Dark-interval relaxation kinetics (DIRK) of absorbance changes as a quantitative probe of steady-state electron transfer

We introduce a new, non-invasive technique to measure linear electron trans fer in intact leaves under steady-state illumination. Dark-interval relaxat ion kinetic or 'DIRK' analysis is based on measurements of the initial rate s of relaxation of steady-state absorbance signals upon a rapid light-dark transition. We show that estimates of electron flux by DIRK analysis of abs orbance signals, reflecting redox changes in the photosynthetic electron tr ansfer chain, can yield quantitative information about photosynthetic flux when the light-dependent partitioning of electrons among redox components o f the electron transfer chain are considered. This concept is modeled in co mputer simulations and then demonstrated in vivo with tobacco plants under non-photorespiratory conditions resulting in linear relationships between D IRK analysis and gross carbon assimilation (A(G)). Estimation based on DIRK analysis of the number of electrons transferred through the photosynthetic apparatus for each CO2 fixed was within 20% of the theoretical value. Poss ible errors and future improvements are discussed. We conclude that the DIR K method represents a useful tool to address issues such as plant stress an d photosynthetic regulation.