Effect Of Distribution Of Excited Energy In Leaves Of Heat-tolerant Brassica Cabbage Seedlings Under High Temperature Stress

Two types of Brassica cabbage cultivars(cv.DNX,heat-tolerant;cv.WFS, heat-sensitive) were used as materials to study their difference in photosynthesis,distribution of excited energy.The main results are as follows:1.The diurnal variations of Pn and chlorophyll fluorescence parameters such as Fo,Fv/Fm,Fv'/Fm',NPQ were observed during fine days when leaf temperature was 16℃～32℃.The diurnal variation of Pn in cv.DNX and cv.WF8 was double peaks curves.Photosynthetic rates of cv.DNX and cv.WF8 reached maximum at 10:00 and 14:00,respectively.Pn of cv.DNX and cv. With increase of light intensity and leaf temperature,Pn decreased to minimal levels at about 12:00.Decrease of Pn at noon mainly resulted from nonstomatal limitations.2.Two types of cabbage cultivars showed significant difference in photosynthetic rate under short-term high temperature stress in the high light. Photosynthetic rate of cv.DNX was approximate 30%higher than that of cv.WF8 under 1200μmol·m^-2·s^-1PFD when leaf temperature was 40℃. Compared with at 28℃,their photosynthetic rates decreased 7%and 24%, respectively.3.Recovery process of cv.DNX and cv.WF8 after heat stress(40℃) for 5 hours under 800μmol·m^-2·s^-1 light intensities showed that high temperature had little effect on cv.DNX.Compared with at 25℃,Pn decreased 27%,but Pn of cv.WF8 decreased 43%.The recovery trends ofφPSâ…¡and Fv'/Fm' was similar to that of Pn.Pn,φPSâ…¡,Fv'/Fm' and Fv/Fm recovered completely after 24 hours.4.When heat stress removed,φPSâ…¡of DNX and WF8 had already recovered obviously,but Pn still decreased,oxygen evolution rate still decreased.This suggested the decrease in photosynthesis induced by heat stress is mainly associated with the non-stomal limitation.It also indicated that high temperature majorly inhibited Calvin cycle.We presumed high temperature stress damaged the key enzyme of Calvin cycle. 5.After treatment at different temperatures under 800μmol·m^-2·s^-1 PFD, the effects of shading(30%of natural light intensity,3d) on Brassica cabbag seedlings(cv.DNX,field-grown for 1 month) were studied,qP andφPSâ…¡of control seedlings distinctly higher than shaded seedlings when leaf temperature was 25℃～35℃.qP andφPSâ…¡of control seedlings were obviously higher than that of shaded seedlings,while qP andφPSâ…¡of control were lower than that of shaded seedlings when leaf temperature was above 33℃.High temperature stress had little effect on shaded seedlings,for example at 41℃,compared with at 25℃,qP andφPSâ…¡of shaded seedlings decreased 20%and 54%,and the control decreased 74%and 91%, respectively;High temperature had little effect on Fv/Fm of shaded seedlings. There was an obvious decrease in Fv/Fm when leaf temperature was 39℃,but the control decreased at 33℃.Fv'/Fm' continued to decline during the treatment.Fv'/Fm' of shaded seedlings was much higher than that of the control when leaf temperature was above 39℃.6.15mmol/l exogenous Ca²⁺ pretreatment could alleviate the damage caused by high temperature stress.After high temperature stress for 10h,Pn of the exogenous Ca²⁺ pretreated seedlings decreased 10.4%,but the control decreased 31.2%.Ci of the control seedlings was much higher than that of the pretreated seedlings,which indicated that the decrease in Pn of the control seedlings under heat stress resulted from nonstomatal limitations and had no relation with decline of Ci and Gs caused by exogenous Ca²⁺ pretreatment. Fv'/Fm' and Fv/Fm were 41%and 8%higher than that in the control, respectively.Fo was 12%lower than that in the control.Exogenous Ca²⁺ pretreatment could alleviate photodamage of PSâ…¡reaction center from photoinhibition under high temperature stress,which was consistent with its high photosynthetic rate.