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The distribution of photosynthetic activity over the area of a leaf and its change with time was determined (at low partial pressure of 02) 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, qN, were computed pixel by pixel. Because rates of photosynthetic electron transport are positively correlated with (1 -qN), computed images of (1 -qN) 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.Until recently, photosynthesis in leaves was studied under the assumption of a uniform distribution of photosynthetic activity. However, there is evidence that this is not always the case. For instance, Farquhar et al. (8) (5) showed the appearance of such patterns during the assimilation of CO2 by leaves that had been given 'ABA by starch-printing or '4C-autoradiography. Areas differing in apparent photosynthetic activity coincided with the areoles which, in heterobaric leaves,

Esthation of leaf photosynthetic rate (A) from leaf nitrogen content (N) is both conceptually and numerically important in models of plan~ecosystem and biosphere responses to global change. The relationship between A and N has been studied extensively at ambient C02 but much less at elevated C@.This study was designed to (1) assess whether the A-N relationship was more similar for species within than between community and vegetation types, and (2)