RELATIONSHIP OF LEAF SPECTRAL REFLECTANCE TO CHLOROPLAST WATER-CONTENT DETERMINED USING NMR MICROSCOPY

Nuclear Magnetic resonance (NMR) microscopy and reflectance spectroscopy were used to evaluate the response of leaf spectral reflectance to chloroplast relative water content (RWC). For sun and shade leaves of maple (Acer platanoides L. var Emerald Queen), NMR determined the distribution of water through the leaf thickness and changes in distribution as leaves dried from 100% RWC to approximately 70%, 50%, and 30% RWC. Chloroplast water could be resolved only in shade leaves. Leaf and chloroplast RWC were similar at leaf RWC of 80% or greater, but chloroplast RWC was as much as 12% greater than leaf RWC when leaf RWC decreased below 50%. In contrast, the RWC of nonchloroplast water compartments was as much as 12% less than leaf RWC. Reflectance responses to leaf water content were greatest in the water absorption bands near 1450 nm, 1950 nm, and 2500 nm wavelengths, and were minimal in the near-infrared from approximately 760-1300 nm. In the 400-760 nm (visible) range, reflectance sensitivity increased substantially as chloroplast RWC decreased. Peak visible reflectance sensitivity occurred near 695 nm and 670 nm for adaxial and abaxial surfaces, respectively, and was more closely related to chloroplast than to nonchloroplast or whole leaf RWC.