735 
Work which has been done on trying to relate the red edge to canopy parameters appears to be controversial. Some 
have found that the wavelength of the red edge was a good indicator of leaf chlorophyll in the laboratory' on single 
leaves but where unable to demonstrate the same effect conclusively for vegetation canopies, because when leaves 
overlap in canopies the spectral signal responds to the chlorophyll content of more than one single leaf. (Horler et 
ai 1980 (44). Steven et al., 1990 (49)). It has also been said that the red edge shift is due to a physical 
mechanism which gives the appearance of a red shift. (Schutt et al., 1984 (1)) and others say the red edge shift is 
due to wind (Vanderbelt et al., 1988 (48)). However other work by Collins, 1978 (45) say that there is 
physiological reasoning behind the red edge and was observed to happen in the field 
4. EXPERIMENTAL SET-UP 
The overall conclusion that can be drawn about the optical properties of leaves and plant canopies, is that there are 
many factors all inter-related and which must be identified both qualitatively and quantitatively if remote sensing 
is going to be of use in identifying and monitoring vegetation health. 
The effect of stresses on the plant response are not unique; many different stresses may give rise to similar 
responses. However a particular stress may also cause more than one physiological change within the plant, 
thereby giving rise to a combination of changes in the plant response. (Epstein, 1972 (43)). In order to classify 
stresses, it is therefore necessary to have a clear understanding of the underlying mechanisms of plants to different 
stresses. By looking at the spectral response, it is hoped to identify critical wavelength bands whose combined 
signature allows source stresses to be identified. It is hoped to extend the scope of vegetation monitoring to 
conditions where the stress affects the efficiency of photosynthesis, and to examine spectral signatures during the 
early stages of stress, before its effects on leaf area and light interception are significant. Therefore as far as 
possible data should collected which relates single sources of stress (uncoupled from other effects) to changes in 
spectral response. Therefore plants are to be grown in controlled water, nutrient, lighting, humidity and 
temperature environments. 
The spectral response of single leaves is to be measured using a dedicated experimental set-up which will allow the 
bi-directional transmittance and reflectance to be measured at a variety of angles. This equipment will allow the 
rotational positions of the light source, leaf, and detector to be independently adjusted so that measurements can 
be taken between the angles of 0°-70° for all three (thereby allowing the adaxial and abaxial sides of leaves to be 
measured). Currently the experimental set-up is nearing completion. Measurements will be taken using a 
Personal Spectrometer II in the visible and near-infrared region (400-1000nm). 
5. FUTURE WORK 
The project is split into two parts :- 
5.1. Laboratory Experiments 
The first stage of the programme is to perform experiments on plants in highly controlled conditions to study' the 
effects of different stresses on the bidirectional reflectance and transmittance of stressed leaves. The plants to be 
studied are wheat barley and sugar beet and the different stresses to be investigated are nitrogen, potassium and 
phosphorous deficiency/toxicity and water stress. Leaves will then be sampled, from the top, middle and bottom of 
the plants at different stages of growth and their bidirectional reflectance and transmittance distributions over the 
exitant hemisphere measured, to see the effects of the different stresses and age on individual leaves. 
The bidirectional reflectance & transmittance distributions of the leaves will also be obtained at various positions 
along the leaves and on the abaxial aswell as the adaxial side of the leaf. After the spectral measurements have 
been taken the leaves will then be analysed for various chemical constituents including starch, protein, water, 
nutnent and pigment concentrations. 
The side and position on a leaf where reflectance measurements are taken also affects the reflectance spectrum of 
the leaf, (Schutt et al., 1984 (1)). The reason generally cited for the differences is the inter-cellular structure of the