| Citrus was low photosynthetic ability and had often been suffered from damaging of strong light in natural environment. Then the quantity and quality of citrus had been limited. Studies on photoinhibition and protective mechanisms in citrus leaves contributed to approach the main limited point of photosynthetic efficiency during photosynthesis,reveal the law of regulating and controlling photosynthetic efficiency and offer theoretical basis fer enhancing photosynthetic efficiency in citrus leaves and producing much more good fruit. Taking Mandarin Citrus leaves as the samples,we determined diurnal variation of light intensity,photosynthetic electron transport rate (ETR),fluorescence parameters Fo,Fm,Fv/Fm,non-photochemical quenching of fluorescence and three phases in it of chloroplast a,photo absorptance in 505 nm,the variation of middle-phase and slow-phase of non-photochemical quenching of fluorescence in leaves controlled by DTT and NaF and fluorescence induction dynamics of fast-phase. At the same time,we studied the change of fluorescence parameters Fo,Fm,Fv/Fm,the rate of generating rate of active oxygen,the change of active oxygen scavenging enzymes after two citrus species of Mandarin Citrus and Navel Orange were treated by low temperature and strong light and the recovery of fluorescence parameters after using antioxidant in leaves. And the purpose was to show the protective mechanisms during photoinhibition in citrus leaves.Experiment 1. On clear days,the diurnal variation in chlorophyll fluorescence parameters and the value of light absorbency on 505nm was studied in Satsuma Mandarin leaves. The results were as follows:with light intensity increasing,electron transport rate (ETR),non-photochemical quenching of chlorophyll fluorescence (qN),both of its three components fast-phase (qNf) and slow-phase (qNs) and the value of light absorbency on 505 nm increased,but photo-chemical efficiency of PS II (Fv/Fm),maximal fluorescence (Fm) and initial fluorescence (Fo) decreased. After treated by DTT,slow-phase (qNs) of non-photochemical quenching was much lower than control,but initial fluorescence (Fo) was much higher than control. These results indicated that xanthophyll cycle-dependent non-radiative energy dissipation and A pH-dependent heat dissipation existed and played an important protective role against photo-damage to the photosynthetic apparatus in citrus leaves during diurnal variation of photosynthetic efficiency.Experiment 2. After treating Mandarin Citrus leaves with different light intensity,we determined middle-phase in the non-photochemical quenching of fluorescence,state transition inducted by far-red light,fluorescence induction dynamics of fast-phase. The result was the photosynthetic apparatus of citrus leaves was in state 2 in low light of 100u mol m-2 s-1 and was in state 1 in high light of 1000 u mol m-2 s-1. After NaF treated leaves,the state 2-state 1 transition of photosynthetic apparatus in high light intensity was inhibited. After analyzing the fluorescence induction dynamics of fast-phase,we found that PQ pool was in reduction state because of QA reduced,which made energy chang from PS II to PS I . Over much light irradiance caused PS II reaction center reversible inactivation and PQ pool could not .get electron and was in oxidation,photosynthetic apparatus changed from state 2 to state 1.Experiment 3. Taking the control of 25 and darkness as the contrast,we determined the change of fluorescence parameters Fo,Fm,Fv/Fm,PSH, co2,and the rate of psn/coi to the two species of "Mandarin Citrus" and "Navel Orange" in both of 4,dark treatment and 4,300 u mol m-2 s-1 treatment. At the same time,we determined generating rate of active oxygen,active oxygen scavenging enzyme and the recovery of fluorescence parameters Fo,Fm,Fv/Fm,PSII- The results were as follows:in the treatment of 4 and darkness the changes of fluorescence parameters were not obvious in Mandarin Citrus leaves,decreased about by 75%. Fo increased clearly compared with the contrast in Navel Orange l... |
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