Changes Of PSⅡ Function And Photoprotection Mechanism In Soybean Leaves During Senescence

By measurement of chlorophyll fluorescence, gas exchange, protein and pigment, etc., changes of PSII function and photoprotection mechanism in soybean leaves during senescence were investigated in this paper. The main results are as follows:1. With the senescence in soybean leaves, photorespiration and photosynthesis decreased markedly. In contrast, Mehler reaction dependent electron transport increased during the first three days of ethylene treatment then decreased to a low level after 3 days of ethylene treatment, and the ratio of Mehler reaction dependent electron transport to the total electron transport increased throughout the senescence.2. NPQ, qf and (Z+A)/(Z+A+V) exhibited two distinct phases, with an initial increase (during the first 3 days) followed by a dramatically decrease after 3 days of ethylene treatment during senescence. However, increase of qI was observed from beginning to the end.At the beginning of senescence, (1-qP)/NPQ (used to estimate the fraction of excess photons) showed a little difference compared to the control; however, it increased to a high level when leaf became3. severely senescent.4. In PSII, the decline of primary photochemical activity was observed during senescence. The rate and extent of QA reduction increased markedly. The QB site was changed reflected by the fluorescence transient and Vt curve, which was the key factor leading to inhibition of electron transporting to QB from QA resulting in the decline of primary photochemistry.5. Photons absorbed by per excited cross-section (ABS/CSo) and used to move electron transport beyond QA- per excited cross-section (ETO/ CSo) decreased throughout the senescence. Accompanied with the decline in the density of active reaction center per excited cross-section (RC/CSO), the dissipated flux per excited cross-section (DIO/CSO) was enhanced. 6. It was clear to observe that the fraction of photons used to move electron transport beyond QA- per reaction center (ETO/RC) and trapped photons per reaction center(TRo/RC) increased markedly during the senescence, which was attribute to that the absorbed photons by inactive reaction center was transported to active reaction centers and resulted in charge separation. 7. Besides, chlorophyll content, soluble protein, thylakoid membrane lipids fatty acid instauration and activity of Violaxanthin de-epoxidase(VDE)were all decreased with the progress of senescence.Photoinhibition in soybean leaves grown in the field was mainly attributed to reversible inactivation of PSII reaction center. However,8. the inactivated degree did not increase with the senescence.In summary, structure and function of PSII changed, and this resulted in inhibition of PSII acceptor side in soybean leaves during senescence induced by ethylene. Photoprotective mechanisms exhibited dynamic change: xanthophyll cycle related thermal dissipation showed a biphasic change, with an initial increase followed a final decrease, throughout the senescence; energy quenching site shifted gradually from antenna to reaction center.