24-epibrassinolide improved chilled tomato photosyntheticperformance by stabilizing electron transport chain and function of photosystem II

Abbreviations : ABS/CS - absorption flux per CS; ABS/RC - absorption flux (exciting PS II antenna of Chl a molecules) per RC; AQY - apparent quantum yield; BRs - brassinosteroids; CEF - cyclic electron transport around PS I; c i - intercellular CO 2 concentration; CS - cros section; E - transpiration rate; EBR - 24-epibrassinolide; ET o /CS – electron transport flux per CS; ET o /RC - electron transport flux (further than Q A- ) per RC; ETR - electron transport rate; F m - maximal fluorescence yield; F o - minimal fluorescence yield; F v /F m - maximal quantum yield of PS II photochemistry; g s - stomatal conductance; M o - approximated initial slop (in ms -1 ) of the fluorescence transient normalized on the maximal variable fluorescence F v ; NPQ - nonphotochemical quenching coefficient; OEC - oxygen-evolving complex; OJIP curve - Chl a fluorescence transient; PI ABS - performance index for energy conservation from photons absorbed by PS II until the reduction of intersystem electron acceptors; P m - maximum P700 oxidation; P N - net photosynthetic rate; P N,max - maximum net photosynthetic rate; PPFD - photosynthetic photon flux density; PS I - photosystem I; PS II - photosystem II; qP - photochemical quenching coefficient; RC/CS - density of Q A -reducing PS II RCs per CS; RCs - PS II reaction centers; ROS - reactive oxygen species; TR o /CS - trapped energy flux per CS; TR o /RC - trapped energy flux (leading to Q A reduction) per RC; V J - relative variable fluorescence at the J-step; W K - normalized relative variable fluorescence at the K step; Y(I) - effective photochemical quantum yield of PS I; Y(II) - effective PS II quantum yield; Y(NA) - quantum yield of non-photochemical energy dissipation of reaction centers due to PS I acceptor side limitation; Y(ND) - quantum yield of non-photochemical energy dissipation in reaction centers due to PS I donor side limitation; Y(NPQ) - quantum yield of regulated energy dissipation; Y(NO) - quantum yield of nonregulated energy dissipation; φ Eo - quantum yield for electron transport (ET); φ Po - maximum quantum yield for primary photochemistry; Ψ o - probability that a trapped exciton moves an electron into the electron transport chain beyond Q A- . Abstract To explore the protective mechanisms of brassinosteroids in the chill-induced photoinhibition in tomato ( Solanum lycopersicum ), we studied the effect of foliar sprayed 24-epibrassinolide (EBR, 0.1µM) on the gas exchange, chlorophyll fluorescence characteristics, and chlorophyll a fluorescence transient in tomato seedlings under chilling stress (a temperature of 8 ℃ and an irradiance of 200 µmol m -2 s -1 ) for 4 d. Results showed that chilling significantly inhibited CO 2 assimilation and induced photoinhibition of photosystem II (PS II). However, photosystem I (PS I) was relatively tolerant to chilling stress, which was due to the downregulation of PS II activity and increase of cyclic electron transport around PS I (CEF). Chilling led to the inactivation of PS II reaction centers (RCs) and blocked the electron transport at the PS II acceptor side, but did not affect the oxygen-evolving complex (OEC) on the donor side of PS II. Exogenous EBR could alleviate chill-induced PS II photoinhibition mainly by the increase of CO 2 assimilation and thermal dissipation of excitation energy in the PS II antennae, while the protective effect of CEF was relatively smaller. This study demonstrated that EBR maintained the stability of the electron transport chain and the function of PS II in chilled tomatoes. EBR promoted the absorption (ABS/CS), trapping (TR o /CS), and electron transport (ET o /CS) per leaf area in tomatoes under chilling stress, which was due to increasing the density of active reaction centers (RC/CS), rather than the activity of active RCs.