  Citation: Shi, Q.; Sun, H.; Timm, S.; Zhang, S.; Huang, W. Photorespiration Alleviates Photoinhibition of Photosystem I under Fluctuating Light in Tomato. Plants 2022, 11, 195. https:// doi.org/10.3390/plants11020195 Academic Editors: Pirjo Mäkelä, Mercè Llugany, Peter A. Roussos and Mumtaz Cheema Received: 4 December 2021 Accepted: 31 December 2021 Published: 12 January 2022 Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affil- iations. Copyright: © 2022 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/). plants Article Photorespiration Alleviates Photoinhibition of Photosystem I under Fluctuating Light in Tomato Qi Shi 1,2,† , Hu Sun 1,2,† , Stefan Timm 3 , Shibao Zhang 1 and Wei Huang 1, * 1 Kunming Institute of Botany, Chinese Academy of Sciences, Kunming 650201, China; shiqi@mail.kib.ac.cn (Q.S.); sunhu19@mails.ucas.ac.cn (H.S.); sbzhang@mail.kib.ac.cn (S.Z.) 2 University of Chinese Academy of Sciences, Beijing 100049, China 3 Plant Physiology Department, University of Rostock, D-18051 Rostock, Germany; stefan.timm@uni-rostock.de * Correspondence: huangwei@mail.kib.ac.cn † These authors contributed equally to this work. Abstract: Fluctuating light (FL) is a typical natural light stress that can cause photodamage to photosystem I (PSI). However, the effect of growth light on FL-induced PSI photoinhibition remains controversial. Plants grown under high light enhance photorespiration to sustain photosynthesis, but the contribution of photorespiration to PSI photoprotection under FL is largely unknown. In this study, we examined the photosynthetic performance under FL in tomato (Lycopersicon esculentum) plants grown under high light (HL-plants) and moderate light (ML-plants). After an abrupt increase in illumination, the over-reduction of PSI was lowered in HL-plants, resulting in a lower FL-induced PSI photoinhibition. HL-plants displayed higher capacities for CO 2 fixation and photorespiration than ML-plants. Within the first 60 s after transition from low to high light, PSII electron transport was much higher in HL-plants, but the gross CO 2 assimilation rate showed no significant difference between them. Therefore, upon a sudden increase in illumination, the difference in PSII electron transport between HL- and ML-plants was not attributed to the Calvin–Benson cycle but was caused by the change in photorespiration. These results indicated that the higher photorespiration in HL- plants enhanced the PSI electron sink downstream under FL, which mitigated the over-reduction of PSI and thus alleviated PSI photoinhibition under FL. Taking together, we here for the first time propose that photorespiration acts as a safety valve for PSI photoprotection under FL. Keywords: photorespiration; cyclic electron flow; photoinhibition; photoprotection; photosystem I 1. Introduction Growth light significantly affects photosynthetic performance in plants. Plants usually modulate their biochemical composition and leaf morphology to acclimate to the specific growth light conditions [1–5]. In general, plants grown under high light (HL-plants) have higher content of proteins and enzymes involving in photosynthetic electron flow and the Calvin–Benson cycle than plants grown under low light [6,7]. These characteristics favors the higher photosynthetic capacity in HL-plants. Concomitantly, the rate of ribulose- 1,5-bisphosphate (RuBP) oxygenation is also increased in HL-plants due to the higher ribulose-1,5-bisphosphate carboxylase/oxygenase (RubisCO) content [8]. Photorespiration is essential for the normal photosynthesis ambient CO 2 and oxygen [9]. A stronger electron flow for photorespiration can protect photosystem II (PSII) by consuming the excess light energy [9,10]. However, the role of photorespiration in protecting photosystem I (PSI) under fluctuating light is not well known. In natural habitats, leaves usually experience fluctuations of illumination owing to cloud, wind and changing leaf sun angle [11,12]. Under fluctuating light (FL), light absorption and PSII electron flow rapidly increased after an abrupt increase in light inten- sity [13,14]. Meanwhile, stomatal opening and the activation of the Calvin–Benson cycle Plants 2022, 11, 195. https://doi.org/10.3390/plants11020195 https://www.mdpi.com/journal/plants