Optical and fluorescence properties of corn leaves from different nitrogen regimes

The important role of nitrogen (N) in limiting or enhancing vegetation productivity is relatively well understood, although the interaction of N with other environmental variables in natural and agricultural ecosystems needs more study. In 2001, a suite of optical, fluorescence, and biophysical measurements were collected on leaves of corn (Zea Mays L.) from field plots provided four N fertilizer application rates: 20%, 50%, 100% and 150% of optimal N levels. Two complementary sets of high-resolution (< 2 nm) optical spectra were acquired for both adaxial and abaxial leaf surfaces. The first was comprised of leaf optical properties (350-2500 nm) for reflectance, transmittance, and absorptance. The second was comprised of reflectance spectra (500-1000 nm) acquired with and without a long pass 665 nm filter to determine the fluorescence contribution to "apparent reflectance" in the 670-750 nm spectrum that includes the 685 and 740 nm chlorophyll fluorescence (ChlF) peaks. Two types of fluorescence measurements were also made on adaxial and abaxial surfaces: 1) fluorescence images in four 10 nm bands (blue, green, red, far-red) resulting from broadband irradiance excitation; and 2) emission spectra at 5 nm resolution produced by three excitation wavelengths (280, 380, and 532 nm). The strongest relationships between optical properties and foliar chemistry were obtained for a "red-edge" optical parameter versus C/N and chlorophyll b. Select optical indices and ChlF parameters were correlated. A significant contribution of steady-state ChlF to apparent reflectance was observed, averaging 10-25% at 685 nm and 2-6% at 740 nm over the range of N treatments. From all measurements assessing fluorescence, higher ChlF was measured from the abaxial leaf surfaces.