Diurnal response of effective quantum yield of PSII photochemistry to irradiance as an indicator of photosynthetic acclimation to stressed environments revealed in a xerophytic species

•Monitoring chlorophyll fluorescence (ChlF) has been of growing interest.•Lack of stable indicators challenge the application of ChlF in the field.•Seasonal changes in diurnal ΦPSII-PAR regression is associated with soil moisture.•Y-intercept of the diurnal ΦPSII-PAR regression can serve as a good eco-indicator.•Regression slopes reflect the sensitivity to environmental stress. Monitoring responses of plants to stresses in situ using the chlorophyll-fluorescence (ChlF) technique has been of growing interest, but challenged by lack of stable daytime indicators, limiting the method’s application. Seasonal changes in diurnal responses of effective quantum yield of photosystem II (PSII) photochemistry (ΦPSII) to photosynthetically active radiation (PAR) have been assumed to reflect changes in physiological status of plants in changing environments. We examined the responses of diurnal ΦPSII to PAR in relation to environmental factors through continuous season-long monitoring of ChlF of PSII in a desert shrub species, Artemisia ordosica. Response data were analysed with a modification of the diurnal regression method by Durako (2012). Diurnal variation in ΦPSII for A. ordosica was largely controlled by PAR, decreasing with increasing PAR. The rates of the response of diurnal ФPSII to PAR (slopes) were down-regulated by seasonal changes in environmental factors, in particular, high values for daily mean air temperature (Ta) and vapor pressure deficit (VPD), and up-regulated with increases in soil water content (SWC). The y-intercepts of diurnal ФPSII-PAR regressions were linearly related to maximal quantum yield of PSII photochemistry (Fv/Fm) during the growing season, increasing with increasing SWC. Our results confirm the suggestion by Durako (2012) that the y-intercept of diurnal ФPSII-PAR relationship serves as a good proxy for Fv/Fm in non-invasive monitoring of the physiological status of plants with the ChlF technique.