A new method to estimate photosynthetic parameters through net assimilation rate-intercellular space CO₂...

Gas exchange (GE) and chlorophyll fluorescence (CF) measurements are widely used to noninvasively study photosynthetic parameters, for example the rates of maximum Rubisco carboxylation (V_cmax ), electron transport rate (J), daytime respiration (R_d ) and mesophyll conductance (g_m ). Existing methods for fitting GE data (net assimilation rate-intercellular space CO₂ concentration (A-C_i ) curve) are based on two assumptions: g_m is unvaried with CO₂ concentration in the intercellular space (C_i ); and light absorption (α) and the proportion of quanta absorbed by photosystem II (β) are constant in the data set. These may result in significant bias in estimating photosynthetic parameters. To avoid the above-mentioned hypotheses, we present a new method for fitting A-C_i curves and CF data simultaneously. This method was applied to a data set obtained from cucumber (Cucumis sativus) leaves of various leaf ages and grown under eight different light conditions. The new method had significantly lower root mean square error and a lower rate of failures compared with previously published methods (6.72% versus 24.1%, respectively) and the effect of light conditions on V_cmax and J was better observed. Furthermore, the new method allows the estimation of a new parameter, the fraction of incoming irradiance harvested by photosystem II, and the dependence of g_m on C_i .