Use of a prototype instrument to detect short-term changes in solar-excited leaf fluorescence
A prototype instrument was used under clear skies to determine the efficacy of the Fraunhofer Line-Depth Principle in detecting short-term (less than one day) changes in solar-excited fluorescence (F) as an indicator of plant physiological status. Corn (Zea mays L. var. Shoe Peg) and soybean (Glycine max L. var. Hutcheson) plants grown in pots outdoors were either assigned as controls or treated with bromacil, a photosystem II herbicide. The Plant Fluorescence System (PFS) measured the radiant flux of F and total upwelling radiant flux (M) from individual leaves in 10 nm bandwidths centred at 690 nm and 760 nm. The herbicide lowered (p = 0.01) net photosynthetic CO2 assimilation rates to negative values in both species. In corn, this corresponded with increases in mean F to 5.9 mW m(-2) nm(-1) and 3.8 mW m(-2) nm 21 at 690nm and 760 nm, respectively, approximately twice control values. In soybean, mean F at 760 nm increased from 2.6 mW m(-2) nm(-1) to 3.8 mW m(-2) nm(-1), whereas F at 690nm was unaffected. Leaf chlorophyll contents were unaffected by treatment. Although the stress induced was drastic, these changes in F increased M by only 1-2%, demonstrating the high radiometric sensitivity required for detection. It is expected that, for whole plant canopies, a still greater variability in F among leaves would further reduce its apparent contribution to M.