Reflectance measurements of leaves for detecting visible and non-visible ozone damage to crops

Spring wheat (Triticum aestivum cv. Turbo), white clover (Trifolium repens cv. Karina) and maize (Zea mays cv. Bonny) plants were exposed for 20-30 days in open-top-chambers to charcoal-filtered air (CF, control) and CF air supplied with O3 for 8-12 h per day in the concentration range of 180-240 ug O3/cubic metre (8-12 h/day treatment mean). At the end of the O3 treatment spectral reflectance measurements were made on single leaves of all 3 species and on canopies of wheat and clover using a CCD (Charged Coupled Device) camera and wavelength filters with 11 wavelength bands ranging from 450 nm to 950 nm. Different vegetation indices such as the normalized difference vegetation index (NDVI) and the "main inflection point" (MIP) were calculated. Based on these results it was shown that visible O3 damages were correlated to the spectral reflectance changes: Both leaves and canopies showed an increased reflectance of visible light after ozone treatment. While clover and maize leaves as well as clover and wheat canopies showed a decreased near infrared (NIR) reflectance, the NIR reflectance of wheat leaves did not change, even if the leaves had visible symptoms. A decreased infrared reflectance was detectable for all clover leaves after O3 treatment although for part of the leaves no visible foliar damage symptoms could be observed.