Phytotoxic effects of allelochemicals and herbicides on photosynthesis, growth and carbon isotope discrimination in Lactuca sativa

We compared the effects of two allelochemicals [ferulic (FA) and p-hydroxybenzoic acids (HBA)] and two herbicides [pendimethalin (PML) and S-metolachlor (MTR)] on physiological processes (growth, photosynthetic efficiency, yield, non-photochemical fluorescence quenching, leaf water relations, carbon isotope discrimination and leaf protein contents) in Lactuca sativa cv. Great Lakes. HBA and FA significantly reduced the shoot length, leaf, root fresh biomass and root length. There was no significant effect of both allelochemicals and herbicides on relative water content (RWC) and leaf osmotic potential (LOP) in lettuce. The allelochemical FA significantly reduced the quantum efficiency (Fv/Fm) of open photosystem II (PSII) reaction centers in dark-adapted leaves and this reduction was stronger than herbicides. Both allelochemicals significantly decreased the effective quantum yield (Φ PSII) of PSII, while herbicides did not effect the Φ PSII. Both allelochemicals also drastically decreased (3-folds less than control) the photochemical fluorescence quenching (qP) and non-photochemical fluorescence quenching (NPQ). In HBA treated plants, carbon isotope ratios (δ¹³C) were significantly less negative (−26.93) than control (−27.61). Lettuce plants treated with FA and MTR, δ¹³C was significantly less negative (−26.90) and (−26.88), respectively as compared to control (−27.61). Carbon isotope discrimination (Δ ¹³C) values were significantly less (19.40) after treatment with FA and HBA (19.25) at 1.5 mM concentration over the control (20.17). The ratios of intercellular CO₂ concentration from leaf to air (ci/ca) were significantly less after treatment with FA, HBA and MTR than control. The FA, HBA (1.5 mM), PML and MTR (10⁻¹ mM) significantly reduced the leaf protein contents of lettuce.