Accounting for the decrease of photosystem photochemical efficiency with increasing irradiance to estimate quantum yield of leaf photosynthesis

Maximum quantum yield for leaf CO2 assimilation under limiting light conditions (I broken vertical bar (CO2LL)) is commonly estimated as the slope of the linear regression of net photosynthetic rate against absorbed irradiance over a range of low-irradiance conditions. Methodological errors associated with this estimation have often been attributed either to light absorptance by non-photosynthetic pigments or to some data points being beyond the linear range of the irradiance response, both causing an underestimation of I broken vertical bar (CO2LL). We demonstrate here that a decrease in photosystem (PS) photochemical efficiency with increasing irradiance, even at very low levels, is another source of error that causes a systematic underestimation of I broken vertical bar (CO2LL). A model method accounting for this error was developed, and was used to estimate I broken vertical bar (CO2LL) from simultaneous measurements of gas exchange and chlorophyll fluorescence on leaves using various combinations of species, CO2, O-2,O- or leaf temperature levels. The conventional linear regression method under-estimated I broken vertical bar (CO2LL) by ca. 10-15 %. Differences in the estimated I broken vertical bar (CO2LL) among measurement conditions were generally accounted for by different levels of photorespiration as described by the Farquhar-von Caemmerer-Berry model. However, our data revealed that the temperature dependence of PSII photochemical efficiency under low light was an additional factor that should be accounted for in the model.