Relationship between photosystem 2 electron transport and photosynthetic CO2 assimilation responses to irradiance in young apple tree leaves

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 (P-N) was saturated at photosynthetic photon flux density (PPFD) 600-1 100 mumol m(-2) s(-1), while the PS2 non-cyclic electron transport (P-rate) showed a maximum at PPFD 800 mumol m(-2) s(-1). With PPFD increasing, either leaf potential photosynthetic CO2 assimilation activity (F-d/F-s) and PS2 maximal photochemical activity (F-v/F-m) decreased or the ratio of the inactive PS2 reaction centres (RC) [(F-i - F-0)/(F-m - F-0)] and the slow relaxing non-photochemical Chl fluorescence quenching (q(s)) increased from PPFD 1 200 imol M s, but cyclic electron transport around photosystem I (RFp), irradiance induced PS2 RC closure [(F-s - F-0('))/(F-m(')- F-0)], and the fast and medium relaxing non-photochemical Chl fluorescence quenching (q(f) and q(M)) increased remarkably from PPFD 900 mumol m(-2) s(-1). Hence leaf photosynthesis of young apple leaves saturated at PPFD 800 mumol m(-2) s(-1) and photoinhibition occurred above PPFD 900 mumol, m(-2) s(-1). During the photoinhibition at different itradiances, young apple tree leaves could dissipate excess photons mainly by energy quenching and state transition mechanisms at PPFD 900-1 100 mumol m(-2) s(-1), but photosynthetic apparatus damage was unavoidable from PPFD 1 200 mumol 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 F-v/F-m as a definition of saturation irradiance and photoinhibition of plant leaves.