Phenolic acids in plant-soil interactions: a microcosm experiment

In soil naturally occurring phenolic acids act as allelochemicals and are involved in several plant-soil interactions. In the present study, a microcosmstyle experiment was used to investigate the allelopathic responses induced by individually applied trans-cinnamic and ferulic acids and coumarin on the root system of maize seedlings grown for 288 h (12 days) under controlled conditions on a sterile 70/30 (v/v) soil/perlite mixture. Preliminary soil incubation assays were carried out to assess the potential bioavailability of these compounds as affected by abiotic sorption/desorption processes. The concentration-dependent sorption coefficients (Freundlich) showed that phenolic acids were rapidly adsorbed onto the soil surfaces following two distinctive sorption mechanisms. Further, recoveries from a 96-h depletion experiment indicated that the bioavailable amount of trans-cinnamic and ferulic acids markedly (-97 and -94%, respectively) and continuously decreased over time; whereas available coumarin strongly decreased (-92%) soon after addition remaining unchanged over time. According to these findings, single (coumarin) and pulsed (trans-cinnamic and ferulic acids) applications were established during the microcosm experiment to provide resident soil concentration at 0.02-0.15 μmol g-1 still able to induce plant responses. Total root length, biomass allocation, root fineness and tissue density of the whole root system and different root types (radicle, seminal, nodal, lateral of the radicle and seminal roots) of maize seedlings were evaluated. Coumarin appeared to be the strongest phytotoxic compound reducing total root length and completely inhibiting the elongation of radicle root and the formation of the other root types. Trans-cinnamic acid exerted a marked and rapid action in reducing the total root length showing also a selective action towards the different root types more than ferulic acid. Further, trans-cinnamic acid affected both biomass allocation and structural parameters; whereas ferulic acid mainly influenced the root fineness. Results suggested: i) a specific response of maize root system in relation to the allelochemical and time of exposure; ii) maize root types showed a different response to the phenolic acids, and iii) their morphological parameters were selectively affected.