Effect Of Different Temperatures Coupled With Different Light Intensity On The Photochemical Activities Of PSâ…  And PSâ…¡ In Leaves Of Chilling-Sensitive Plant Cucumber And Chilling-Tolerant Plant Rumex K-1

The effects of different temperatures coupled with different light intensity on activity of photosynthetic apparatus in leaves of a chilling sensitive plant cucumber (Cucumis sativus L. cvs. Jinchun No4) and a chilling-tolerant plant Rumex K-1(Rumex Patientia×R. tianschaious) were studied by measurements of photosynthetic gas exchange, oxygen- evolution, modulated chlorophyll a fluorescence, 820nm transmission signal paralleling the chlorophyll a fluorescence transient, changes in reactive oxygen species (ROS) and activity of ROS scavenging enzymic system, ultrastructure of chloroplasts and so on. The main results obtained are followed:1. With treatment of chilling temperature (4℃)coupled wth 300μmolm^-2s^-1 light, the sharp decreases were observed in the light-saturated net CO₂ fixation and maximal oxygen evolution rate of cucumber. The maximal efficiency of PSI (â–³I/Io) reduced significantly, coupled with the decline of maximal photochemistry (Fv/Fm) in cucumber leaves. However, the redox states of PSI activity (â–³I/Io) was decreased more severely than the efficiency that a trapped exciton can move an electron into the electron transport chain beyond Q_A^- (ΨO) in both cucmuber and Rumex K-1 intact leaves. These results suggested that PSI should be the primary target of chilling-photoinhibition in the both chilling-sensitve and chilling-tolerant plants.2. The efficiency of excitation capture by open PSII centers (Fv'/Fm'), and actual photochemical efficiency of PSII (ФPSII) decreased considerably, at the same time, the reduction of electron transfer chain (1-qP) increased greatly in cucumber leaves. The close correlation of excitation pressure on PSII and PSI photoinhibition were observed in both the cucumber and Rumex K-1 leaves, indicating that the reduction state of QA is dependent of the extent of PSI photoinhibition. The accumulation of QA- could lessen electron flow to PSI,this might be important for protection against the photoinhibition of PSI.3. The content of H2O₂, O 2 and MDA in leaves increased largely. The activity of catelase (CAT), ascorbate peroxidase (APX), glutathione reducase (GR) and dehydroascorbate reductase (DHAR) were decreased in the leaves of cucumber treated with chilling temperature under moderate light, while activity of monodyhydroascorbate reductase (MDHAR) remained unchanged, and the activity of superoxide dismulate (SOD) increased. These data suggested that although the activity of SOD was increased, the activitity of antioxidant system was generally decreased in chilling-sensitive plant cucumber treated with chilling temperatures under moderate light, so that they were not able to scavenge the accumulated reactive oxygen species (ROS), resulting in cellular oxidative damage in cucumber leaves.4. Compared with the chilling–tolerant plant Rumex K-1, the membrane lipid composition of cucumber changed more noticeably and the polyunsaturated fatty acids index of PC, PG and SQDG decreased more remarkable in cucumber treated with chilling temperature combined with moderate light, which might be one of the important reseaons resulting in chilling-sensitivity of photochemical activity of PSI, PSII and OEC in cucumber leaves.5.Compared with the chilling–tolerant plant Rumex K-1, chloroplasts of cucumber were more vulnerable to chilling-light stress. After chilling treatment under moderate light, the thylakoid progressivly became distorted, swelled, and small vesicles were formed from inner membrane of chloroplasts. The chloroplast envelopes were eventually disintegrated, as a result, stromal contents mixed with the cytoplasm. While a striking chloroplasts reoriention was observed in Rumex K-1 leaves. The chloroplasts of Rumex K-1 were not affected obviously under chilling-light treatment. This observations supported the view that chloroplast avoidance movement play an important role in decreasing the light absorbation by photosystems, reducing oxidantive pressure on chloroplasts under excess light .6. Photosynthetic oxygen evolution, PSI and PSII photochemistry was maintained steady in cucumber leaves exposed to high temperature (40℃) in the dark , while OEC activity and PSII photochemistry as well as the rate of P700⁺ reduction were markedly reduced in Rumex K-1 leave exposed to high temperature (40℃) in the dark.7. The PSI and PSII photochemistry were hardly affected in cucumber leaves treated with 40℃under low and moderate light intensities. The OEC activity, PSII photochemistry and the rate of P700+ reduction were not obviously chaged in Rumex K-1 leaves eoxposed to 40℃under low light. Whereas, under 300μmol m^-2 s^-1 PFD combined with 40℃, photochemistry of PSII, activity of OEC and P700+ reduction were slightly inhibited in Rumex K-1. These results indicated that the photosynthetic apparatus of chilling-sensitive cucumber was thermotolerant, however, the OEC complex and PSII photochemisty of Rumex K-1 were heat-sensitive, the severely impaired electron donation from PSII limited the P700+ reduction in Rumex K-1.