Spectral reflectance from a soybean canopy exposed to elevated CO₂ and O₃

By affecting the physiology and structure of plant canopies, increasing atmospheric CO₂ and O₃ influence the capacity of agroecosystems to capture light and convert that light energy into biomass, ultimately affecting productivity and yield. The objective of this study was to determine if established remote sensing indices could detect the direct and interactive effects of elevated CO₂ and elevated O₃ on the leaf area, chlorophyll content, and photosynthetic capacity of a soybean canopy growing under field conditions. Large plots of soybean (Glycine max) were exposed to ambient air (~380 μmol CO₂ mol⁻¹), elevated CO₂ (~550 μmol mol⁻¹), elevated O₃ (1.2x ambient), and combined elevated CO₂ plus elevated O₃ at the soybean free air gas concentration enrichment (SoyFACE) experiment. Canopy reflectance was measured weekly and the following indices were calculated from reflectance data: near infrared/red (NIR/red), normalized difference vegetation index (NDVI), canopy chlorophyll content index (chl. index), and photochemical reflectance index (PRI). Leaf area index (LAI) also was measured weekly. NIR/red and LAI were linearly correlated throughout the growing season; however, NDVI and LAI were correlated only up to LAI values of ~3. Season-wide analysis demonstrated that elevated CO₂ significantly increased NIR/red, PRI, and chl. index, indicating a stimulation of LAI and photosynthetic carbon assimilation, as well as delayed senescence; however, analysis of individual dates resolved fewer statistically significant effects of elevated CO₂. Exposure to elevated O₃ decreased LAI throughout the growing season. Although NIR/red showed the same trend, the effect of O₃ on NIR/red was not statistically significant. Season-wide analysis showed significant effects of O₃ on PRI; however, analysis of individual dates revealed that this effect was only statistically significant on two dates. Elevated O₃ had minimal effects on the total canopy chlorophyll index. PRI appeared to be more sensitive to decreased photosynthetic capacity of the canopy as a whole compared with previously published single leaf gas exchange measurements at SoyFACE, possibly because PRI integrates the reflectance signal of older leaves with accumulated O₃ damage and healthy young, upper canopy leaves, enabling detection of significant decreases in photosynthetic carbon assimilation which have not been detected in previous studies which measured gas exchange of upper canopy leaves. When the canopy was exposed to elevated CO₂ and O₃ simultaneously, the deleterious effects of elevated O₃ were diminished. Reflectance data, while less sensitive than direct measurements of physiological/structural parameters, corroborate direct measurements of LAI and photosynthetic gas exchange made during the same season, as well as results from previous years at SoyFACE, demonstrating that these indices accurately represent structural and physiological effects of changing tropospheric chemistry on soybean growing in a field setting.