Zeaxanthin Formation and Energy-Dependent Fluorescence Quenching in Pea Chloroplasts under Artificially Mediated Linear and Cyclic Electron Transport |
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| Authors: | Gilmore A M Yamamoto H Y |
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| Affiliation: | University of Hawaii, Department of Plant Molecular Physiology, 3190 Maile Way, St. John 503, Honolulu, Hawaii 96822. |
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| Abstract: | Artificially mediated linear (methylviologen) and cyclic (phenazine methosulfate) electron transport induced zeaxanthin-dependent and independent (constitutive) nonphotochemical quenching in osmotically shocked chloroplasts of pea (Pisum sativum L. cv Oregon). Nonphotochemical quenching was quantitated as Stern-Volmer quenching (SVN) calculated as (Fm/F′m)-1 where Fm is the fluorescence intensity with all PSII reaction centers closed in a nonenergized, dark-adapted state and F′m is the fluorescence intensity with all PSII reaction centers closed in an energized state. Reversal of quenching by nigericin and electron-transport inhibitors showed that both quenching types were energy-dependent SVN. Under light-induced saturating ΔpH, constitutive-SVN reached steady-state in about 1 minute whereas zeaxanthin-SVN continued to develop for several minutes in parallel with the slow kinetics of violaxanthin deepoxidation. SVN above the constitutive level and relative zeaxanthin concentration showed high linear correlations at steady-state and during induction. Furthermore, Fo quenching, also treated as Stern-Volmer quenching (SVO) and calculated as (Fo/F′o)-1, showed high correlation with zeaxanthin and consequently with SVN (Fo and F′o are fluorescence intensities with all PSII reaction centers in nonenergized and energized states, respectively). These results support the view that zeaxanthin increases SVN above the constitutive level in a concentration-dependent manner and that zeaxanthin-dependent SVN occurs in the pigment bed. Preforming zeaxanthin increased the rate and extent of SVN, indicating that slow events other than the amount of zeaxanthin also affect final zeaxanthin-SVN expression. The redox state of the primary electron acceptor of photosystem II did not appear to determine SVN. Antimycin, when added while chloroplasts were in a dark-adapted or nonenergized state, inhibited both zeaxanthin-SVN and constitutive-SVN induced by linear and cyclic electron transport. These similarities, including possible constitutive Fo quenching, suggest that zeaxanthin-dependent and constitutive SVN are mechanistically related. |
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