Relationship Between Photosystem 2 Electron Transport and Photosynthetic CO<sub>2</sub> Assimilation Responses to Irradiance in Young Apple Tree Leaves

Abstract

The responses to irradiance of photosynthetic CO2 assimilation and photosystem 2 (PS2) electron transport were simultaneously studied by gas exchange and chlorophyll (Chl) fluorescence measurement in two-year-old apple tree leaves (Malus pumila Mill. cv. Tengmu No.1/Malus hupehensis Rehd). Net photosynthetic rate (PN) was saturated at photosynthetic photon flux density (PPFD) 600-1 100 (μmol m-2 s-1, while the PS2 non-cyclic electron transport (P-rate) showed a maximum at PPFD 800 μmol m-2 s-1. With PPFD increasing, either leaf potential photosynthetic CO2 assimilation activity (Fd/Fs) and PS2 maximal photochemical activity (Fv/Fm) decreased or the ratio of the inactive PS2 reaction centres (RC) [(Fi - Fo)/(Fm - Fo)] and the slow relaxing non-photochemical Chl fluorescence quenching (qs) increased from PPFD 1 200 μmol m-2 s-1, but cyclic electron transport around photosystem 1 (RFp), irradiance induced PS2 RC closure [(Fs - Fo')/Fm' - Fo')], and the fast and medium relaxing non-photochemical Chl fluorescence quenching (qf and qm) increased remarkably from PPFD 900 (μmol m-2 s-1. Hence leaf photosynthesis of young apple leaves saturated at PPFD 800 μmol m-2 s-1 and photoinhibition occurred above PPFD 900 μmol m-2 s-1. During the photoinhibition at different irradiances, young apple tree leaves could dissipate excess photons mainly by energy quenching and state transition mechanisms at PPFD 900-1 100 μmol m-2 s-1, but photosynthetic apparatus damage was unavoidable from PPFD 1 200 μmol m-2 s-1. We propose that Chl fluorescence parameter P-rate is superior to the gas exchange parameter PN and the Chl fluorescence parameter Fv/Fm as a definition of saturation irradiance and photoinhibition of plant leaves.