Regular paper A simple chlorophyll fluorescence parameter that correlates with the rate coefficient of photoinactivation of Photosystem II Luke Hendrickson 1 , Britta Fo¨rster 2 , Barry J. Pogson 2 & Wah Soon Chow 1, * 1 Research School of Biological Sciences, The Australian National Universtiy, GPO Box 475, Canberra, ACT 2601, Australia; 2 School of Biochemistry and Molecular Biology, The Australian National University, Canberra, ACT 0200, Australia; *Author for correspondence (e-mail: chow@rsbs.anu.edu.au; fax: +61-2- 6125-8056) Received 30 September 2004; accepted 18 November 2004 Key words: Capsicum annuum, energy flux analysis, non-photochemical quenching, photochemistry, rate coefficient of photoinactivation, thermal dissipation. Abstract A method of partitioning the energy in a mixed population of active and photoinactivated Photosystem II (PS II) complexes based on chlorophyll fluorescence measurements is presented. There are four energy fluxes, each with its quantum efficiency: a flux associated with photochemical electron flow in active PS II reaction centres (J PS II ), thermal dissipation in photoinactivated, non-functional PS IIs (J NF ), light-regulated thermal dissi- pation in active PS IIs (J NPQ ) and a combined flux of fluorescence and constitutive, light-independent thermal dissipation (J f,D ). The four quantum efficiencies add up to 1.0, without the need to introduce an ‘excess’ term E, which in other studies has been claimed to be linearly correlated with the rate coefficient of photoinacti- vation of PS II (k pi ). We examined the correlation of k pi with various fluxes, and found that the combined flux (J NPQ + J f,D =J pi ) is as well correlated with k pi as is E. This combined flux arises from F s =F 0 m , the ratio of steady-state to maximum fluorescence during illumination, which represents the quantum efficiency of combined non-photochemical dissipation pathways in active PS IIs. Since F s =F 0 m or its equivalent, J pi , is a likely source of events leading to photoinactivation of PS II, we conclude that F s =F 0 m is a simple predictor of k pi . Abbreviations: Chl – chlorophyll; E – ‘excess’ energy not accounted for by photochemistry or non-photo- chemical dissipation; F o , F m – relative Chl fluorescence yield for open and closed PS II reaction centres, respectively, in dark-treated samples; F 0 o – minimum fluorescence yield corresponding to open reaction centers during illumination; F s , F 0 m – steady-state or maximum fluorescence yield during illumination, respectively; F v /F m – intrinsic quantum efficiency of PS II photochemistry; I A -, PAR absorbed by the leaf; J pi =J NPQ + J f,D ; k pi – rate coefficient of photoinactivation of PS II; NPQ – non-photochemical quenching; PAR – photosynthetically active radiation; PS II – Photosystem II; F f,D , J f,D – combined quantum efficiency or flux of fluorescence and constitutive thermal dissipation, respectively; F NF ,J NF – quantum yield or flux of thermal dissipation in non-functional PS II, respectively; F NPQ ,J NPQ – quantum yield or flux of light- dependent and DpH- and xanthophyll-mediated regulated thermal dissipation, respectively; F PS II ,J PS II – quantum yield or flux of PS II electron flow, respectively; q P – photochemical quenching coefficient Introduction Quantifying the fluxes of utilization and dissi- pation of light energy absorbed by Photosystem II (PS II) using chlorophyll (Chl) fluorescence non-intrusively has been of considerable interest. Several methods have been proposed, evolving primarily from the work by Genty et al. (Cailly Photosynthesis Research (2005) 84: 43–49 Ó Springer 2005