Abstract: In order to reveal the protective roles of endogenous salicylic acid（SA）on photosynthetic
systems in cucumber seedlings under low temperature stress，the seedlings were pretreated with 100
μmol · L^-1 Paclobutrazol（Pac，an inhibitor of SA biosynthesis）for 24 hours and then cultured at 10 ℃.
After 10 hours of temperature treatment，parts of materials were fed with exogenous SA（50 μmol · L^-1）.
The endogenous SA content，chlorophyll fluorescence parameters，and the relative expression of Rubisco
small subunit（RbcS）and Carbonic anhydrase（CA）genes were determined at different treatment times.
The results exhibited that low temperature stress could induce endogenous SA accumulation in cucumber
leaves，and this accumulation could be prevented by spraying with Pac. Low temperature resulted in a
reduction in maximum photochemical efficiency of PSⅡ（F v/F m），effective photochemical quantum yield
of PSⅡ（Φ PSII），potential activities of PSⅡ（F v/F o），and electron transport rate（ETR）. Pac spraying
caused a greater reduction in PSⅡefficiency. Low temperature stress led to a decreased allocation of light
absorbed by PSⅡantenna to the photochemical reaction and an increased allocation of excessive energy，
and Pac-treatment caused a much more allocation of light to dissipation as excessive energy. Also，the
expression of RbcS and CA genes was down-regulated by low temperature stress，and the reduction was
greater in Pac-treated seedlings. Furthermore，application of exogenous SA to Pac-treated seedlings
alleviated the reduction of photosynthetic efficiency and rescued the repressed gene expression of RbcS
and CA. All of these results suggested that low temperature-induced SA accumulation was required for
maintenance of photosynthetic efficiency and carbon assimilation capacity，and thereby protected
cucumber seedlings against low temperature-induced damages.

Key words: cucumber , salicylic acid , low temperature stress , photosystemⅡ （ PS Ⅱ ）, ribulose-1,5-bisphosphate carboxylase/oxygenase（Rubisco）