DOI: 10.1007/s11099-016-0190-1 PHOTOSYNTHETICA 54 (3): 359-366, 2016 359 Diminution of photosynthesis in rice (Oryza sativa L.) seedlings under elevated CO 2 concentration and increased temperature S. PANIGRAHI * , M.K. PRADHAN * , D.K. PANDA ** , S.K. PANDA *** , and P.N. JOSHI *,+ Laboratory of Biophysics and Biochemistry, Anchal College, Padampur, P.O.: Rajborasambar, Dist: Bargarh, 768036 Odisha, India * Women's College, Padampur, P.O.: Rajborasambar, Dist: Bargarh, 768036 Odisha, India ** Royal College of Pharmacy and Health Sciences, Berhamapur, P.O.: Berhamapur, Dist: Ganjam, 760002 Odisha, India *** Abstract The photosynthetic responses to elevated CO 2 concentration (EC) at ambient and ambient +4°C temperature were assessed in the second leaf of rice (Oryza sativa L.) seedlings. The duration of different leaf developmental phases, as characterised by changes in photosynthetic pigment contents and photochemical potential, was protracted in the seedlings grown under EC. On the other hand, a temporal shift in the phases of development with an early onset of senescence was observed in the seedlings grown under EC at ambient +4°C temperature. The contents of carotenoids, β-carotene, and xanthophyll cycle pigments revealed that EC downregulated the protective mechanism of photosynthetic apparatus against oxidative damages, whereas this mechanism assumed higher significance under EC at ambient +4°C temperature. We observed an enhancement in electron transport activity, photochemical potential, and net photosynthesis in spite of a loss in photostasis of photosynthesis under EC. On the other hand, the loss in photostasis of photosynthesis was exacerbated under EC at ambient +4°C temperature due to the decline in electron transport activity, photochemical potential, and net photosynthesis. Additional key words: chlorophyll fluorescence; gas exchange; intrinsic water-use efficiency; lutein; violaxanthin; zeaxanthin. Introduction Alterations in structure and function of chloroplasts in re- sponse to increased atmospheric CO 2 concentration have been a matter of concern (Griffin et al. 2001, Zuo et al. 2002, Zhang et al. 2012). The elevated CO 2 concentration (EC) affects the developmental characteristics of the photo- synthetic organelles in general (Nie et al. 1995, Miller et al. 1997). It induces a change in the volume density of thylakoids (Kutík et al. 1995) and increases the number and size of starch grains in chloroplasts (Zhang et al. 2012), besides increasing the ratio of stromal to granal thylakoids (Griffin et al. 2001). A higher number of plasto- globuli is observed in chloroplasts of leaves developed under EC (Velikova et al. 2009). CO 2 at a high concentration is known to improve the activities of antioxidant enzymes and it reduces the accumulation of malondialdehyde (MDA) (Zhang et al. 2012). Although contents of total chlorophyll (Chl) and Chl per unit fresh mass of leaf are reduced under EC, the rate of photosynthesis is increased (Zhang et al. 2012). Higher CO 2 concentration is also known to bring changes in the structure and composition of thylakoid membranes during development of the organelle (Robertson and Leech 1995). High temperature, another major environmental stress factor, is also known to modulate the chloroplast functions (Joshi et al. 2013b, Mathur et al. 2014). The stress dis- mantles the thylakoid membrane, uncouples the light- harvesting complex II, reduces the oxygen evolution (Wen et al. 2005), and affects the activity of PSII (Pospíšil and ——— Received 11 February 2015, accepted 6 November 2015, published as online-first 29 November 2015. + Corresponding author; e-mail: padmanava.joshi@gmail.com Abbreviations: A – antheraxanthin; ACT – ambient CO2 under ambient temperature; Car – carotenoids; Chl – chlorophyll; CS – cross section; DAE – days of the experiment; DI – dissipation; EC – elevated CO2 concentration; ET – ambient temperature +4°C; Fm – maximum fluorescence; Fv – variable fluorescence; gs – stomatal conductance; HPLC – high performance liquid chromatography; IPCC – Intergovernmental Panel for Climate Change; Lut – lutein; MDA – malondialdehyde; PN – net photosynthetic rate; PSA – photosynthetic apparatus; ROS – reactive oxygen species; V – violaxanthin; WUEi – intrinsic water-use efficiency; Z – zeaxanthin; β-Car – β-carotene. Acknowledgement: Financial assistance (MRP F.35-161/2008(SR) Dt 18 Feb 2010) from UGC, New Delhi, is gratefully acknowledged.