Most plants growing in temperate desert zone exhibit brief temperature-induced inhibition of photosynthesis at midday in the summer. Heat stress has been suggested to restrain the photosynthesis of desert plants like Alhagi sparsifolia S. It is therefore possible that high midday temperatures damage photosynthetic tissues, leading to the observed inhibition of photosynthesis. In this study, we investigated the mechanisms underlying heat-induced inhibition of photosynthesis in A. sparsifolia, a dominant species found at the transition zone between oasis and sandy desert on the southern fringe of the Taklamakan desert. The chlorophyll (Chl) a fluorescence induction kinetics and CO 2 response curves were used to analyze the thermodynamic characters of both photosystem II (PSII) and Rubisco after leaves were exposed to heat stress. When the leaves were heated to temperatures below 43ºC, the initial fluorescence of the dark-adapted state (F o ), and the maximum photochemical efficiency of PSII (F v /F m ), the number of active reaction centers per cross section (RCs) and the leaf vitality index (PI) increased or declined moderately. These responses were reversed, however, upon cooling. Moreover, the energy allocation in PSII remained stable. The gradual appearance of a K point in the fluorescence curve at 48ºC indicated that higher temperatures strongly impaired PSII and caused irreversible damage. As the leaf temperature increased, the activity of Rubisco first increased to a maximum at 34ºC and then decreased as the temperature rose higher. Under high-temperature stress, cell began to accumulate oxidative species, including ammoniacal nitrogen, hydrogen peroxide (H 2 O 2 ), and superoxide (O 2˙¯) , suggesting that disruption of photosynthesis may result from oxidative damage to photosynthetic proteins and thylakoid membranes. Under heat stress, the biosynthesis of nonenzyme radical scavenging carotenoids (Cars) increased. We suggest that although elevated temperature affects the heat-sensitive components comprising of PSII and Rubisco, under moderately high temperature the decrease in photosynthesis is mostly due to inactivation of dark reactions.

During the period of senescence of desert plant Alhagi sparsifolia Shap. the maximum photochemical quantum yield measured as variable to maximum fluorescence ratio (F v /F m ) 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 (Q A ) 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.