Topography of Photosynthetic Activity of Leaves Obtained from Video Images of Chlorophyll Fluorescence

The distribution of photosynthetic activity over the area of a leaf and its change with time was determined (at low partial pressure of O2) by recording images of chlorophyll fluorescence during saturating light flashes. Simultaneously, the gas exchange was being measured. Reductions of local fluorescence intensity quantitatively displayed the extent of nonphotochemical quenching; quench coefficients, $q_{N}$, were computed pixel by pixel. Because rates of photosynthetic electron transport are positively correlated with (1 - $q_{N}$), computed images of (1 - $q_{N}$) represented topographies of photosynthetic activity. Following application of abscisic acid to the heterobaric leaves of Xanthium strumarium L., clearly delineated regions varying in nonphotochemical quenching appeared that coincided with areoles formed by minor veins and indicated stomatal closure in groups.