Use of a prototype instrument to detect short-term changes in solar-excited leaf fluorescence

Abstract

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⁻¹ at 690 nm 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 690 nm 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.

Acknowledgments

The NASA Earth Science Applications Directorate, formerly the Commercial Remote Sensing Program and Earth System Science Office, at Stennis Space Center, funded this study. The PFS was developed under a NASA, Stennis Space Center, Small Business Innovative Research (SBIR) grant to Aerodyne, Research Inc. The NASA SBIR program at Kennedy Space Center provided additional support. We also thank the US Forest Service, Southern Institute of Forest Genetics, Saucier, Mississippi, for providing experimental facilities and technical assistance.

Notes

^*Present address: Gulf Coast Geospatial Center, The University of Southern Mississippi, PO Box 7000, 703 East Beach Drive, Ocean Springs, MS 39564, USA; e-mail: [email protected]

Additional information

Notes on contributors

G. A. Carter*

*Present address: Gulf Coast Geospatial Center, The University of Southern Mississippi, PO Box 7000, 703 East Beach Drive, Ocean Springs, MS 39564, USA; e-mail: [email protected]