Abstract: Taking Malus domestica Borkh as test object, this paper studied the primary photochemical reactions and xanthophyll cycle in its fruit peel in respons
e to the diurnal changes of incident photon flux density (PFD) and air temperature in sunny days. With the increase of PFD and air temperature in a daily cycle, a severe photo-inhibition of the primary photochemical reactions in M. domestica fruit peel occurred from 12:00 to 14:00. The relative variable fluorescence at 300 μs of chlorophyll a fluorescence transient (W_k) did not change significantly through the day, which indicated that the activity of oxygen evolving complex (OEC) was not damaged by strong light and high temperature. However, the efficiency that a trapped exciton moved an electron into the electron transport chain beyond Q_A- (Ψ_o) was reduced from 12:00 to 14:00, indicating that the acceptor side of PSⅡ in apple fruit peel was damaged. Strong light decreased the density of PSⅡ reaction centers per excited crosssection (RC/CS), which induced the increase of the energy absorption per active reaction center (ABS/RC). However, the excited energy was not able to be efficiently used via photochemical reaction (TR_o/RC), resulting in an increase in non-photochemical energy dissipation per active reaction center (DI_o/RC). Along with the appearance of photo
inhibition, the de-epoxidation level of xanthophyll pigment pool (PRI) increased markedly, showing that the xanthophyll cycle in fruit peel was enhanced by strong light to dissipate excess excitation energy to prevent photosynthetic apparatus from further damage. Both strong light and high temperature enhanced the photoinhibition in apple fruit peel, and the effect of strong light was significantly more prominent than that of high temperature during a day.

Key words: Malus domestica Borkh, fruit peel, photo-inhibition, primary photochemical reaction, xanthophyll cycle, spectral reflectance, cotton, salinity stress, fertilization, nitrogen utilization.