Introduction:
One phase of our remote sensing research at Weslaco, Texas concerns
studies relating the interaction of plant leaves and canopies
with electromagnetic radiation. Experiments are designed to speci-
fy wavelengths for differentiating plant stress conditions, to
distinguish among plant species and range sites, to determine
green and nongreen biomass, and to understand the causes of light
reflectance, transmittance, and absorptance by leaves and their
components. A particular spectral signature must be understood
to predict responses for uncharacterized plant species of interest
and to project generalized findings for global applications.
We will consider reflectance primarily over the 0.4- to 2.5-um
waveband. For ease of interpretation, this waveband can be par-
titioned into: the 0.4- to 0.75- um region, affected primarily
by pigments; the 0.75- to 1.35 -um region, affected mostly by
leaf structure; and the 1.35- to 2.5-um region, affected strongly
by leaf water content.
Literature Synopsis
Reflectance measurements in the visible region can be used to:
follow changes in leaf chlorophyll content (Benedict and Swidler,
1961), quickly estimate the nitrogen status of sweet pepper plants
(Thomas and Oerther, 1972), evaluate turf color (Birth and McVey,
1968), measure amounts of green and dry biomass (Tucker et al.,
1973, 1975, and 1977a), and predict yield (Thomas and Gerbermann,
1977). However, correlations of leaf reflectance with chlorophyll
concentration are sometimes poor (Gausman et al., 1975 c) because
reflectance from leaf cell wall-air interfaces interacts with
reflectance associated with leaf chlorophyll concentration. In-
terest has been recently stimulated in leaf carotenoids (carotenes
and xanthophylls) because of their apparent usefulness in measuring
dry biomass (Tucker, 1977a) and their relationship to leaf reflec-
tance in the 0.35- to 0.50-um region (Tucker and Garratt, 1977).
Near-infrared light (IR) reflectance (0.75 to 1.35 um) usually
increases as the number of air spaces in leaf mesophylls increase
(Allen et al., 1971; Gates et al., 1965; Gausman et al., 1969a:
Hememger, 1977; Hoffer and Johannsen, 1969; Knipling, 1970; Moss,
1951; Pearman, 1966; Shull, 1929; Sinclair, 1968; Thomas, Wiegand,
and Myers, 1967; Wiegand et al., 1972; and Willstätter and Stoll,
1918).
Near-infrared light is scattered or reflected from leaves by re-
fractive index discontinuities (Gausman, 1974a). The most important
discontinuity is the cell wall/air-space interface. If IR light
travels at a critical angle from a hydrated cell wall with a re-
fractive index of about 1.425 (Gausman et al., 1974b) to an air
space with a refractive index of 1.0, the IR light is scattered
or reflected. (Early researchers (reviewed by Kumar, 1972) found
that reflectance was decreased and transmittance was increased
by replacing air with water by vacuum infiltration.) Refractive
index discontinuities among cellular constituents (cell walls,
chloroplasts, cytoplasm, membranes, nuclei) are also of some
importance in causing reflectance of IR light (Gausman, 1973a,
1973b; Woolley, 1971).
