Abstract: The pepper cultivation in greenhouse is affected by suboptimal temperature and light （temperature at about 18 ℃，light 200～400 umol·m^-2·s^-1），leading to decreased photosynthetic rate，delayed growing development or developmental disorder and reduced yield and quality，etc. In particular，it has become the main ecological factor of pepper cultivation production in north-west regions of China because of effects of continuous cloudy weather. It has been reported that anti-stress inducers may alleviate plant growing disorder，which include 5-aminolevulinicacid （ALA） and chitosan （CTS）. Although，the detrimental effects of suboptimal temperature and light on photoinhibition，physiological-biochemical characteristics and photosynthetic rate of pepper have been extensively studied，the effect of exogenous anti-stress inducers to alleviate suboptimal temperature and light damage is unclear，including inducers kinds，method of application，effectiveness. The effect of ALA and CTS was investigated on photosynthetic properties，chlorophyll florescence parameters，chlorophyll content and antioxidant enzyme of pepper in simulation environment of suboptimal temperature and light，to illustrate the mechanism of ALA and CTS improving plant endurance to suboptimal temperature and light. The treatment of 0.5 mg/L ALA and 0.25% CTS was performed on Longjiao NO.2 pepper for three replication cultured in simulation environment of suboptimal temperature and light （day/night 18 ℃/12 ℃，photon flux density 300 μmol·m^-2·s^-1） through phytotron with control of distilled water. The photosynthetic pigments，gas exchange parameters，chlorophyll fluorescence parameters and photosynthetic enzyme activities including superoxide dismutase （SOD），peroxidase （POD） and catalase （CAT） were determined during vegetation process of 8 d clarity their effects on increasing photosynthesis of pepper seedlings. It was found that net photosynthetic rate （Pn），stomatal conductance （Gs），transpiration rate （Tr），the actual photochemical efficiency （ΦPSII），light energy capture efficiency of PSII reaction center，photochemical quenching coefficient （qP），Fv/Fm，chlorophyll content and the activities of superoxide dismutase （SOD），peroxidase （POD） and catalase （CAT） of ALA，CTS and control groups in simulation environment of suboptimal temperature and light decreased with prolonged cultivation time in exception of increasing intercellular CO₂ concentration[（Ci）.] Compared to control group，after 6 d chlorophyll a （Chla） and b （Chlb） contents were higher in ALA group （1.57 and 1.98 times，respectively） and CTS grous （1.15 and 1.40 times，respectively）；and after 8 d Pn，SOD and POD were also higher in ALA group （1.69，2.45 and 1.68 times，respectively） and CTS groups （1.31，2.05 and 1.27 times，respectively）. The treatment of ALA and CTS could alleviate the decrease of Pn，Gs，Tr，ΦPSII，qP，Fv/Fm，SOD，POD and CAT of pepper in simulation environment of suboptimal temperature and light. It was also found that [Pn] has significant positive correlations with Gs，Tr，Chla and Chlb，and significant negative correlation with Ci in ALA and CTS groups，indicating that significant inhibition of photosynthetic capacity was due to non-stomatal limitation factors. The treatment of ALA and CTS at appropriate concentration could improve photosynthesis and increase activity of antioxidant enzymes of pepper under suboptimal temperature and light condition with increased chlorophyll contents. It is shown that exogenous ALA and CTS treatment would promoted metabolism of active oxygen in pepper seedlings mainly by alleviating chlorophyll loss and increasing SOD，CAT and POD activities to protect photosynthetic characteristics of cell and increase tolerance.

Key words: pepper, ALA, CTS, suboptimal temperature and light intensity, photosynthesis, antioxidant systems