Atrazine [6-chloro-N-ethyl-N'-(1-methylethyl)-1,3,5-triazine-2,4-diamine] inhibits photosystem II (PSII) and is commonly used to control weeds in maize. It has been found that addition of sodium salicylate (sodium 2-hydroxybenzoate; NaSA) increased the postemergence herbicidal activity of atrazine against dicotyledonous weeds. NaSA also potentiated the activity of bentazon, another PSII-inhibiting herbicide. NaSA increased atrazine activity when applied either as a tank mix or up to 96 h prior to atrazine application. Other salicylates and the plant disease resistance inducers acibenzolar-S-methyl [benzo-(1,2,3)-thiadiazole-7-carbothioic acid S-methyl ester] and 2,6-dichloroisonicotinic acid also increased atrazine activity. Among the compounds tested, 3-chloro-5-fluorosalicylate, 4-chlorosalicylate, or 2,6-dichloroisonicotinic acid combined with atrazine yielded the greatest increase in herbicidal activity. Potentiation of atrazine by NaSA was greater at higher temperatures (35 and 25 > 15 degrees C). Also, greater potentiation was observed as the light level decreased. In darkness, NaSA alone or in combination with atrazine caused plant death, whereas atrazine alone had little effect. NaSA increased atrazine activity on npr1-2, an Arabidopsis mutant compromised in SA-induced disease resistance. Atrazine activity was also potentiated by NaSA on the ethylene insensitive mutant ein2-1. This indicates that atrazine potentiation is independent of either salicylate-induced disease resistance or ethylene perception.