Experimental and simulated CO2 responses of photosynthesis in leaves of Hippophae rhamnoides L. under different soil water conditions

CO2 concentrations and soil moisture conditions seriously affect tree growth and physiological mechanisms. CO2 responses of photosynthesis are an important part of plant physiology and ecology research. This study investigated the photosynthetic CO2 responses in the leaves of two-year-old Hippophae rhamnoides L. under eight soil water conditions in a semi-arid loess hilly region, and discussed the quantitative relationship between CO2 responses and soil moisture. CO2 response curves and parameters were fitted using a rectangular hyperbola model, non-rectangular hyperbola model, exponential equation, and modified rectangular hyperbola model. Results revealed that the relative soil water content (RWC) required to maintain a high photosynthetic rate (Pn) and carboxylation efficiency (CE) ranged from 42.8% to 83.2%. When RWC fell outside these ranges, the photosynthetic capacity (Pnmax), CE, and CO2 saturation point (CSP) decreased. CO2 response curves and three parameters, CE, CO2 compensation point ({Gamma}), and photorespiration rate (Rp), were well fitted by the four models when RWC was appropriate. When RWC exceeded the optimal range, only the modified rectangular hyperbola model fitted the CO2 response curves and photosynthetic parameters better.