Chlorophyll fluorescence measured using the Fraunhofer line‐depth principle and relationship to photosynthetic rate in the field

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Abstract
A field study was conducted to determine the relationship of solar‐excited chlorophyllafluorescence to net CO2assimilation 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 CO2and 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‐2nm‐1as CO2assimilation rate increased from 1 to 8 μmol m‐2s‐1(r2= 0.520). In the C3Liquidambar styracifluaandPinus taeda, fluorescence decreased from 6 to 2 mW m‐2nm‐1as assimilation rate increased from 2 to 5 or 0 to 2 μmol m‐2s‐1(r2= 0.44 and 0.45. respectively). The Fraunhofer line‐depth principle enables the simultaneous measurement of solar‐excited fluorescence and CO2assimilation rate in individual leaves, but also at larger scales. Thus, it may contribute significantly to field studies of the relationship of fluorescence to photosynthesis.