The flower-inducing activities of benzoic and salicylic acids added to the medium differ with the species (Lemna paucicostata and L. minor), and even with the strains used. The type and pH of the medium used, full or 1/10 strength M medium at pH 3.8, 4.4 or 5.1, or 1/2 or 1/20 strength NH4+-free Hutner's medium at pH 5.0, 6.0 or 7.0, also modify their activity. L. paucicostata, strain 151 is the most sensitive of the strains used to both benzoic and salicylic acids followed by strain 381. Such dramatic flowering responses were not obtained with the other strains, but even strain 321, reportedly insensitive to benzoic acid, could be induced to flower by adding benzoic acid to a modification of the medium. Benzoic acid is more effective than salicylic acid for all strains of L. paucicostata, but the contrary is true for two L. minor strains tested. A higher percentage of flowering is obtained in L. paucicostata in 1/2 strength NH4+-free Huter'sn medium than in M medium, except for strain 151. When diluted, both media enhance flowering in all L. paucicostata strains. Generally, a lower concentration of benzoic acid or salicylic acid is enough to induce flowering when the pH of the medium is lower.
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Kaihara et al. (1981) studied this question.