Relation of light reflectance to histological and physical evaluations of cotton leaf maturity

Cotton plants were grown hydroponically with controlled environment. Third, growth chamber grown true leaves of cotton plants were tagged on the day they became macroscopically visible. Beginning 3.0 days after tagging,five leaf harvests representing maturity dates were made at successive 2- or 3-day intervals. Measurements with a spectrophotometer made on the leaves showed that the largest increase in reflectance, about 5%, and decrease in transmittance, about 8%, occurred between average values for after-tagging-ages of 3.5 days and 8.0 days over the 0.75-1.35-micro wavelength interval. Between after-tagging-ages of 3.5 days and 8.0 days, leaves expanded approximately fivefold, numbers of intercellular spaces approximately doubled, and thicknesses increased 14%. The theory of diffuse reflectance and transmittance of a compact leaf of equivalent water thickness (EWT) specified by D is generalized to include also the noncompact leaf characterized by many intercellular air spaces, can be regarded as a pile of N compact layers separated by infinitesimal air spaces. The void area index (VAI) of a noncompact leaf is given by N - 1, where N is not necessarily an integer. Predictions from the generalized theory include a measure of the water, air, and plant pigments in a leaf. An effective dispersion curve associated with the leaf surfaces is also obtained. A derived parameter D/N largely determines the reflectance and transmittance of a typical leaf over the 1.40-2.50-micro spectral range. A cotton leaf is highly compact when it first unfolds. At this point D/N ~ 180 micro. This value is essentially the leaf thickness. Intercellular air spaces develop rapidly during the next few days, and D/N decreases in value to about 130 micro Subsequently, the leaf cells increase in size with no substantial further increase in the number of intercellular air spaces. This final growth phase is characterized by a slight increase in D/N to a maximum value of about 140 micro. Maximum reflectance of the leaf corresponds to a minimum value of D/N. The parameter D/N is highly correlated with the amount of intercellular air spaces in a leaf.