Chilling Tolerance Study In Kosteletzkya Virginica (L.) Presl

Seedlings of Kosteletzkya virginica （L.） were treated under low temperatures with differentconditions to study the changes of their photosynthetic characteristics. The study mainly dealedwith three aspects:（1） The critical temperature for chilling injury in Kosteletzkya virginica.（2）The recovery of Kosteletzkya virginica under light or darkness conditions after chilling stress.（3）The comparative effect on photosystem of low temperature with weak light and low temperaturewith strong light. The paper mainly researched the mechaniosm of photo-inhibition by lowtemperature. The results are as follows.1. The critical temperature of chilling injury in Kosteletzkya virginicaLow temperature treatment decreased the net photosynthetic rate （Pn） of Kosteletzkyavirginic. The stomatal conductance declined with the decline of temperature. Under lowtemperatures above of13℃, the decline ofPn was chiefly caused by stomatal limitation factors,at temperatures below10℃, the stomatal limitation value increased significantly; implyingnon-stomatal limitation factors such as the damage of photosynthetic apparatus or the mesophylllimitations decreased Pn in Kosteletzkya virginica.Under low temperature treatments, the actual photochemical efficiency （ΦPS Ⅱ） ofKosteletzkya virginica decreased significantly with the decrease of temperature, while theminimum fluorescence yield （Fo） increased gradually. Conclusions from the Arrhenius’s plotssuggested that10.6℃was thecritical temperature for Fo change.After chilling stress, ΦPo and ΦEo decreased as the temperature decreased, while ΦDo ofKosteletzkya virginica increased obviously, which indicated the increase of heat dissipation canhelp Kosteletzkya virginica gain resistance to low temperature damage. Different lowtemperature treatments decreased the density of active reaction centers （RC） per excitedcross-section （RC/CSo） of Kosteletzkya virginica. Under low temperature stress, the energyabsorbed and captured by per unit leaf area used in electron transfer （ETo/CSo） declined, whilethe energy absorbed and captured by per RC used in electron transfer （ETo/RC） increased. Atboth per unit leaf area and per RC bases, the heat dissipation increased under low temperaturetreatments.Low temperature stress also decreased PIabs significantly, especially at the temperature of7℃,4℃a nd1℃. The critical temperature for PIabs was11.2℃. Based on the results above, we can conclude that the critical temperature for lowtemperature tolerance of Kosteletzkya virginica was about10to11℃.2. The recovery of Kosteletzkya virginica after low temperature treatments.The recovery of net photosynthetic rate （Pn） after chilling injury was investigated, we foundthe Pn of Kosteletzkya virginica return to the pre-treatment level in48h when the treatingtemperature was13℃.Similar result was found at10℃treatment; however, when thetreatment temperature reached7℃, which is lower than critical temperature, restoration was notfound in48h.The recovery of actual photochemical efficiency （ΦPS Ⅱ） ofKosteletzkya virginica in48hwas similar to Pn. Under light condition, the ΦPSⅡ ofKosteletzkya virginica treated with13℃and10℃restored to normal in48h，while ΦPSⅡ treated with7℃can’t go back topre-treatment level in48h, no matter under light or dark conditions.The chilling treatment of13℃didn’t increase Fo. However,10℃treatment increased Fo,but it returned to normal in48hours. Furthermore, after7℃treatment,the Fo increasedsignificantly and could’t return to normal level even after48hours.The recovery of maximum photochemical efficiency in PSII （Fv/Fm） of Kosteletzkyavirginica followed the same trend. The Fv/Fm of Kosteletzkya virginica treated under13℃and10℃treatment all restotred to pre-treatment level in48hours, while that under7℃treatmentcan’t return to noraml no matter under light or dark conditions.All the above results proved that10℃was the critical temperature for chilling injury inKosteletzkya virginica.3. The photoinhibition under low temperature and the recovery under light and darkconditions in Kosteletzkya virginicaAfter treated by low temperature and dark conditions (6℃,0μmol·m^-2·s-1), there was nosignificant decline in PSII activity （Fv/Fm） and PSI oxidoreductive activity （△I/Io） inKosteletzkya virginica treatment, so it was with Fo.After4h treatment by low temperature with weak light (6℃,200μmol·m^-2·s-1), the Fv/Fmdecreased by2.5%, and the△I/Io decreased by18.5%.After4h treatment by low temperature with strong light (6℃,800μmol·m^-2·s-1), the Fv/Fmof Kosteletzkya virginica decreased by10.37%, while the△I/Io decreased by7.57%.The PSII activity （Fv/Fm） of Kosteletzkya virginica could fully recover in8h after lowtemperature with weak light (6℃、200μmol·m^-2·s-1) treatment, while the oxidoreductive activityof PSI （△I/Io） could not recover after48h. The recovery of PSI under dark was better thanunder light condition. The recovery of Pn in Kosteletzkya virginica was similar to that of PSI, which could not recover in48h, suggesting that after low temperature with weak light stress, therestriction factor for photosyntheis recovery mainly lied in PSI.During the recovery after low temperature with strong light stress, however, the Fv/Fm and△I/Io was in consistent with that of Pn, which all returned to normal level completely in48h.And the recovery was better in light than in adrkness. The result suggested that the recovery afterlow temperature with strong light stress was both resticted by PSII and PSI recovery.From the experiments we can conclude that （1） Kosteletzkya virginica was relativelysensitive to low temperature stress and its critical temperature for chilly injury was about10to11℃.（2） Low temperature with weak light stress mainly injured PSI, while PSII was relativelystable.（3） The injury to PSII was much greater than to PSI by low temperature with strong lightstress.（4） After the treatments with low temperature and weak light, the recovery of PSII wasfaster in the light than in darkness, while the recovery of PSI was the opposite.（5）After lowtemperature and high light treatments, the recovery of PSII was faster in darkness than in thelight, and the recovery of PSI showed oppsitely.