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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 (Phi(PSII)) 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 Phi(PSII) to PAR in relation to environmental factors through continuous season-long monitoring of ChlF of Phi(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 Phi(PSII) for A. ordosica was largely controlled by PAR, decreasing with increasing PAR. The rates of the response of diurnal Phi(PSII) to PAR (slopes) were down-regulated by seasonal changes in environmental factors, in particular, high values for daily mean air temperature (T-a) and vapor pressure deficit (VPD), and up-regulated with increases in soil water content (SWC). The y-intercepts of diurnal Phi(PSII)-PAR regressions were linearly related to maximal quantum yield of PSII photochemistry (F-v/F-m) during the growing season, increasing with increasing SWC. Our results confirm the suggestion by Durako (2012) that the y-intercept of diurnal Phi(PSII)-PAR relationship serves as a good proxy for F-v/F-m, in non-invasive monitoring of the physiological status of plants with the ChlF technique. (C) 2016 Elsevier Ltd. All rights reserved.