Cotton leaves from nodes 2 to 6 down from plant apices (basipetally) were used to obtain absolute radiometric data (spectrophotometrically measured diffuse reflectance and transmittance) of adaxial (upper) surfaces of single and stacked leaves over the 500- to 2,500-rim wavelength interval (WLI). These data were used calculate three optical constants: absorption coefficient, k; scattering coefficient, s; and infinite reflectance, R∞.
Youngest leaves had approximately 10% lower single leaf reflectance over the 750- to 1,350-nm WLI than the older lower leaves. Reflectance of the older leaves slowly but progressively increased 2 to 3% going from nodes 3 down to 6.
Leaves were stacked to measure R∞. Youngest leaves had the lowest R∞ particularly over the 750- to 1,350-nm WLI. R∞ increased approximately 5% for older leaves from the 3rd node. R∞ increased at a slower rate for leaves from the 4th, 5th, and 6th nodes.
Second-node leaves gave the lowest s over the entire 500- to 2,500-nm WLI; then s greatly increased for leaves from the 3rd node, followed by a smaller but progressive increase in s for leaves from nodes 4 to 6.
Youngest leaves had the lowest k over the 500- to 750-nm visible range, the highest k over the 750- to 1,350-nmn earinfrared range and the lowest k above 1,350 nm. Absorption progressively increased in the visible range as leaves aged (nodes 3 to 6), and generally did so above 1,350 nm, but this pattern was inconsistent over the 750- to 1,350-nm WLI.
Leaf water contents were positively correlated with thicknesses (r2 = 95%), and each increased with aging going from node 2 down to 6. The absorption coefficient k was positively correlated with leaf water content over the 1,350- to 2,500-nm WLI.
These results show that leaf age affects the interaction of light with cotton leaves (Gossypium hirsutum L.). As investigations of light interactions within plant canopies advance, more attention will need to be given to optical properties of leaves.