Research article Damage of photosynthetic apparatus in the senescing basal leaf of Arabidopsis thaliana: A plausible mechanism of inactivation of reaction center II Pranab Kishor Mohapatra a, 1 , Padmanava Joshi b , Nemmara Krishnan Ramaswamy c , Mukesh Kumar Raval d , Udaya Chand Biswal a , Basanti Biswal a, * a School of Life Sciences, Sambalpur University, Jyoti Vihar, 768019, Odisha, India b Department of Physics, Anchal College, Padampur, P.O. Rajborasambar, 768036 Bargarh, Odisha, India c Nuclear Agriculture and Biotechnology Division, Bhabha Atomic Research Center, Trombay, 400485 Mumbai, India d Department of Chemistry, Gangadhar Meher College (Autonomous), Sambalpur 768004, Odisha, India article info Article history: Received 15 May 2012 Accepted 6 October 2012 Available online 14 November 2012 Keywords: Leaf senescence Oxygen evolution Photochemical quenching Photosystem II Redox homeostasis Thermoluminescence abstract Significant decline in oxygen evolution and DCPIP photoreduction and a marginal restoration of the later with DPC as an electron donor suggest the inactivation of reaction center of photosystem II. The declines in the height of thermoluminescence bands support the view and the damage of reaction center II could be central to the senescence process in Arabidopsis leaves. The enhancement in the number of reduced quinones, signifying a loss in redox homeostasis in the electron transport chain between photosystem II and I leads to the creation of an energy imbalance. The view is supported by the decline in actual quantum yield of photosystem II in the light adapted state and maximum quantum yield of primary photochemistry in the dark adapted state of chlorophyll fluorescence. An increase in chlorophyll a fluorescence polarization and decline in carotenoid to chlorophyll energy transfer efficiency suggest the perturbation in thylakoid structure. A plausible mechanism illustrating the senescence mediated inac- tivation of oxygen evolving complex has been proposed. Ó 2012 Elsevier Masson SAS. All rights reserved. 1. Introduction It is well established that alterations in the structure and func- tion of chloroplasts occur during leaf senescence [1e 7]. The alter- ations include the degradation of photosynthetic pigments and proteins, inactivation of both photosystems (PS) I and II, and down regulation of enzyme activities associated with Calvin-Benson cycle [5,6,8e11]. Reports demonstrating the damage of PS II [7,12e14], with oxygen evolving complex (OEC) as one of the initial events during leaf senescence in many plant systems are exquisitely available [6,7,15]. On the other hand, perusal of relevant literature reveals that the study on alterations in the structure and function of chloroplasts during senescence of leaf in Arabidopsis thaliana, a plant extensively used as a model system for senescence related genomic study, is meager. In a recent work from our laboratory, senescence-mediated decline in O 2 -evlution, photochemical potential of PS II, stomatal conductance, CO 2 fixation and up regulation of certain enzymes have been reported in this plant system [16]. However, no attempt has yet been made to understand the mode of inactivation of PS II during senescence of the leaf. In fact, in course of this work we observed a marginal restoration of DCPIP photoreduction with DPC as an electron donor letting us to believe that the damage of reaction center (RC) II may not be inconsequential in the inactiva- tion of OEC. Thus our objectives have been to examine the inacti- vation of RC II during senescence of Arabidopsis leaf and if the inactivation has a role in the decline in O 2 -evolution and the disorganization of thylakoid membrane. 2. Results 2.1. Changes in Chl and protein contents Fig. 1 describes the changes in the content of total Chl and total leaf protein in course of growth and development of basal leaf of Arabidopsis seedlings. Both these parameters increased, although Abbreviations: Chl, chlorophyll; DCMU, 3(3,4-dichlorophenyl)-1,1-dimethylurea; DCPIP, 2,6-dichlorophenolindophenol; DPC, 1,5-diphenylcarbazide; F v /F m , maximum quantum yield of primary photochemistry in the dark adapted state; F 0 , initial fluorescence; TL, thermoluminescence; qP, photochemical quenching; MDA, malondialdehyde; OEC, oxygen evolving complex; PSII, photosystem II; ROS, reac- tive oxygen species; RC II, reaction center II; V PSII , quantum yield of photosystem II in the light adapted state. * Corresponding author. Tel.: þ91 663 2432060; fax: þ91 663 2430158. E-mail address: basanti_b@hotmail.com (B. Biswal). 1 Present address: Department of Chemistry, CV Raman College of Engineering, Bidya Nagar, Mahura, Janla, Bhubaneswar 752054, Odisha, India. Contents lists available at SciVerse ScienceDirect Plant Physiology and Biochemistry journal homepage: www.elsevier.com/locate/plaphy 0981-9428/$ e see front matter Ó 2012 Elsevier Masson SAS. All rights reserved. http://dx.doi.org/10.1016/j.plaphy.2012.10.018 Plant Physiology and Biochemistry 62 (2013) 116e121