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The reflectance spectra from 500 to 2600 nm were measured for the leaves of six agronomic crops. Leaf samples were collected at three periods during the growing season; water content was determined and cross sections of the internal leaf structure were observed microscopically. The reflectance spectra of all fresh, green leaves were very similar. However, reflectance at all wavelengths increased as the crop matured and their leaves senesced. Decreased absorption by chlorophyll apparently increased the reflectance of the visible wavelengths (500 to 700 nm). Similarly, water loss accompanying senescence resulted in an increased reflectance in the far infrared wavelengths (1300 to 2600 nm). Changes in the internal structure of leaves caused increases in the near infrared wavelengths (700 to 1300 nm).

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The Willstätter and Stoll theory explains leaf reflectance as critical or total reflection of light at cell wall‐air interfaces of spongy mesophyll tissue, and is based on observations of the spectral properties of leaves in the visible wavelengths. Our data, obtained in the 0.72 to 1.3 μm (reflective infrared) wavelengths, showed that unexpectedly high levels of reflectance occurred from palisade tissue and from dehydrated leaves that had collapsed internal structures. A modification of the Willstätter‐Stoll theory, termed the diffuse reflectance hypothesis, allows a satisfactory explanation of the observed spectral properties of leaves in the reflective infrared, as well as the visible wavelengths.

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