Aims Kobresia pygmaea is a typical low temperature-tolerant arid mesophyte,and mainly distributes at low slope and high mountains ranging from 3 000 to 5 960 m on the Qingzang Plateau.Low temperature is a frequently occurring abiotic stress factor during the plants growing season on the Qingzang Plateau.The objectives of this study were to analyze the photochemical and non-photochemical energy distribution characteristics of the photosystem Ⅱ(PSⅡ)reaction center of K.pygmaea leaves,and explore their quenching protection mechanism in response to low temperature stress.Methods Turf blocks(30 cm × 30 cm)of K.pygmaea meadow were collected from the Alpine Grassland Ecosystem Research Station of the Resource of Three Rivers,moved and kept in a culture room with air temperature of 24 ℃/18 ℃(day/night)at a diurnal photoperiod of 12 h and relative humidity of 45％;being irradiated with artificial LED light source of 500 μmol·m-2·s-1light intensity.When the turf black had been kept in culture room for one day,the measurements of chlorophyll fluorescence were performed immediately using chlorophyll fluorescence imager with built-in protocol.The trial of light response curves at different leaf temperatures,temperature response at a steady-state light intensity and light-temperature interaction effects were performed at controlled temperature using thermostatic control instruments.Based on the"Lake Model",the relative variation of the PSⅡ actual photochemical efficiency(ΦPSⅡ),the quantum yield of regulated energy dissipation(ΦNPQ)and non-regulated energy dissipation(ΦNO)were investigated.Furthermore,the interaction effects of low temperature and high light intensity was analyzed by two-way ANOVA of the general linear model(GLM).Important findings The maximum quantum efficiency of PSⅡ photochemistry(Fv/Fm and 1/Fo-1/Fm)were higher at 10 ℃ and their coefficient of variation(CV)was smaller relative to other temperatures.The rapid light-response curves of the PSⅡ relative electron transfer rate(rETR)showed a downward as a whole with the decreasing of temperature,and their initial slope(a)also decreased accordingly.Low temperature caused a decrease in ΦPSⅡ and ΦNO,and an increase in ΦNPQ,accompanied with enhancement of CV values.Under 1 000μmol·m-2 s-1 steady-state light intensity,the relative ratios of ΦPSⅡ∶ΦNPQ∶ΦNO at the temperature of 20,10,5,0,and-5 ℃were in turn 23:57:20,18∶63∶19,15∶68∶17,11∶75∶14,and 8∶80∶12,showing remarkable decrease inΦPSⅡ and increase in ΦNPQ The relative limitation of PSⅡ photochemical efficiency(LPPFD)increased gradually with the increase of light intensity,and also with the decrease of temperature.The two-way ANOVA of GLM showed that there were no interaction effects of low temperature and high light intensity in both PSⅡphotochemical and non-photochemical energy dissipation processes.Therefore,the excited energy absorbed by PSⅡ antenna pigments can be transferred and dissipated effectively by the photochemical energy conversion ofΦPsn and protective regulatory mechanisms of ΦNPQ avoiding the exacerbation of ΦNO,which exhibited a strong tolerance and adaptation to alpine habitats.However,the increased instability of photosynthetic activities at low temperatures indicated that the photosynthetic apparatus of K.pygmaea still suffer the stress while maintaining normal operation.It can be inferred that the low temperature is an important factor affecting the photosynthetic physiological process and limiting growth and development and its distribution.
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