Foliar spraying low concentration La(NO3)3 on ryegrass seedlings under NaHCO3 stress helps protect their photosynthetic system. It reduces the negative effects of stress on photosynthesis and enhances antioxidant enzyme activities. This treatment improves the efficiency of photosynthesis and helps the plant cope with stress by promoting the Mehler reaction and xanthophyll cycle. However, using a high concentration of La(NO3)3 does not have the same positive effects. Overall, applying the right amount of La(NO3)3 can help ryegrass seedlings maintain their photosynthetic rate and protect them from damage caused by stress.