Effects of Irradiance on the in Vivo CO(2):O(2) Specificity Factor in Tobacco Using Simultaneous Gas Exchange and Fluorescence Techniques.

The effects of gas phase O(2) concentration (1%, 20.5%, and 42.0%, v/v) on the quantum yield of net CO(2) fixation and fluorescence yield of chlorophyll a are examined in leaf tissue from Nicotiana tabacum at normal levels of CO(2) and 25 to 30 degrees C. Detectable decreases in nonphotochemical quenching of absorbed excitation occurred at the higher O(2) levels relative to 1% O(2) when irradiance was nearly or fully saturating for photosynthesis. Photochemical quenching was increased by high O(2) levels only at saturating irradiance. Simultaneous measurements of CO(2) and H(2)O exchange and fluorescence yield permit estimation of partitioning of linear photosynthetic electron transport between net CO(2) fixation and O(2)-dependent, dissipative processes such as photorespiration as a function of leaf internal CO(2) concentration. Changes in the in vivo CO(2):O(2) ;specificity factor' (K(sp)) with increasing irradiance are examined. The magnitude K(sp) 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.