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Barriers to continuous cropping of poplar plantations are closely related to the accumulation of phenolic acids in the soil. The purpose of this study was to explore the mechanism through which phenolic acid stress affects poplar productivity. The results showed that phenolic acids had a significant inhibitory effect on the photosynthesis of poplar. The inhibition of photosynthesis due to phenolic acids occurred mainly because, during electron transfer at the PSII reaction center, the primary quinone acceptor of PSII (QA) in the oxidized state was reduced, and excess light energy was lost in the form of heat dissipation; thus, poplar productivity decreased. At low phenolic acid concentrations (0X--1.5X), the activity of the PSII reaction center was temporarily inactivated, mainly because of stomatal limiting factors. At high phenolic acid concentrations (1.5X--3.0X), the PSII reaction centers were damaged, and photoinhibition occurred, mainly because of nonstomatal limiting factors. [ABSTRACT FROM AUTHOR]