CHLOROPHYLL FLUORESCENCE MEASURED USING THE FRAUNHOFER LINE-DEPTH PRINCIPLE AND RELATIONSHIP TO PHOTOSYNTHETIC RATE IN THE FIELD

Abstract. A field study was conducted to determine the relationship of solar‐excited chlorophyll a fluorescence to net CO2 assimilation rate in attached leaves. The Fraunhofer line‐depth principle was used to measure fluorescence at 656.3 nm wavelength while leaves remained exposed to full sunlight and normal atmospheric pressures of CO2 and O2. Fluorescence induction kinetics were observed when leaves were exposed to sunlight after 10 min in darkness. Subsequently, fluorescence varied inversely with assimilation rate. In the C4Zea mays, fluorescence decreased from 2.5 to 0.8 mW m‐2 nm‐1 as CO2 assimilation rate increased from 1 to 8 μmol m‐2 s‐1 (r2= 0.520). In the C3Liquidambar styraciflua and Pinus taeda, fluorescence decreased from 6 to 2 mW m‐2 nm‐1 as assimilation rate increased from 2 to 5 or 0 to 2 μmol m‐2 s‐1 (r2= 0.44 and 0.45. respectively). The Fraunhofer line‐depth principle enables the simultaneous measurement of solar‐excited fluorescence and CO2 assimilation rate in individual leaves, but also at larger scales. Thus, it may contribute significantly to field studies of the relationship of fluorescence to photosynthesis. Copyright © 1990, Wiley Blackwell. All rights reserved