Seasonal changes in photosynthesis, antioxidant systems and ELIP expression in a thermonastic and non-thermonastic Rhododendron species: A comparison of photoprotective strategies in overwintering plants

Leaves of overwintering evergreen rhododendrons are typically exposed to freezing temperatures and high light during winters which can potentially result in photon flux exceeding that required for photochemistry. This excess energy, if not dissipated as heat or fluorescence, may cause photooxidative damage to PSII. The goal of this study is to compare the photoprotection strategies during seasonal cold acclimation (CA) in two Rhododendron species (R. catawbiense Michx. and R. ponticum L.) that are divergent in their leaf freezing tolerance and thermonastic behaviour (temperature-induced leaf movement). R. catawbiense exhibits thermonasty while R. ponticum does not. Differences in leaf freezing tolerance (LT50), photosynthesis, photoinhibition, early light-induced proteins (ELIPs) gene expression, and accumulation of antioxidantmetabolites and enzymes during seasonal CAwere investigated. During seasonal CA, maximum photosynthetic rate (Pmax) and maximum quantum efficiency of PSII (Fv=Fm) were significantly down-regulated. Compared with R. catawbiense, R. ponticum showed less photoinhibition and higher overall accumulation (in magnitude) of antioxidant systems while R. catawbiense exhibited more efficient up-regulation of ELIP expression and antioxidant system (i.e., greater efficiency of increasing this pool inwintermonths relative to the summer levels). The two species respond differently to winter conditions and have evolved strategies to avoid, reduce and/or tolerate photooxidative stress in winter. These include down-regulation of photosynthesis and up-regulation of ELIPs and antioxidant systems, together with specialized leaf anatomy and thermonasty behaviour. Published by Elsevier Ireland Ltd. Abbreviations: APX, ascorbate peroxidase; CAT, catalase; CA, cold acclimation; ELIP, early light-induced protein; Fv=Fm , maximum quantum efficiency of PSII photochemistry; LSP, light saturation point; LFT, leaf freezing tolerance; MDA, malondialdehyde; NA, non-acclimation; PAR, photosynthetically active radiation; Pmax, maximum photosynthetic rate; Pn, net photosynthetic rate; PCR, polymerase chain reaction; POD, peroxidase; PPFD, photosynthetic photon flux density; PSII, photosystem II; ROS, reactive oxygen species; RT, reverse transcriptase; SOD, superoxide dismutase; AsA, ascorbate acid; GSH, glutathione. * Corresponding author. Tel.: +1 515 294 0031; fax: +1 515 294 0730. E-mail address: [email protected] (R. Arora).