| In flue-cured tobacco production in Heilongjiang province, the seedlings often were cultivated in greenhouse or plastic film covered shed, and then transplanted to the field. Photosynthetic adaptations in leaves of flue-cured tobacco seedlings following transfer of seedlings grown in greenhouse under high temperature and low light suddenly to low temperature and high light field environment must be changed. In this paper, taking the seedlings of flue-cured tobacco (Nicotiana tabacum L.) variety "Longjiang 911" as experimental materials, using gas exchange and chlorophyll fluorescence technology, photosynthetic and fluorescence characteristics were studies during low-temperature acclimation, ligh-light acclimation, and ABA and CaCl2 treatment in order to enhance the survival rate and reduce the transplanting seedling time. Results showed as follows:1. The changes of net photosynthetic rate (Pn) in leaves of flue-cured tobacco seedlings to different time for low temperature and high light instensity acclimation were different. The leaf net photosynthetic rate (Pn) for 1 h,2 h and 4 h acclimated was decreased significantly with stomatal conductance (Gs) in the initial stages of low temperature and high light, it was showed the stomatal limitation to Pn. Then Pn in leave of flue-cured tobacco began to rise with the acclimated time, but leave for 8 h acclimated maintained higher Pn and Fv/Fm were no significant difference with the nonacclimated, it indicated that the suitable cold and high light instensity acclimation would not cause photoinhibition of PSⅡreaction centre. The active photochemical efficiency (ΦPSⅡ) of seedlings to different time were reduced at different amplitude with the acclimated time go. The original chlorophyll fluorescence (Fo) was showed a increased tendency by 2 h and 4 h acclimation.For 8 h acclimated first was increased, then decreased and was no significant difference with the control. For 1 h acclimated Fo maintain nearly the same compared with the nonacclimated. The results show that:the suitable cold and high light instensity acclimation can significantly enhance the photosynthetic capacity of seedlings to improve the cold resistance and the capacity to resist photoinhibition,and would not damage and destruct PSⅡreaction centre.1 h and 8 h were the optimal acclimated time.2. Under cold and high light instensity, apparent quantum yield (AQY), RuBP carboxylic efficiency (CE) and photosynthetic rate at saturation for intercellular CO2 concentration (CSP) in leaves of flue-cured tobacco seedlings were decreased to reducing carbon assimilation. Exogenous CaCl2 could effectively alleviate the decline of stomatal conductance (Gs) induced by low temperature and high light, but it reduced the stomatal resistance to maintain a higher intercellular CO2 concentration (Ci). Exogenous ABA could promot stomatal closure to prevent leaves from water loss and increase leaf water potential of tobacco seedlings. Exogenous ABA and CaCl2 both maintained a higher original photochemical efficiency (Fv/Fm) and PSⅡactivity. It showed that the appropriate concentration of exogenous CaCl2 and ABA application to flue-cured tobacco seedlings enhanced plant cold resistance and the capacity to resist photoinhibition to improve the photosynthetic capacity.3. The result indicated that effects of ABA on tobacco seedlings photosynthesis had theconcentration effect, environmental effect and time effect. At the normal temperature, ABA could induce stomatal closure to reduce carbon assimilation, cause the inhibition of photosynthetic apparatus and decrease PSⅡactivity to inhibit photosynthesis in leaves of tobacco.Under cold and high light instensity stress, ABA could alleviate effectively the decline ofΦPSⅡand Fv/Fm in leaves of seedlings, and the rise of Fo to enhance PSⅡactivity. Pn in leaves of seedlings treated with 1μmol·L-1 ABA was significantly higher than that of control, Pn in leaves of seedlings with 10μmol·L-1 ABA was lower than that of the control, but the difference between 1μmol·L-1 ABA and 10μmol·L-1 ABA treatments was not significant, and Pn treated with 100μmol·L-1 ABA was significantly lower than that of control. It indicated that ABA regulated and improved the physiological functions in seedlings, and enhances resistant capacity to photoinhibition under cold and high light stress. But higher concentration in ABA did not play a role. Pn treated with 100μmol·L-1 ABA was first increased, and then decreased. There were two possibilities, one is the high concentration of ABA reducing activity of CO2 assimilation enzyme and inhibiting carbon assimilation, thus reducing the photosynth-etic rate under stress. Then due to ABA solution was gradually diluted as time, the inhibition was reduced and photosynthetic capacity was gradually improved. Another may be the high concentration of ABA has a lagging process on defense to cold and high light instensity by itself.4. Under the normal temperature, spraying CaCl2 had no significant influence on PSⅡreaction centre and Pn,but could improve Gs. Under cold and high light instensity, exogenous CaCl2 could largely increase Gs,Ci,Pn, Fv/Fm andΦPSⅡ, and inhibit the rise of Fo, enhance cold resistance and the capacity to resist photoinhibition.5. There was cross adaptation to drought and chilling in tobacco seedlings. Although chilling could not increase the photosynthetic rate of seedlings under drought stress, not damage the photosynthetic systems and membrane lipids, but chilling could increase the activities of antioxidative enzyme SOD, APX, CAT, and the content of soluble sugar and soluble protein, reduce the damage of membrane lipid by reactive oxygen species, to enhance the drought resistance of tobacco seedling.
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