Abstract: Effects of exogenous salicylic acid ( SA) and oxalate (OA) on photosynthesis, chlorophyll fluorescence and xanthophyll cycle in cucumber leaves under photooxidative stress induced bymethyl viologen (MV) were studied. The cucumber leaveswere sp rayed with SA 0.2 mmol·L ^{- 1} and OA 5 mmol·L ^{- 1} solution for 2 days before photooxidative stress by smearing MV 500 μmol·L^-1 on leaf surface. The results showed that both SA and OA pretreatment could inhibit the decreases in photosynthetic rate ( Pn) , transp iration rate (Tr) , quantum yield of PSⅡ (ΦPSⅡ) , chlorophyll photochemical quenching ( qP) and the increases in intercellular CO₂ concentration (Ci) , chlorophyll non-photochemical quenching (NPQ) and (A + Z) / (V +A + Z) ( the de-epoxidation extent of xanthophyll cycle) caused by photooxidative stress, but had little effect on stomatal conductance (Gs) and intrinsic photochemical efficiency of PSⅡ ( Fv/Fm). The decreases in xanthophyll cycle pool size, chlorophyll and carotenoid contents caused by photooxidative stress were also inhibited by SA and OA pretreatment. These results suggested that both SA and OA has the protective role against the damage of photosynthetic system during photooxidative stress.

Key words: Cucumber, Salicylic acid, Oxalate, Photooxidative stress, Chlorophyll fluorescence, Xanthophyll cycle