Synergistic inhibitory effects of vanillic andp-hydroxybenzoic acids on radish and grain sorghum

Radish and grain sorghum germination and sorghum growth were inhibited in a synergistic manner by combinations of vanillic andp-hydroxybenzoic acids. At threshold inhibition levels, 2.5 × 10^–3 M vanillic acid-treated radish seeds had 71 % of control germination after 24 hr and 2.5 × 10^–3 Mp-hydroxybenzoic acid-treated radish yielded 95% germination. A mixture of 2.5 × 10^–3 M of each of these two phytotoxins showed 52% germination after 24 hr. Equimolar mixtures of 5 × 10^–3 M vanillic andp-hydroxybenzoic acids allowed sorghum germination of 60% of untreated seeds after 24 hr, whereas separate treatments of individual phenols had 93% and 96% of control seed germination. Sorghum root and shoot elongation and total seedling growth were more sensitive than germination to vanillic andp-hydroxybenzoic acid treatments, and synergistic effects also were apparent. A combination of 5 × 10^–3 M vanillic with 5 × 10^–3 Mp-hydroxybenzoic reduced root length more than either did individually, and a mixture of 5 × 10^–4 M vanillic with 5 × 10^–4 Mp-hydroxybenzoic acid reduced sorghum seedling growth to approximately that resulting from a 10^–3 M concentration of either phenol alone. Phytotoxin levels inhibitory to sorghum growth caused small increases in lower leaf surface diffusive resistance, but did not close stomates, and this effect was not judged to be the cause of reduced sorghum growth.