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

CO 2 concentrations and soil moisture conditions seriously affect tree growth and physiological mechanisms. CO 2 responses of photosynthesis are an important part of plant physiology and ecology research. This study investigated the photosynthetic CO 2 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 CO 2 responses and soil moisture. CO 2 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 ( P n ) and carboxylation efficiency ( CE ) ranged from 42.8% to 83.2%. When RWC fell outside these ranges, the photosynthetic capacity ( P nmax ), CE , and CO 2 saturation point ( CSP ) decreased. CO 2 response curves and three parameters, CE , CO 2 compensation point ( Γ ), and photorespiration rate ( R p ), 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 CO 2 response curves and photosynthetic parameters better.