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 leav es of Washingtonia robusta (palm) and Vitis vinifera (grape). Under cl ear skies, the FLR utilized the Fraunhofer line-depth principle (FLDP) to detect F within the O-2 absorption band centered at 687 nm wavelen gth. On 20 April, the morning after soils were treated with the photos ystem II herbicide DCMU, F increased significantly (p = 0.05) in palm, while Leaf reflectance at 687 nm (R) did not change. By 21 April, lea f herbicide concentrations were lethal. Increased F and R in palm corr esponded with net photosynthetic rates near zero. Photosynthesis in gr ape also decreased greatly, but F and R remained similar to the contro ls. Results for palm indicate that F could be measured while leaves re mained exposed to full sunlight, and support earlier work to indicate a potential role for the FLDP in remotely sensing F and photosynthetic capacity.