Photosystem II (PSII) plays an especially important role in the photosynthetic response of higher plants to environmental perturbations and stresses. In this study, a pot experiment was conducted to investigate the differences in the photochemical efficiency and activity of PSII between Bothriochloa ischaemum and Lespedeza davurica in mixtures under three soil water regimes [80 ± 5 % FC (field capacity) (HW), 60 ± 5 % FC (MW) and 40 ± 5 % FC (LW)]. The maximum PSII quantum yield (Fv/Fm), non-photochemical quenching (NPQ), photochemical quenching (qP) and rapid light curve-derived parameters (rETRmax, maximum relative electron transport rate; Ik, minimum saturating irradiance; α, initial slope of the curve) of each species were investigated during the heading period (HP), flowering period (FP) and mature period (MP). The results showed that under HW and MW regimes, the averaged Fv/Fm values of B. ischaemum in mixtures at the HP were significantly higher than in monoculture, and the mean rETRmax values of B. ischaemum during the HP and FP in mixtures were significantly higher than in monoculture. Fv/ Fm values of B. ischaemum in the HP were significantly lower than in the other two growth periods under LW regime. During the MP, the averaged Fv/Fm values of L. davurica in mixtures were significantly higher than in monoculture under the HW regime, and the mean rETRmax values of L. davurica in mixtures were significantly higher than in monoculture under each water regime. In the same mixture ratio, NPQ values of B. ischaemum were significantly higher than those of L. davurica, but the rETRmax and Ik values of B. ischaemum were significantly lower than those of L. davurica under each water regime. The results indicated that application of mixture planting enhanced the photosynthetic performance of both species depending on the developmental stage of the individual plant. B. ischaemum showed the maximal photosynthetic performance in the HP and FP while L. davurica in the MP under both sufficient water supply and water stress conditions.
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