RESPONSE OF PHOTOSYNTHETIC RATE AND STOMATAL CONDUCTANCE OF RICE TO LIGHT INTENSITY AND CO_2 CONCENTRATION IN NORTHERN CHINA

The response of photosynthetic rate and stomatal conductance of rice (Oryza saliva var. Japonica) to changes in light intensity and CO2 concentrations was studied using a Li-6400 in Northern China. In general, photosynthetic rates increased with light intensity and CO2 concentrations and could be expressed by a Michaelis-Menten function. Apparent quantum yield increased with CO2 concentrations but decreased slightly when CO2 concentrations exceeded 800 mol·mol-1. Similarly, apparent carboxylation efficiency increased with light intensity but decreased slightly when light intensity exceeded 1600 mol·m-2·s-1. The response of stomatal conductance to light intensity can also be expressed by a Michaelis-Menten function, whereas the response to CO2 concentrations can be expressed by a hyperbola. If the combined effects of light intensity and CO2 concentrations are considered, the photosynthetic rate can be estimated by a Michaelis-Menten equation with a maximum photosynthetic rate of 71.74 mol·m-2·s-1. Apparent quantum yield was 0.056 0 mol CO2·mol-1 photons and carboxylation rate was 0.1031 mol·m-2·s-1/mol·mol-1. The response of stomatal conductance (Gsw) to light intensity can be expressed by a Michaelis-Menten function too, but the response to CO2 concentrations (Cs) can be simulated by the equation:Gsw=Gmax,c/(1+Cs/Cs0) where Gmax,c is maximum stom- atal conductance of stomatal response to CO2 under a defined light intensity and CS0 is a constant, because the stomatal conductance decreases with increases in CO2 concentrations, stomatal conductance can be estimated by Gsw=Gmax(PFD/PFDc)/[(1+PFD/PFDc)(1+Cs/Cs0)]+Gct in response to the combined effects of CO2 concentration and light intensity (Ⅰ) . The potential maximum stomatal conductance, Gmax, can reach 0.670 9 mol·m-2·s-1 under saturated light levels and CO2 near 0 mol·mol-1. Ball-Berry model and its revised form can still be used to express the coupled relationship of stomatal conductance and photosynthesis. The simulation precision will be improved if saturation vapor pressure deficit, Ds, at the leaf surface was used in the Ball-Berry model instead of relative humidity.