| The plants of cut flower chrysanthemum cv.'Shenma'plants were selected as material to be treated with the combination of lower temperature and poor light (16℃/12℃, PFD100μmol·m-2·s-1), critical temperature and poor light (12℃/8℃, PFD60μmol·m-2·s-1) stress for 11 days and then transferred to normal conditions of CK (22℃/ 18℃, PFD450μmol·m-2·s-1) for 11 days, to study the mechanism of responsing to stress of low temperature and poor light in chrysanthemum. And the effects of acetylsalicylic acid (ASA) and Ca2+ treatments on its photosynthetic system and antioxidant enzyme activities under the stress of low temperature and poor light were investigated, and the possible physiological mechanisms of ASA and Ca2+ was discussed. The results showed as follows:1. Studies on Mechanism of responsing to stress of low temperature and poor light in chrysanthemum(1) The stress of lower temperature and poor light induced excessive vegetative growth but the stress of critical temperature and poor light restrained the growth of plant. The growth of shoots of chrysanthemum well under the stress conditions of lower temperature and poor light, but its were restrained under the stress conditions of critical temperature and poor light. The two treatments restrained flower bud differentiation of chrysanthemum, which was more significant under the stress of critical temperature and poor light compared to the stress of lower temperature and poor light.(2) The results showed that net photosynthetic rate (Pn) and stomatal limitation value (Ls) decreased, however intercellular CO2 concentration (Ci) increased.The cause of reduction of Pn in chrysanthemum leaves under the stress of low temperature and poor light might be mainly non-stomata limitation.The stress of combination of poor light and lower temperature did not affect maximal photochemical efficiency of PSⅡin the dark (Fv/ Fm) and initial fluorescence (Fo), and led to a little decrease during the early period of stress and recover slowly during the later period of stress in maximal photochemical efficiency of PSⅡin the light (Fv′/Fm′); But Fo and Fv/Fm increased as well as Fv′/Fm′decreased evidently under the stress of critical temperature and poor light;ΦPSⅡ, qP and Fv'/Fm' decreased with stress degree increasing and time prolonging of low temperature and poor light, these parameters rapidly arose to a value similar to control for the treatment of poor light and low temperature, but recovered slowly for the stress of critical temperature and poor light after being released from the stress; At the same time, Prate decreased, but Drate and Ex increased under the low temperature and poor light. Drate was the main pathway of superfluous light allocation.Chla content, Chlb content, Chl (a+b) content, Chl (a/b) and Car content all reduced after treatment for 11 days. Decrease of contents of chlorophyll and carotenoid under the stress of low temperature and poor light maybe is one mainly cause resulting in the reducing of Pn.All in all, the changes of the above parameters under the stress of critical temperature and poor light were more significant than those under the stress of lower temperature and poor light.(3) SOD activities continually increased during the stress for days under stress of lower temperature and poor light, but SOD activities increased in the earlyer stages (1～5d) and decreased in the later stages (5～11d). Membrane permeabilities,POD activities increased, while CAT activities decreased and membrane lipid peroxidization pricked up, following MDA accumulation and cell leakage increasing in chrysanthemum leaves with stress degree increasing and time prolonging. The stress of lower temperature and poor light had opposite effects than the stress of critical temperature and poor light. Each index had reversed by different degree.(4) Contents of soluble sugar appeared durative increase with stress degree increasing and time prolonging. Soluble protein and proline increased during the stress for days under stress of lower temperature and poor light, which increased in the earlyer stages (1～5d) and decreased in the later stages (5～11d). Each index had reversed by different degree.2. Studies on mechanism of alleviatory effect of ASA and Ca2+ under stress of low temperature and poor light in chrysanthemum Under the stress of low temperature and poor light, combination application of ASA and CaCl2 addition greatly inhibited the netphotosynthesis rate (Pn) , carboxylation efficiency (CE), apparent quantum yield (AQY), maximal photochemical efficiency of PSⅡin the dark (Fv/Fm), quantum yield of PSⅡe lectron transport (ΦPSⅡ) and photochemical quenching (qP) compared with those of controls, single application of ASA or CaCl2. The combination application of ASA and CaCl2 obviously improved the chorophyll content, relatively improved the number and size of chloroplasts and starch grainins in the leaves of chrysanthemum under the stress of low temperature and poor light. The results suggested that ASA and CaCl2 could regulate the leaf photosynthetic functions in the leaves of chrysanthemum, and enhance the resistance against to the low temperature and light intensity. ASA and CaCl2 could reduce relative conductance rate and injury rate of leaves of chrysanthemum and increase the activities of SOD, POD and CAT and the contents of soluble protein compared with those of the controls, indicating that ASA and CaCl2 had positive regulation effects on the defense enzyme activities in chrysanthemum leaves, which could protect the photosynthetic system of the leaves of chrysanthemum under the stress of low temperature and poor light to a certain degree. Induced resistance effects of ASA and CaCl2 mixture was better than single application of ASA or CaCl2.
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