Resolving chlorophyll a fluorescence images of photosynthetic efficiency into photochemical and non-photochemical components – calculation of qP and Fv-/Fm-; without measuring Fo-;

Imaging of chlorophyll a fluorescence from leaves has enabled the spatial resolution of the fluorescence parameter, ΔF/Fm-;. Although this parameter provides a reliable estimate of photosynthetic efficiency under most conditions, the extent to which this efficiency is defined by (i) competition with other energy-dissipating processes operating at photosystem II and (ii) by processes on the reducing side of photosystem II, such as carbon assimilation, requires the use of additional parameters. Of particular value are qP, which quantifies the photochemical capacity of photosystem II, and Fv-;/Fm-;, which quantifies the extent to which photochemistry at photosystem II is limited by competition with thermal decay processes. Imaging of both qP and Fv-;/Fm-; requires measurement of Fo-; (the minimum fluorescence yield in the light-adapted state), which cannot be imaged with existing systems. In this paper, a method is described which estimates Fo-; through a simple equation involving the minimum fluorescence yield in the dark-adapted state (Fo), the maximum fluorescence yield in the dark-adapted state (Fm), and the maximum fluorescence yield in the light-adapted state (Fm-;). This method is tested here, through comparison of measured and calculated values of Fo-;. An example of the application of this method to analysis of photosynthetic performance in leaves, from images of chlorophyll a fluorescence, is also presented.