title: Photoinactivation of Photosystem II in wild-type and chlorophyll b-less barley leaves: which mechanism dominates depends on experimental circumstances.
abstract: Action spectra of photoinactivation of Photosystem II (PS II) in wild-type and chlorophyll b-less barley leaf segments were obtained. Photoinactivation of PS II was monitored by the delivery of electrons from PS II to PS I following single-turnover flashes superimposed on continuous far-red light, the time course of photoinactivation yielding a rate coefficient k i. Susceptibility of PS II to photoinactivation was quantified as the ratio of k i to the moderate irradiance (I) of light at each selected wavelength. k i/I was very much higher in blue light than in red light. The experimental conditions permitted little excess light energy absorbed by chlorophyll (not utilized in photochemical conversion or dissipated in controlled photoprotection) that could lead to photoinactivation of PS II. Therefore, direct absorption of light by Mn in PS II, rather than by chlorophyll, was more likely to have initiated the much more severe photoinactivation in blue light than in red light. Mutant leaves were ca. 1.5-fold more susceptible to photoinactivation than the wild type. Neither the excess-energy mechanism nor the Mn mechanism can explain this difference. Instead, the much lower chlorophyll content of mutant leaves could have exerted an exacerbating effect, possibly partly due to less mutual shading of chloroplasts in the mutant leaves. In general, which mechanism dominates depends on the experimental conditions.
authors.affiliation: Natural Sciences & Science Education, National Institute of Education, Nanyang Technological University, 1 Nanyang Walk, Singapore, 637616, Singapore.
authors.forename: Jie
authors.initials: J
authors.lastname: He
authors.affiliation: College of Life Sciences, Northwest A&F University, Yangling, 712100, Shaanxi, China.|Division of Plant Science, Research School of Biology, The Australian National University, 46 Biology Place, Canberra, ACT, 2601, Australia.
authors.forename: Wenquan
authors.initials: W
authors.lastname: Yang
authors.affiliation: Natural Sciences & Science Education, National Institute of Education, Nanyang Technological University, 1 Nanyang Walk, Singapore, 637616, Singapore.
authors.forename: Lin
authors.initials: L
authors.lastname: Qin
authors.affiliation: State Key Laboratory of Vegetation and Environmental Change, Institute of Botany, The Chinese Academy of Sciences, Beijing, 100093, China.|Division of Plant Science, Research School of Biology, The Australian National University, 46 Biology Place, Canberra, ACT, 2601, Australia.
authors.forename: Da-Yong
authors.initials: DY
authors.lastname: Fan
authors.affiliation: Division of Plant Science, Research School of Biology, The Australian National University, 46 Biology Place, Canberra, ACT, 2601, Australia. Fred.Chow@anu.edu.au.
authors.forename: Wah Soon
authors.initials: WS
authors.lastname: Chow
mesh_terms: D002734:Chlorophyll / Q000378:metabolism; D002736:Chloroplasts / Q000378:metabolism; D005453:Fluorescence; D001467:Hordeum / Q000378:metabolism / Q000528:radiation effects*; D008027:Light; D045332:Photosystem II Protein Complex / Q000378:metabolism / Q000528:radiation effects*
chemical_list: D045332:Photosystem II Protein Complex; D002734:Chlorophyll; C037184:chlorophyll b
keywords: Chlorophyll fluorescence; P700; Photoinactivation of Photosystem II; Photoinhibition; Photosystem II