two sides of the leaf and position along the leaf. However, the positional differences could also be due to the 
chlorophyll, nutrient and water contents differing with position along the leaf. Therefore another set of 
experiments will look at the inter-cellular structure & measure the chlorophyll, water and nutrient content of a leaf 
as a function of position along the leaf i.e. moving from the tip of the leaf along to the end of the leaf that is joined 
to the stem to see how much they' change along the leaf. 
With the nutrient & water stress, investigations will be done similar to Cohen. 1991 (31). An investigation will be 
done on the variance of the reflectance values of leaves with the same water/nutrient contents and different 
water/nutrient contents. This will be done for all the nutrients investigated. For a correlation to exist between 
water or nutrient stress and spectral response, the reflectance variation of the leaves with similar water/nutrient 
contents should be less than the reflectance variance of leaves with different water/nutrient contents in at least one 
wavelength. 
Work on the effects of nutrient deficiency/toxicity on a leafs reflectance has tried to correlate the amount of 
nutrient given to the plant with the reflectance value of the leaf. This is not the correct way of studying the effects 
of nutrient deficiency/toxicity because nutrients interact with each other i.e. one has to consider the "antagonistic" 
actions of nutrients. (Epstein. 1972 (43)). For example, if the level of nitrogen is reduced it may affect the 
concentration of the other nutrients. The plant may consequently absorb more or less potassium and/or calcium 
than it would normally do because it cannot absorb the nitrogen it requires. Therefore the corresponding change m 
reflectance of a nitrogen deficient leaf might be due to the higher/lower concentration of potassium and/or calcium 
instead of the lower concentration of nitrogen. It is therefore necessary when investigating the effects of nutnent 
toxicity/deficiency' on the bidirectional reflectance and transmittance distributions of single leaves, to measure the 
concentration of all the nutrients in the leaf and not just the one that is being varied. 
5.2. Field Experiments 
As field conditions do not compare directly with laboratory conditions, the following field experiments are 
proposed 
Crops of winter wheat, spring barley' and sugar beet will be subjected to a range of nitrogen, potassium and 
phosphorous levels. Measurements of reflectance in visible and near infrared will be taken so as to assess the 
effects of varying the nutrient levels on the plant canopy and spectral response. This will be repeated at several 
stages of plant growth. 
The following physical measurements will be done on each canopy; 
1. Plant Height 
2. Plant Cover/Percentage Cover 
3. Plant Density 
4. Leaf Area Index (LAI). 
5. Plant Biomass 
6. Plant Geometry/ Leaf Angle Distribution. 
Generally the measurements will be done on bright cloudless days. However, use of the technique under sub- 
optimal conditions might still be useful as these are the conditions under which it would have to be used 
commercially. 
Also the height of the sensor relative to the canopy will be studied. After the spectral measurements have been 
taken, leaves will be sampled from the plots and analysed for their constituent chemical contents including water, 
chlorophyll and nutrient content. The inter-cellular structure & chlorophyll, water and nutrient content of a leaf 
will be measured as a function of position along the leaf. It is hoped that statistical analysis similar to Cohen, 1991 
(31) can also be carried out for the field experiments.