| In the present studies, Seedlings of a white cut chrysanthemum cultivar'Jinba'(Dendranthema grandiflora'Jinba') were treated by extreme high temperature (40/35℃) or sub-high temperature (33/28℃) for different time respectively to study effects of high temperature stress on both operation of photosynthesis organization and physiological response in chrysanthemum plants. Net photosynthesis rate, gas exchange, chlorophyll fluorescence and the response of active oxygen scavenging enzyme system to high temperature were detect as the main indexes. Through the study on recovery of chrysanthemum seedlings after different menace dosage, the possible response mechanism of high temperature on photosynthesis and activities of light system were discussed. In addition, the effects of exogenous Ca2+ on photosynthetic organization and active oxygen scavenging enzyme system as well as relation with high temperature in chrysanthemum plants were explored in the aim of providing alternative method for chrysanthemum growers. The main results of this study are as follows:1. The net photosynthesis rate (Pn) of chrysanthemum decreased gradually treated by 33/28℃, stomatal conductance (Gs) decreased evidently at 5th day; Pn and Gs decreased dramatically in plants treated by 40/35℃. Rise of intercellular CO2 concentration (Ci) at early stage under the given high temperatures showed that the inhibition of photosynthesis by high temperature stress was resulted from nonstomatal limitations, 9 days later, stomatal limitation mainly induced the decrease of Pn.2. With the prolonging of the high temperature stress, light saturation point (LSP ) , max Pn (Pm), apparent quantum yield (AQY) and corboxylation efficiency (CE) evidently all decreased; meanwhile, both CO2 compensation point (CCP) and light compensation point (LCP) increased. The result showed that the capabilities of chrysanthemum seedling to use weak light and high intensive light were lower than the control. The activity of Rubisco and the capability of using lower concentration of CO2 were lower too. Decreased of contents of chlorophyll and carotenoid under high temperature stress maybe is one mainly cause resulting in the reducing of Pn. Middle leaf is revolute and lowest leaves drooped under lower high temperature dosage; middle leaves drooped with yellow spot and lowest leaves perished under high stress dosage.3. The intrinsic photochemical efficiency (Fv/Fm), quantum yield of PSⅡ(ΦPSⅡ),the efficiency of excitation energy capture by open PSⅡreaction center(Fv'/Fm')of plants that were treated by 33/28℃and 40/35℃all decreased with antenna heat dissipation increasing,showed that react center were protected by decreased light capture and efficiency of electron transfer through PSⅡ. Photochemical quenching (qP) of plants under 33/28℃descended first and then turned to rise, suggested that the electron transfer was firstly restrained by the stress; contrastively, qP rise continuously under 40/35℃, indicating oxygen-evolving complex (OEC) was the most sensitive location to extreme high temperature in chrysanthemum photosynthesis apparatus.4. All the antioxidation enzymes of SOD, POD, CAT, APX was activised by Short-term 33/28℃, SOD and APX raised slightly under 40/35℃, with the durative reduce of CAT and POD;antioxidation matter AsA and GSH are both sensitive to high temperature stress,appeared durative reduce under given two high temperature intensity. With the prolonging of stress, membrane lipid peroxidization prick up, following MDA accumulation and cell leakage increasing.5. Pn,Fv/Fm,ΦPSⅡof chrysanthemum leaves treated by 33/28℃within 7 d could recovered after transfer to 23/18℃; the above parameters could recovered to 70% of control treated beyond 7 d; contrastly, the photosynthesis of chrysanthemum leaves treated by 40/35℃beyond 5 d can not recovery , that treated beyond 9 d even keep on reducing transfer to control condition, light react center is permanent injured.6. Additions of exogenous Ca2+ significantly enhanced the contents of chlorophyll a, chlorophyll b and carotenoid of chrysanthemum leaves under high temperature stress. All of net photosynthetic rate (Pn), photochemical efficiency of PSⅡ(Fv/Fm) and quantum yield of PSⅡelectron transport (ΦPSⅡ) increased, whereas initial fluorescence (Fo) decreased. Ca2+ treatment significantly enhanced the activities of SOD, POD and CAT. In conclusion, addition of Ca2+ efficiently protected chrysanthemum leaves against photosynthetic organization damage under high temperature stress conditions, and the active oxygen was scavenged by the enhanced antioxidant enzymes, which hence significantly alleviated the membrane lipid peroxidization and MDA accumulation.
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