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efficient in photosynthesising incident light. Secondly stressed vegetation often have altered geometry thus 
affecting the amount of light intercepted and absorbed. Steven et al., 1990 (49) claim that large number of studies 
of crop growth under stress has e found that most stresses have a larger effect on the fraction of light intercepted 
t han on the efficiency and explains that this observation accounts for the degree of success that some conventional 
vegetation indices have had (which essentially measure percentage ground cover). 
Steven et al.. 1990 (49) go on to list requirements for monitoring stressed vegetation. These include being able to: 
1. measure canopy' density or light absorption for photosynthesis. 
2. establish the presence of significant stress. 
3. distinguish different classes of stress 
4. measure the degree of stress and its effect on productivity. 
3.2. Leaf Reflectance 
Incident radiation on a leaf can be reflected, absorbed or transmitted. It is necessary to consider all three 
components when modelling the reflected spectral response of vegetation. The reflectance spectra of all types of 
vegetation are remarkably similar in the visible to middle infrared region. The differences that do occur appear in 
the magnitude of the reflectance. Three spectral domains can be considered according to the different leaf optical 
properties of leaves. (Knipling. 1970 (2), Gausman. 1974 (14). Woolley 7 . 1971 (27));. 
The Visible Domain (400-700nm) : In this domain leaf reflectance is low (less than 15%) and leaf transmittance 
is generally very low. Most incident radiation is absorbed by 7 leaf pigments such as chlorophyll, xanthophyll, 
carotenoids and anthocyanins. The main pigments affecting leaf absorption are chlorophylls a and b which exhibit 
two absorption bands centred in the blue and red, (Gates et al., 1965 (3), Knipling. 1970 (2), Gausman, 1974 (14). 
The Near Infrared Domain (700nm-1300nm) : In this spectral domain leaf pigments and cellulose walls are 
transparent so little or none of the infrared radiation in the wavelength range 0.7 - 1.3 pm is absorbed internally 
About 40%-60% of the incoming radiation is scattered upward through the surface of incidence and is designated 
diffuse reflected radiation, whereas the remainder is scattered downward and is designated transmitted radiation 
This internal scattering mechanism accounts for the similarity in the shape of the reflectance and transmittance 
spectra, (Gates et al., 1965 (3), Knipling, 1970 (2), Gausman . 1974 (14),Woolley 7 . 1971 (27)). 
The Middle Infrared Domain (I300-2500nm) : In this domain leaf optical properties are mainly affected by their 
water content. (Knipling. 1970 (2)). Beyond 1300nm strong water absorption bands at 1450, 1950 and 2500nm 
produce leaf reflectance minima. But between these bands, water absorption still exists and affects leaf optical 
properties. The lev els of the two relative maxima therefore vary according to leaf w ater content. 
The leaves of a given species tend to have a characteristic surface which is made up of irregular facets. Each facet 
may specularly 7 reflect intercepted radiation directly, internally refract and reflect radiation diffusely, or absorb 
radiation. (Woolley. 1971 (27)). Radiation is internally scattered by the leaf as it encounters varying cellular 
structures at each refractive discontinuity. (Gausman. 1977 (13)). Specularly reflected visible and near-infrared 
radiation is dependent on the surface characteristics of the leaf and therefore there is no interaction with the tissue 
below cuticle of the leaf, (Grant et al.. 1987 (21)). 
3.2.1. Single Leaf Reflectivity and Stress. Principal factors which affect a single leafs reflectance are :- 
1. The age of the leaf. 
2. The leaf nutrient content. 
3. The leaf water content. 
4. The leaf pigment content. 
5. The leaf surface. 
6. The leaf thickness. 
7. The leaf internal structure. 
These factors are inter-related and change when the leaf becomes stressed. (Gausman et al.. 1971 (6),Thomas & 
Oerther. 1972 (10), Hinzman et al.. 1980 (52). Milton et al.. 1991 (30), Al-Abbas et al., 1974 (11). Horler et al..