Plant and Cell Physiology Advance Access originally published online on September 7, 2009
Plant and Cell Physiology 2009 50(10):1815-1825; doi:10.1093/pcp/pcp123
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The Involvement of Dual Mechanisms of Photoinactivation of Photosystem II in Capsicum annuum L. Plants
1Plant Sciences, Department of Biological Sciences, Graduate School of Science, University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo, Japan
2Photobioenergetics Group, Research School of Biology, College of Medicine, Biology and Environment, Australian National University, Canberra, ACT 0200, Australia
*Corresponding author: E-mail, riichi{at}biology.tohoku.ac.jp; Fax, +81-22-795-6699.
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Abstract |
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For plants, light is an indispensable resource. However, it also causes a loss of photosynthetic activity associated with photoinactivation of photosystem II (PSII). In studies of the mechanism of this photoinactivation, there are two conflicting hypotheses at present. One is that excess energy received by leaves, being neither utilized by photosynthesis nor dissipated safely in non-photochemical quenching, causes the photoinactivation. The other involves a two-step mechanism in which excitation of Mn by photons is the primary cause. In the former hypothesis, photoinactivation of PSII should not occur in low light that provides little excess energy, but in the latter hypothesis it should. Therefore, we tested these two hypotheses in different irradiances. We used a system that can measure the fraction of functional PSII complexes under natural conditions and over a long period in intact leaves, which were attached to a plant treated with lincomycin taken up via the roots. The leaves were photoinactivated in low, medium or high light (30, 60 or 950 µmol m–2 s–1) with white, blue, green or red light-emitting diode arrays. Our results showed that the extent of photoinactivation per photon exposure was higher in high light than in low light, consistent with the abundance of excess energy. However, photoinactivation did occur in low light with little excess energy, and blue light caused the greatest extent of photoinactivation followed by white, green and red light in this order, an order that can be predicted from the Mn absorbance spectrum. These results suggest that both mechanisms occur in the photoinactivation process.
Keywords: adaxial and abaxial side of leaves - chlorophyll fluorescence - excess energy hypothesis - intact leaves - photoinhibition - two-step hypothesis
Abbreviations:
chl, chlorophyll; ETR, rate of electron transport; Fm (Fm'), maximum fluorescence in the fully relaxed state (in the light); Fo (Fo'), minimum fluorescence in the fully relaxed state (in the light); Fs', steady state fluorescence in the light; Fv, variable fluorescence in the fully relaxed state (in the light), Fv = Fm – Fo (Fv' = Fm' – Fo'); Fv/Fm, maximum photochemical efficiency of photochemistry in photosystem II of dark-adapted leaves; Kpi, rate coefficient of photoinactivation; LED, light emitting diode array; NPQ, non-photochemical quenching; PAM, pulse amplitude modulated; 
PSII, quantum yield of PSII electron transport; qP, photochemical quenching coefficient; PSII, photosystem II.
3Present address: Graduate School of Life Sciences, Tohoku University, Aoba, Sendai, 980-8578 Japan
(Received May 26, 2009; Accepted September 1, 2009)
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