[Response of strawberry leaves photosynthesis to strong light and its mechanism]

In this paper, PAM-2000 portable chlorophyll fluorometer and HCM-1000 photosynthesis measurement system were applied to measure the apparent quantum efficiency (AQY), initial fluorescence (Fo), maximal photochemical efficiency of PSII (Fv/Fm), maximal fluorescence (Fm), photochemical quenching (qP), non-photochemical quenching (qN), actual quantum yield of PS II electron transport (phiPSII), electron transport rate (ETR), amount of inactive PS II reaction centers (Fi-Fo), Q(A) reduction rate, proportion of Q(B)-non-reducing PS II reaction centers [(Fi-Fo)/(Fp-Fo)], and energy-dependent quenching (qE), photoinhibitory quenching (qI) and state-transition quenching (qT) of non-photochemical quenching of strawberry leaves. The results showed that the Fv/Fm, Fm, Fi-Fo and Q(A) reduction rate decreased in strong light and increased during subsequent dark recovery, but (Fi-Fo)/(Fp-Fo) was in reverse. They changed drastically within the first 10 minutes in strong light or in subsequent darkness. In strong light, phiPSII, ETR and qP increased firstly and then decreased, while qN decreased drastically firstly and then increased slightly. After exposure to strong light for 4 hours, the AQY of two varieties "Toyonoka" and "Houkouwase" decreased by 20.9% and 37.5%, respectively, and qE was 89.1% and 87.1%, respectively in NPQ (qE + qI + qT). In strong light, "Toyonoka" showed less changes than "Houkouwase" in Fo, Fv/Fm, Fm, ETR, phiPSII and AQY. After treated with DTT, the Fv/Fm and Fm were lower, but Fo was much higher than control. It is deduced that in strong light, xanthophyll cycle-dependent non-radiative energy dissipation and pH-dependent heat dissipation could play an important protective role against photo-damage to the photosynthetic apparatus in strawberry leaves.