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During the period of senescence of desert plant Alhagi sparsifolia Shap. the maximum photochemical quantum yield measured as variable to maximum fluorescence ratio (Fv/Fm) remained relatively high, although the number of active reaction centres per cross section (RCs) decreased significantly. The efficiency of electron acceptors beyond the primary quinone acceptor (QA) decreased. The effect of temperature and irradiance on photosystem activity was maximum after 6 d. Our results suggest that: 1) the down-regulation of photosystem activity was due to the decline of both RCs and electron acceptance between plastoquinone (PQ) and cytochrome (cyt) b6/f; 2) photosystem activity presented negative correlation with daily mean temperature, and 3) reduction of daily sunshine period and increase of temperature at noon can stimulate the speed of senescence.