An apparatus is described for studying the interaction between soil fungi and the growth of higher plants in the presence of specific phytotoxins. At concentrations above 1 mm, caffeic and vanillic acids inhibited the growth of Pisum sativum cultured in nutrient solutions under axenic conditions. Soil fungi capable of using phenolic acids as energy sources were isolated from soil on kaolin aggregates into which a phenolic acid had been incorporated. Some of the fungi (Volutella ciliata, Gliocladium roseum and a Penicillium sp.) were isolated on to agar plates and grown in nutrient solutions containing a specific phenolic acid. One of the fungal isolates, V. ciliata, was compared with an XAD-4 resin, for effectiveness in reducing a phytotoxic concentration of vanillic acid towards the growth of pea seedlings. Reducing the concentration of vanillic acid from 1 to 0.2 mm enhanced the growth of the main root and increased the number of laterals so that the root system resembled that of control plants without the phenolic acid. The precise effect depended on the age of the plant when the phytotoxic concentration of the vanillic acid was reduced. At a 1 mM concentration of NO3-N (10% of concentration in the usual nutrient solution), concentrations as low as 10 μm vanillic acid were phytotoxic towards the growth of pea seedlings, and this effect was also ameliorated by V. ciliata. The V. ciliata produced no phytotoxins per se. Reducing phytotoxic concentrations of vanillic acid during the first 3 days of culture was more effective than subsequent reduction, and after 8 days the plants did not recover subsequently.
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