Abstract: This paper studied the effects of foliar spraying different concentration La(NO₃)₃ on the photosynthesis, chlorophyll fluorescence parameters, Mehler reaction, and xanthophyll cycle of ryegrass seedlings under the stress of 150 mmol NaHCO₃·L^-1. Foliar spraying low concentration (0.05 mmol·L^-1) La(NO₃)₃ could significantly decrease the decrement of net photosynthesis rate (P_n), stomatal conductance (G_s), transpiration rate (T_r), and stomatal limited value (L_s) and the increment of intercellular CO₂ concentration (C_i) under NaHCO₃ stress, efficiently alleviate the inhibitory effects of NaHCO₃ stress on PSⅡ photochemical quenching (q_P), actual photochemical efficiency (Φ_PSⅡ), photosynthetic carbon assimilation dependent electron transport rate (ETR_p), and Mehler reaction- dependent electron transport rate (ETR_m), enhance the activities of superoxide dismutase, peroxidase, and ascorbate peroxidase, the non-photochemical energy dissipation (NPQ), the xanthophyll cycle pool (V+A+Z), and the de-epoxidation extent of xanthophyll cycle(A+Z)/(V+A+Z),and thereby, alleviate the damage of photosynthetic apparatus caused by NaHCO₃ stress. However, treating with
high concentration (0.5 mmol·L^-1) La(NO₃)₃ had no obvious alleviation effects. It was suggested that foliar spraying an appropriate concentration La(NO₃)₃ could not only alleviate the decrease of ryegrass seedling’s photosynthetic rate induced by non stomata factors and the inhibition of photochemical efficiency, but also accelerate the Mehler reaction under NaHCO₃ stress. With the accelerated Mehler reaction, excessive excitation energy could directly be consumed, and the xanthophyll cycle- dependent thermal dissipation could be promoted to efficiently protect the photosynthetic apparatus against photo-damage under NaHCO₃ stress. Also, the active oxygen produced by the accelerated Mehler reaction could be scavenged by the enhanced anti-oxidative enzyme activities.

Key words: La(NO₃)₃, NaHCO₃ stress, ryegrass, photosynthesis, chlorophyll fluorescence, Mehler-reaction, xanthophyll cycle