Effects of irradiance on the in Vivo CO2:O2 specificity factor in tobacco using simultaneous gas exchange and fluorescence techniques

Abstract

The effects of gas phase O2 concentration (1%, 20.5%, and 42.0%, v/v) on the quantum yield of net CO2 fixation and fluorescence yield of chlorophyll a are examined in leaf tissue from Nicotiana tabacum at normal levels of CO2 and 25 to 30°C. Detectable decreases in nonphotochemical quenching of absorbed excitation occurred at the higher O2 levels relative to 1% O2 when irradiance was nearly or fully saturating for photosynthesis. Photochemical quenching was increased by high O2 levels only at saturating irradiance. Simultaneous measurements of CO2 and H2O exchange and fluorescence yield permit estimation of partitioning of linear photosynthetic electron transport between net CO2 fixation and O2-dependent, dissipative processes such as photorespiration as a function of leaf internal CO2 concentration. Changes in the in vivo CO2:O2 'specificity factor' (Ksp) with increasing irradiance are examined. The magnitude Ksp was found to decline from a value of 85 at moderate irradiance to 68 at very low light, and to 72 at saturating photon flux rates. The results are discussed in terms of the applicability of the ribulose bisphosphate carboxylase/oxygenase enzyme model to photosynthesis in vivo.