Temporal Dynamics In Photosynthetic Parameters Of Artemisia Ordosica In Semi-arid Area

Dynamics in photosynthetic parameters in Artemisia ordosica, which is a dominant shrub species in northwest semi-arid area, was poorly documented. We measured the leaf gas exchange of A. ordosica from 2012 to 2013 using a portable CO2 analyzer to quantify the temporal variations and its environmental controls. As a result, the diurnal variations in net photosynthesis rate (Pn), transpiration rate (E) and water use efficiency (WUE) were all asymmetric around 12:00, being higher in the morning than in the afternoon. The photosynthetic parameters of A-Ci curves, as maximum carboxylation efficiency (V cmax), maximum rate of electron transport (J max) and dark respiration in light (Rp), gradually declined after first rise appeared, with maximum values appearing in expanded leaf phase. These parameters had positive correlations with air temperature (Ta) in growing season. Apparent quantum yield (AQY) had a siginificantly positive correlation with Ta, and gradually decline after first rise appeared, being highest at the beginning of expanded leaf phase. The dark respiration (Rd) gradually declined in growing season, being highest in leaf expanding period. Maximum photosynthetic capacity Amax) and stomatal conductance (gc) showed similar pattern with Rd. However maximum values of Amax and gc appeared on expanded leaf phase of A. ordosica. With Rd and AQY affecting indirectly, light compensation point (LCP) and light saturation point (LSP) had similar variations that was to increased gradually after decline sharply in growing season. Vcmax and Amax of A. ordosica varied in the range 0～140 μmol·m-2·s-1 and 0～50 μmol CO2·m-2·s-1 in growing season, respectively. Moreover AQY was less than 0.05 μmol·μmol-1 and maximum value of LSP was 1514 μmol·m-2·s-1. LCP changed from 20 μmol·m-2·s-1 to 120 μmol·m-2·s-1. In different phonological phase, maximum averages of parameters, as Vcmax,Jmax, Amax and AQY in natural state, appeared on expanded leaf period. From date of nitrogen addition to early expanded leaf period, parameters of A-Ci and A-PAR curves under nitrogen addition were higher than that in nature, however parameters of A-Ci and A-PAR curves with two treatment were little different in expanded leaf period and leaf defoliation. Because of nitrogen addition, annual averages of Vcmax, Jmax and Amax with nitrogen addition was higher than that in nature. Models of simulating photosynthesis response to light are good tools, but there are more practices for parameter uniqueness.It was concluded that Ta and soil moisture were mainly environmental factors controlling the seasonal variation in photosynthetic parameters of Artemisia ordosica. Nitrogen addition increased the magnitude of photosynthetic parameters of Artemisia ordosica. The photosynthetic characteristics of A. ordosica, such as wide amplitude, low utilization rate of weak light, high CO2 utilization and photosynthetic capacity, reflected an adaptative strategies to semi-arid conditions of low water availability and high summer temperature.