application in current agricultural science as well as in invasion ecology. The present study addresses the major knowledge gaps in this field by investigating (i) how the bioavailability of allelochemicals is altered when they are present in a mixture in the soil matrix and (ii) what are the cues and consequences of allelochemical production in nutrient acquisition by the plant. ^ The role of preferential sorption to soil in altering the chemical composition of plant exudates was studied in a silt loam soil using representative mixtures of plant phenolic acids, namely, hydroxybenzoic acid, vanillic acid, coumaric acid, and ferulic acid. The concentration-dependent sorption coefficient ( Kd) of hydroxybenzoic acid was decreased more by than 90% in the presence of coumaric acid. About 95% of sorbed vanillic acid was displaced into the soil solution in the presence of ferulic acid. Soil organic matter was associated with preferential sorption. The results demonstrate that preferential sorption of phenolic acids to soil can alter the availability of plant exudates in mixtures and thus may mediate their phytotoxic effects. ^ To understand the dynamics of allelochemical mixture in soil matrix, using Centaurea maculosa Lam.as a model source, we investigated how the bioavailability of complex allelochemical mixtures is modified in a soil-microbial system. C. maculosa litter decomposition experiment revealed the existence of allelochemical in complex mixture in soil matrix. We observed the prolonged persistence of allelochemicals in soil matrix when present in complex mixtures. Also, allelochemicals exhibited dynamic nature undergoing simultaneous degradation as well as synthesis. ^ We investigated allelopathy as a corollary effect of resource acquisition mechanism using Centaurea diffusa Lam. as a model system. The exudation of 8-hydroxyquinoline (8HQ) by C. diffusa was not correlated with any of the tested nutrient stresses; however the 8HQ production remained steady under prolonged Fe stress, indicating this as a specific plant response to Fe stress. 8HQ showed high specificity in extracting Fe from both invaded and non-invaded soils. C. diffusa was able to uptake Fe from Fe(OH)3, but addition of carbon resulted in Fe deficiency, indicating a direct role of 8HQ in Fe uptake by this plant. Related (C. maculosa, C. solistitialis) as well as non related species (Zea mays ) of C. diffusa were able to uptake Fe from Fe-hydroxyquinoline complex (FeQ). Combining the above with the root reductase activity of the species, we propose that FeQ because of its hydrophobic nature, could passively diffuse across plasma membrane. In bioassay studies the addition of Fe decreases the toxicity of 8HQ, suggesting the phytotoxic action of 8HQ is via chelation of cellular Fe. ^ In conclusion, the present study revealed the competitive sorption phenomenon of plant secondary metabolites in soil matrix, as well as their dynamic nature. Combining the results from these studies that, 8HQ not only mobilizes the nutrients from soil, but also these nutrient complexes can be taken up by plants, we report an additional role of 8HQ in increasing the competitive ability of this weed - by facilitating the resource acquisition. Also, we report for the first time, the possible existence of a phytosiderophore-like mechanism for Fe acquisition in non-graminaceous species. ^
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