The interacting effect of urea and fenoxaprop-P-ethyl on photosynthesis and chlorophyll fluorescence in Perilla frutescens

We studied the effect of herbicide and nitrogen supply on photosynthesis in Perilla frutescens L. Britt. Plants were exposed to combined treatment of urea and herbicide, fenoxaprop-P-ethyl (FPE), in various concentrations. FPE reduced significantly chlorophyll (Chl) content, photosynthetic rate, and stomatal conductance, but increased significantly intercellular CO₂ concentration; thus, FPE inhibited significantly the photosynthetic capacity. In addition, FPE also decreased significantly the PSII photochemical efficiency, effective quantum yield of photochemical energy conversion in PSII, PSII potential activity, and photochemical quenching of variable Chl fluorescence. It also decreased nonphotochemical quenching. It indicated that FPE impaired PSII and blocked the electron transport in light reaction. The urea treatment at moderate concentration (1–4 g L^?1) could antagonize the negative effect of FPE, while the high urea concentration (8 g L^?1) aggravated this effect. The treatment with urea (4 g L^?1) and then with FPE (1.33 mL L^?1) enhanced Chl content index, photosynthetic rate, and stomatal conductance by 12.5, 36.1, and 28.5% compared to FPE treatment alone. Thus, we suggested to treat plants first with urea (4 g L^?1) and then by FPE (1.33 mL L^?1) as the best and the safest method to balance the fertilization and weeding.