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BIDIRECTIONAL LEAF REFLECTANCE & TRANSMITTANCE
OF STRESSED AND UNSTRESSED LEAVES.
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19 90, Description
mote Sensing, vol.
todeling: the SAIL
Sonia Rubie.
University College London,
Dept of Geography,
26 Bedford Way, London, WC1H OAP
England.
Sira Limited,
Sira/UCL Postgraduate Research Centre,
South Hill, Chislehurst, Kent, BR7 5EH,
England.
email: soniato'sirapgc.demon.co.uk
ABSTRACT
The angular dependence of reflected and transmitted radiation os er the exitant hemisphere for individual stressed
and unstressed leaves will be studied to determine how the spectral response is affected Investigations will be
carried out as to whether it is possible to both distinguish sources of stress and identify' them sufficiently early to
allow corrective action. The reflectance and transmittance will be measured at source incidence angles between 0°
- 70° and view angles between 0° - 70° in the visible and infrared region to obtain the bi-directional reflectance
and transmittance distributions of the abaxial and adaxial surfaces of the leaves which have grown under a variety
of stress conditions in a controlled environment. The leaves will also be analysed for various constituent
concentrations including water, nutrients and pigments. A limited number of field trials are also proposed to study
the bidrectional reflectance and transmittance of stressed canopies.
KEY WORDS: Remote-Sensing, Vegetation, Bidirectional. Stress, Visible. Near-infrared. Water. Nutrient.
1. INTRODUCTION
The detection of stressed crops by remote sensing is based on the premise that their reflectivity is qualitatively and
quantitatively different from that of healthy vegetation before visual signs of stress appear (Richardson & Everitt
1987 (28)). There is experimental evidence and remote sensing data to support this view but in some cases the
physical or physiological basis for the differences has been misinterpreted Both the red and infrared reflectance
are sensitive indicators of physiological stress. Research has shown that the differences in reflectivity that allow
discrimination of stressed plants can be traced to their leaf and canopy characteristics.
2. AIMS
The aim of the work is to investigate the relationship between plant stresses and their spectral responses with the
long-term objective of developing instrumentation of practical benefit to the agricultural community 7 . Although
much work has been done in the field of remote sensing, the correlation between plant stress and changes in the
spectral response is poorly understood Investigating this relationship is an important first step in developing
instrumentation that can give early warning of plant stress, so that corrective action can be taken promptly.
Resultant instrumentation is intended for use in low-level sensing i.e. measurements at most a few metres above
the plant canopy as opposed to satellite or airborne instrumentation.
3. BACKGROUND REVIEW
3.1. Vegetation Stress
Stress may be defined as any 7 factor that reduces the productivity of the plant below its optimal value^ Trie
productivity of stressed vegetation can be impaired for two reasons. Firstlv the stres canopy may ess
