Partitioning of Noncyclic Photosynthetic Electron Transport to O(2)-Dependent Dissipative Processes as Probed by Fluorescence and CO(2) Exchange.

The partitioning of noncyclic photosynthetic electron transport between net fixation of CO(2) and collective O(2)-dependent, dissipative processes such as photorespiration has been examined in intact leaf tissue from Nicotiana tabacum. The method involves simultaneous application of CO(2) exchange and pulse modulated fluorescence measurements. As either irradiance or CO(2) concentration is varied at 1% O(2) (i.e. absence of significant O(2)-dependent electron flow), the quantum efficiency of PSII electron transport (phi(se)) with CO(2) as the terminal acceptor is a linear function of the ratio of photochemical:nonphotochemical fluorescence quenching coefficients (i.e. q(Q):q(NP)). When the ambient O(2) concentration is raised to 20.5% or 42% the q(Q):q(NP) is assumed to predict the quantum efficiency of total noncyclic electron transport (phi'(se)). A factor which represents the proportion of electron flow diverted to the aforementioned dissipative processes is calculated as (phi'(se) - phi(se))/phi'(se) where phi(se) is now the observed quantum efficiency of electron transport in support of net fixation of CO(2). Examination of changes in electron allocation with CO(2) and O(2) concentration and irradiance at 25 degrees C provides a test of the applicability of the Rubisco model to photosynthesis in vivo.