Detection of solar-excited chlorophyll a fluorescence and leaf photosynthetic capacity using a fraunhofer line radiometer

A Fraunhofer Line Radiometer (FLR) measured solar-excited chlorophyll a fluorescence (F) as an indicator of photosynthetic capacity in leaves of Washingtonia robusta (palm) and Vitis vinifera (grape). Under clear skies, the FLR utilized the Fraunhofer line-depth principle (FLDP) to detect F within the O2 absorption band centered at 687 nm wavelength. On 20 April, the morning after soils were treated with the photosystem II herbicide DCMU F increased significantly (p = 0.05) in palm, while leaf reflectance at 687 nm (R) did not change. By 21 April, leaf herbicide concentrations were lethal. Increased F and R in palm corresponded with net photosynthetic rates near zero. Photosynthesis in grape also decreased greatly, but F and R remained similar to the controls. Results for palm indicate that F could be measured while leaves remained exposed to full sunlight, and support earlier work to indicate a potential role for the FLDP in remotely sensing F and photosynthetic capacity.