The use of chlorophyll fluorescence to predict CO ₂ fixation during photosynthetic oscillations

The relation between photosynthetic electron transport and chlorophyll a fluorescence is of fundamental importance and would have immense practical potential in measurement if properly understood. For this reason we have examined some aspects of this relation in the context of recently proposed models. Chlorophyll fluorescence quenching was measured (together with quantum yields of CO$_{2}$ fixation) at different light intensities and during oscillations in photosynthesis induced by abrupt changes in CO$_{2}$ concentration or re-illumination after 1 min darkness. The relation between (i) the quantum yield of open photosystem II (PSII) centres ($\Phi _{\text{p}}$) and non-photochemical quenching (q$_{\text{N}}$), and (ii) the quantum yield of CO$_{2}$ fixation ($\Phi _{\text{s}}$) and the fluorescence parameter F$_{\text{m}}^{\prime}$-F$_{\text{s}}$/F$_{\text{m}}^{\prime}$ were examined for barley and wheat at different light intensities, CO$_{2}$ concentrations, and temperatures. From this data two empirical equations (i) $\Phi _{\text{s}}$ = q$_{\text{P}}\times $ (0.153-0.131 $\times $ q$_{\text{N}}$) and (ii) $\Phi _{\text{s}}$ = ((F$_{\text{m}}^{\prime}$-F$_{\text{s}}$/F$_{\text{m}}^{\prime}$)-0.0085)/7.94, which related $\Phi _{\text{s}}$ to chlorophyll-fluorescence parameters, were obtained. Both equations allowed good predictions to be made of CO$_{2}$ fixation during oscillatory behaviour. However, further examination of the relation between $\Phi _{\text{p}}$ and q$_{\text{N}}$ showed that this is not always linear. Moreover it is complicated by the fact that it is impossible to measure dark-level fluorescence (F$_{0}^{\prime}$) without disturbing photosynthesis. For these and other reasons it was concluded that the relation between $\Phi _{\text{s}}$ and F$_{\text{m}}^{\prime}$-F$_{\text{s}}$/F$_{\text{m}}^{\prime}$ provides a better indicator of photosynthetic electron transport particularly under conditions in which F$_{0}^{\prime}$ and q$_{\text{N}}$ are likely to show large variations.