Interaction Between Photosystem â…  And Photosystem â…¡ Under Chilling-light Stress In Cucumber Leaves

In leaves of higher plant, photosystem I (PSI) and photosystem II (PSII) interacts eachother remarkably due to the electron transport in series between them. The inhibition ofelectron transfer from PSII to PSI interferes with the electron transfer of PSII accept-side andcauses the lack of electron donation to PSI simultaneously. It has been known that thephotoinhibition of PSII is aggravated by the suppression of the electron transfer from PSII toPSI, however, the effect of this suppression to PSI activity is unclear yet. On the other hand,the recovery of PSI activity is much slower than that of PSII activity because PSII exists arapid turnover mechanism, D1protein de novo synthesis, but PSI does not. The activity ofphotochemical reaction depends on the lower activity among the PSI and PSII. PSI willbecome a limiting factor of the photochemical reaction for a long time after PSIphotoinhibition. PSI is the primary site of photoinhibition in chilling-sensitive plant underchilling-light stress. It is of great importance to explore how to avoid PSI photoinhibition inleaves of chilling sensitive plants under chilling-light stress and to accelerate the recovery ofPSI activity after chilling-light stress.In this study, the interaction between PSI and PSII, and especially to study the effect ofthe electron transfer from PSII to PSI on the chilling photoinhibition and the later recoverywere researched. The aim of this study is understand the mechanism of PSI photoinhibitionand later recovery, and to provide theoretical and practical support in winter protectioncultivation and cold-tolerance breeding. The main results obtained are as follows:(1) Under chilling-light treatment, higher light intensity caused higher excess excitationpressure ((1-qP)/NPQ). The maximal photochemical efficiency of PSII (Fv/Fm) continuallydecreased with the increase of excess excitation pressure. The maximum PSI redox activity(△I/Io) also decreased significantly with the increase of excitation pressure when theexcitation pressure was relatively lower. However, once the excess excitation pressureexceeded a certain level, the△I/Io did not increase obviously with the increase of the excessexcitation pressure any more. This result indicates that the electron transfer from PSII to PSI is necessary for PSI photoinhibition caused by chilling-light treatment. Under high lightintensity, sever PSII photoinhibition limited the electron transfer from PSII to PSI andprotected PSI from the future photoinhibition.(3) Under chilling-light treatment, higher photosynthesis rate, lower ROS level, lowerPSI and PSI photoinhibition were observed in Rumex K-1leaves than in cucumber leaves.The relation between PSI photoinhibition and PSII excitation pressure in leaves of cucumberwas similar to that in leaves of Rumex K-1, but the PSII in Rumex K-1leaves is moresensitive to PSII excitation pressure than that in cucumber leaves. This result indicates that,compared to the cucumber, the higher chilling-tolerance of Rumex K-1is because that thePSII excitation pressure in Rumex K-1is much lower due to enhanced stabilization of the PSIin Rumex K-1under light-chilling.(2) The chilling-light treatment inhibited CO2assimilations completely in differentexpanded cucumber leaves. However, the photoinhibition of the two photosystems (PSI andPSII) and the accumulation of H2O2were more severe in young leaves than in full-developedleaves. During chilling-light treatment, PSII photoinhibition was positive correlated with PSIphotoinhibition in leaves，but this correlation did no occur when the electron transport fromPSII to PSI was blocked by3-(3,4-dichlorfenyl)-1,1-dimethylkarbonyldiamid (DCMU).Although the photoinhibition of PSII and PSI in young leaves were more severe, thesensitivity of PSII to excitation pressure was lower in young leaves than in fully expandedleaves. This study demonstrated that the more sever PSII photoinhibition observed in youngleaves was due to the higher PSII excitation pressure caused by the higher sensitivity of PSIin young leaves to chilling-light treatment.The study showed that PSI is the primary site of photoinhibition caused bychilling-light, and the chilling-light tolerance of a plant depends highly on PSI activity. So, inwinter protected cultivation and cold tolerance breeding, attention should not only be paid toPSII photoinhibition but also be paid to avoiding or reducing PSI photoinhibition.(4) The PSI photoinhibition was stopped by blocking the electron transfer from PSII toPSI with DCMU under different light intensities. In leaves from chilling-resistant plantsRumex K-1and chilling-sensitive plants cucumber leaves, when the electron transfer fromPSII to PSI was blocked, The PSI photoinhibition was completely stopped. (5) The photoinhibition of PSI and PSII occurred synchronously under chilling anddifferent light quality (red, green and blue). Under the same light intensity (100μmol m-2s-1),red light caused the weakest photoinhibition whereas blue light caused the severestphotoinhibition. Blue light excited PSII more effectively than the red and green light, so thePSII photoinhibition under blue light was higher than that under other light; the electrontransfer from PSII to PSI is higher under blue light than under red and green light, so the PSIphotoinhibition under blue light was higher than that under other light. This result furthersupports the hypothesis that the PSI photoinhibition under chilling-light depends highly onthe number of electrons transferred from PSII to PSI.Above two researches demonstrated that the electrons transferred from PSII to PSI isnecessary for PSI photoinhibition caused by chilling-light treatment. Appropriate PSIIphotoinhibition reduced the electrons transferred from PSII to PSI, alleviating PSIphotoinhibition. So, in protected cultivation in winter and cold tolerance breeding, unilateralincreasing of PSII activity and chilling-light tolerance in PSII will exacerbate PSIphotoinhibition under chilling-light treatment and impair chilling-tolerance of plant.(6) UV illumination caused PSII photoinhibition but did not directly hurt PSI. However,under chilling-light treatment, UV illumination deteriorated PSII photoinhibition anddecreased the electron transfer from PSII to PSI. Thereby, the existence of UV light alleviatedPSI photoinhibition under chilling-light treatment. This result showed that compared with thechilling-light stress in nature condition, the PSI photoinhibition was overestimated inprevious studies under artificial light source due to lack of UV light in the light source used inlaboratory. In protected cultivation in winter, the UV-conductive greenhouse film instead ofUV-prohibitive glass should be used to alleviate PSI damage caused by chilling-light stress.(7) During the recovery process at room temperature after chilling-light treatment, thePSII activity recovered quickly and was insensitive to high light intensity, while the PSIactivity recovered quickly under weak light intensity (15μmol m-2s-1) but slowly under highlight intensity (200μmol m-2s-1). With the present of DCMU which blocks the electrontransfer from PSII to PSI, the PSI activity recovered quickly even under high light intensity(200μmol m-2s-1). Above results suggest that after the chilling-inducted photoinhibition,reducing electron transfer from PSII to PSI protects the PSI from future inhibition, accelerating the recovery of PSI activity. So, in the breeding of chilling-resistant crops,attention should not only be paid to faster recovery of PSII after chilling-inducedphotoinhibition but more attention should also be paid to the coordinating of PSI and PSIIafter chilling-induced photoinhibition. In winter protected cultivation, after chillinghappened, to reduce light intensity will help the recovery of PSI activity from photoinhibitionso that a faster recovery of the activity of the whole photosynthetic apparatus will beachieved.