Effects of Salicylic Acid and Oxalate on Photosynthetic System and Xanthophyll Cycle in Cucumber Leaves under Photooxidative Stress Effects of Salicylic Acid and Oxalate on Photosynthetic System and Xanthophyll Cycle in Cucumber Leaves under Photooxidative Stress Comparison of stability of photosystem II between two Alternanthera invasive species with different invasion ability