| There are two classes of photoinhibition, chronic and dynamic. Chronic photoinhibition is mainly due to photodamage of the photosynthetic apparatus, and usually occurs in shade-grown plants. Dynamic one is principally associated with energy dissipation processes and occurs often in sun-grown plants. In the past many studies the two classes of photoinhibition are separated each other. Therefore, the relationship between chronic and dynamic photoinhibition is still unclear. In soybean leaves, dynamic photoinhibition is mainly due to reversible inactivation of PSâ…¡, and chronic photoinhibition is attributed to irreversible photodamage of PSâ…¡ reaction centers. We have explored the relationship between reversible inactivation and irreversible photodamage of PSâ…¡ and light intensity, as well as the changes in oligomeric state of PSâ…¡ during the reversible inactivation and irreversible photodamage of PSâ…¡. The aim of this study is to understand the mechanism of reversible inactivation and the relationship between reversible inactivation and irreversible photodamage. 1. Light Intensity-Dependent Reversible Inactivation and Irreversible Damage of PSâ…¡ in Soybean Leaves Light intensity dependence of reversible inactivation and irreversible damage of PSâ…¡ was studied by determination of chlorophyll fluorescence, PSâ…¡ electron transport, and D1 protein level. After illumination with moderate light (700 ?mol photons m-2s-1, saturated for photosynthesis) for 3 h, both the light-saturated PSâ…¡ electron transport rate and D1 protein level had no significant change in soybean leaves grown at 250-300 ?mol m-2s-1. Although PSâ…¡ photochemical efficiency (Fv/Fm) declined significantly, it could recover completely after 4 h in the dark. After illumination with strong light (2000 ?mol m-2s-1) for 3 h, however, the significant decreases in all parameters above were observed, and Fv/Fm could not restore completely after 4 h in the dark. Moreover, the chlorophyll fluorescence parameter F685/F735 measured at 77K dropped significantly in thylakoids from all soybean leaves illuminated at 700 (L2), 1200 (L4) and 2000 (L7) ??mol m-2s-1 for 3 h. After subsequent dark recovery for 3 h, F685/F735 in thylakoids from soybean leaves illuminated with L2 could recover to the level of dark control, but not in those from soybean leaves illuminated with L4 and L7. Those results indicate that the illumination with moderate light results in reversible inactivation of some PSâ…¡s, while illumination with strong light leads to irreversible damage of some PSâ…¡s, namely, both reversible inactivation and irreversible damage of PSâ…¡ are light intensity-dependent.2. The relationship between reversible inactivation and irreversible photodamage of PSâ…¡and PSâ…¡oligomeric state The oligomeric states of PSâ…¡ complex in soybean leaves treated with moderate light (700 ??mol m-2 s-1) and strong light (2000 ??mol m-2 s-1) were studied by nondenaturing PAGE and gel filtration chromatography. The following results were obtained. PSâ…¡ dimers resolved by nondenaturing PAGE accounted for about 75% of total PSâ…¡ complexes and there was no significant difference in the ratio of PSâ…¡ dimer to monomer between samples from moderate light-treated and fully dark-adapted leaves. However, the ratio in soybean leaves declined significantly after treatment with strong light for 3 h. Furthermore, BBY particles were resolved into four chlorophyll-enriched fractions by gel filtration chromatography. From their molecular masses and protein components, these fractions were deduced to be PSâ…¡ dimer, PSâ…¡ monomer, oligomeric LHCâ…¡ and monomeric LHCâ…¡. Also, no change in the proportion of PSâ…¡ dimer in total PSâ…¡s was observed in the granal region of thylakoid membranes from moderate light-illuminated soybean leaves. Based on the above results, it is concluded that (1) the dimer is the predominant natural form of PSâ…¡ in vivo; (2) no monomerisation of PSâ…¡ dimer occurs during moderate light-induced photoinhibition in soybean leaves; and (3) mo...
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