In order to probe into the responses of photosynthesis of C₄ desert species Haloxylon ammodendron and Calligonum mongolicum under different moisture conditions, thus providing a scientific basis for the conservation of desert ecosystem and restoration of damaged regions, the diurnal courses of their net photosynthetic rate (Pn) and photochemical efficiency of PSⅡ(Fv/Fm), and the responses of Pn to different light intensities under three different water conditions were studied in the desert in the middle section of the Hexi Corridor regions in Gansu Province, China (39°20′N， 100°08′E，1380 m a.s.l.). The three water conditions were arid environment (AE), near-ground moist air condition during post-rain period (PR), and condition of artificial supplement of soil water (SW). Adult plants (more than 20-years-old) of H. ammodendron and C. mongolicum, were selected to be studied.
The results showed that the diurnal course of Pn of H. ammodendron and C. mongolicum exhibited a non-typical bimodal pattern with a decline under intensive irradiation in the arid environment, but a unimodal pattern under the PR and SW. The diurnal changes in Fv/Fm exhibited an obvious decline under intensive irradiation in the arid environment, but did not under the other two conditions. Those suggested that there existed photoinhibition under the arid environment, but no under higher air humidity and soil moisture content conditions. Both two species performed higher daily mean Pn under PR and SW than that under arid environment. The daily mean Pn for both species one to two days after SW was higher than that one day after the PR. The mean Pn for the first day after SW was obviously higher than that in the second day. Compared to that under the arid environment, the daily mean Pn of H. ammodendron was 3.1 μmolCO₂•m^-2•s^-1 higher under the PR, and 9.7 (the first day) and 7 μmol CO₂•m^-2•s^-1 (the second day) higher under the SW, respectively. Once water condition was improved, the light compensation points of the two species were reduced, but their light saturation point became higher, light saturated rate of photosynthesis and apparent quantum yield were increased. The apparent quantum yield and the light saturated rate of photosynthesis under the PR were higher and less respectively than that under the SW.
It was concluded that water stress causes the photoinhibition of H. ammodendron and C. mongolicum. Increasing air humidity or soil moisture content can avoid photoinhibition, and thus increase the efficiency for solar energy utilization.