Aims Several local varieties of wheat (Triticum aestivum) developed by Northwest Institute of Plateau Biology, Chinese Academy of Sciences, are widely cultivated in the agricultural regions in Qinghai-Xizang Plateau. These varieties are well adapted to multiple environmental stresses such as low temperature, strong solar radiation, and drought. The objective of this study was to determine the responses of PSII photochemical efficiency to high solar irradiance in leaves of four wheat varieties. We examined whether photo-inhibition was appeared in wheat varieties and analysed variations of quantum yield of quenching due to light-induced and non-light-induced.
Methods Field experiments were conducted on the farmland of Xiangride, which is located in the eastern side of Caidamu Basin, Qinghai Province. Four local wheat varieties were used during the heading stage in 2013. Measurements of photochemical efficiency and quantum yield were made on the abaxial surface of flag leaves facing the Sun by using a FMS-2 fluorometer, and the content of photosynthetic pigments and specific leaf weight (SLW) were concurrently determined. Pulse-modulated in-vivo chlorophyll fluorescence technique was used to obtain rapid information on photosynthetic processes. The maximum quantum efficiency of PSII photochemistry (F_v/F_m) was determined at 8:30, 12:00 and 16:30 on clear days after allowing for 20 min dark adaptation with leaf clips. The PSII maximal and actual photochemical efficiency (F_v′/F_m′ and Φ_PSII), the PSII photochemical and non-photochemical quenching coefficient (q_P and NPQ) were analyzed between morning and afternoon using inner actinic light with photosynthetically active photon flux density at 1120 μmol photons∙m^-2∙s^-1. Furthermore, along with analysis of the fraction of PSII reaction centers that are opened (q_L), the quantum yield of quenching due to light-induced processes (Φ_NPQ) and non-light-induced processes (Φ_NO) were explored.
Important findings There were significant differences in the content of photosynthetic pigments and SLW among the four wheat varieties. Under conditions of clear days, the flag leaves exhibited marked depressions in F_v/F_m at three typical times when determined after 20 min dark adaptation. At a given light intensity, the values of F_v′/F_m′ were significantly reduced in the afternoon due to influences by long-lasting high-light irradiation, and Φ_PSII showed little differences among the four wheat varieties and no difference between morning and afternoon. There were almost similar variations in q_P and NPQ among the four wheat varieties, suggesting that q_P and NPQ belong to instinct property and are influenced by the accumulative stresses of high-light intensity. The fractions of Φ_NPQ were higher than that of Φ_NO in the four wheat varieties and the up-regulatory of Φ_NPQ in the afternoon indicated that the photosynthetic apparatus in these wheat varieties had already acclimated to strong solar irradiation in agricultural regions of Qinghai-Xizang Plateau.