Study On The Damaging And Alleviating Mechanism Of Plant Photosystem Under Chilling Stress With Light

Chilling is a major abiotic factor limiting the productivity and geographical distribution of many species. It has been known that PSI suffers serious damage compared with PSII in chilling sensitive plants under chilling light stress. However,injury mechanism and the way to deal with chilling light stress are still not clearly nowadays.In our research, changes of non-photochemical quenching components have been compared between chilling sensitive plant Cucumis sativus and chilling tolerant plant Atriplex centralasiatica, to clarify the reason why the injury of PSII is always present in chilling tolerant plants and the injury of PSI happening in chilling sensitive plants.Meanwhile, protection of three electron transport chain inhibitors on PSI in Cucumis sativus under chilling light stress has been discussed. The main results are as follows:1. The comparison of photosystem activities and thermal dissipation between Cucumis sativus and Atriplex centralasiatica under chilling light stress(1). Photosynthetic machinery of Cucumis sativus has not been injured under sole chilling (4?, 0 ?mol�m-2�s-1).(2). After 6 h chilling light treatment, PSII activities of Cucumis sativus decreased to 60% of original level, while PSI activities decrease to 20%. In the chilling sensitive plant Cucumis sativus, light treatments at low temperature impaired PSI rather than PSII as the primary target for photoinhibition in vivo. During the early recovery phase(within 3 h), light is harmful for PSII. 3 h later, 25 ?mol�m-2�s-1 illumination was optimum for PSII repairment. After 48 h, recovery of PSII activities was faster than PSI under any light density. We can conclude that PSI was the primary target for photoinhibition in vivo.(3). After 24 h chilling light stress, PSII activities of Atriplex centralasiatica reduced to 65% of original level, while there was no change in PSI activities. This demonstrated that Atriplex centralasiatica PSII was the primary target for photoinhibition in vivo under chilling light stress. Most impressively, the Atriplex centralasiatica PSII activities recovered to original level within 12 h when transplaced to room temperature under 25?, 100 ?mol �m-2�s-1 illumination.There is no doubt that Atriplex centralasiatica is of chilling light tolerance capacity. In order to uncover the mystery, non-photochemical thermal dissipation(NPQ) composition changes between the two species were compared.(4). Within the time course of chilling light stress, obvious variation of NPQ composition in Atriplex centralasiatica leaves can be monitored. Energy-dependent quenching or ApH-dependent thermal dissipation (qE) ratio decreased from 90% to about 30% in 6 h. Whithin 3 h, State transition-dependent thermal dissipation (qT)ratio rose to 20% from less than 2%, and then kept steady. Photoinhibitory quenching or reversible inactivation of PSII reaction center dependent thermal dissipation (qI)ratio increased from 10% to 50% within 6 h.Therefore by comparing the dissipation mode of excess energy, we can conclude that: chilling tolerant plant Atriplex centralasiatica down-regulated PSII activity by reversible inactivation of PSII reaction center dependent thermal dissipation (qI) and depressed electron transport to PSI, thus played a protective effect on the photosystem.For chilling sensitive plant Cucumis sativus,?pH-dependent thermal dissipation (qE)played a major role in dissipation of excess energy. This led to redandancy of electrons flow into photosystem and damage to PSI, even to whole photosynthetic apparatus.(5). During the first 4 h of chilling light stress, significant change were detected in Cucumis sativus leaves. The ApH-dependent thermal dissipation (qE) ratio is promoted to 95% from 60% within 4 h. State transition-dependent thermal dissipation(qT) ratio and reversible inactivation of PSII reaction center-dependent thermal dissipation (qI) ratio presented down-regulation trend and the total percentage were below 5%. However, during the 4 h to 6 h period, there happened a reversal that qE ratio dropped seriously while qE and qI rose.2. The protection mechanisms of different kinds of electron transport chain inhibitors on Cucumis sativus photochemical systemsTo alleviate the photodamage of Cucumis sativus leaves under chilling light stress, appropriate concentration of electron transport chain inhibitors were utilized to limit excess electrons flowing into photosystem. In which way,similar manner to deal with excess energy as chilling tolerant plant Atriplex centralasiatica may emerge. The following experiments were carried out.(6). Through spraying different concentrations of DCMU solution, we can find that 30?mol�L-1 DCMU solution played a protective role for PSI. It may be that DCMU restrained excess electrons to PSI from PSII, and avoided photodamage of PSI.However, DCMU had limitations in practical applications because of its damage to young leaves.(7) Through spraying exogenous DBMIB solution, we found that 1000 ?mol�L-1DBMIB solution can relieve the injury of Cucumis sativus leaves under chilling stress.The good news is that 1000 ?mol�L-1 DBMIB will not injure young leaves of Cucumis sativus. Further study on the protection mechanism of DBMIB against photodamage has been carried out. We found that after spraying exogenous DBMIB solution, the non-photochemical quenching components has changed from depending mainly on qE to qI. It may have good future in applications.In conclusion,this experiment first analyzed the difference of dissipation mechsnism of excess energy between chilling sensitive plant Cucumis sativus and chilling tolerant plant Atriplex centralasiatica, and uncovered the damage mechanism on the photosystem of chilling sensitive plants. Meanwhile, through spraying exogenous DBMIB solution,a practicable method of alleviating photodamage was found.