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Mesures physiques et signatures en télédétection

I. MOYA^ 1 ), Y. GOULASM, Z. CEROVld 1 ), J.M. BRIANTAIS^ 2 ), L. CAMENENK 3 ), G. GUYOT< 3 ) and
G. SCHMUCd 4 )
(1) L.U.R.E, CNRS, bât 209, University of Paris XI, 91405 Orsay, France.
(2) Ecophysiology, CNRS, URA 1492, University of Paris XI, 91405 Orsay, France.
(3) Bioclimatology - I.N.R.A., Avignon, France
(4) C.E.C, J.R.C., Institute for Remote Sensing Applications. Ispra (VA), Italy.
The physiological state of plants is under strong influence of their photosynthetic activity. Many environmental
stresses affect photosynthesis. Therefore, monitoring of vegetation could be done in a large scale by remote
sensing of photosynthesis. It is now well established that photosynthetic activity can be assessed by measuring
the dependence of the quantum yield () of chlorophyll fluorescence on light intensity. In this paper we present
a new method to estimate <)> under outdoor conditions, by measuring the mean fluorescence lifetime (t). We
also present a prototype of a fluorescence LIDAR with a sub-nanosecond time resolution together with
mathematical algorithms that permit to retrieve lifetime parameters from complex targets. Thanks to the
excitation by a UV laser, the LEDAR can measure the emission in the blue-green part of the spectrum, in
addition to chlorophyll fluorescence.
KEY WORDS: Photosynthesis, Chlorophyll, Fluorescence, Lifetime, LEDAR, Remote sensing, Canopy
structure, Blue fluorescence, Flavin nucleotides, Plant phenolics, Pyridine nucleotides.
The biosphere, whether continental or oceanic, plays a major role in the change of our planet's climate.
Continental vegetation in particular is one of the main agents that regulate water and carbon dioxide fluxes
between surface and atmosphere. Extended spectral coverage together with higher spectral resolution will be
provided by a new generation of optical sensors (“Imaging Spectrometer”) to discriminate and identify spectral
signatures associated with the response of vegetation to various stress agents.
A new technique for remote sensing of plant status, is the detection of the chlorophyll laser-induced
fluorescence. Since the chlorophyll fluorescence is only due to the emission from green vegetation, it can be
considered complementary to the existing techniques. Besides this complementarity, the direct link of the
fluorescence emission to the process of photosynthesis can be regarded as a major advantage.
During the photosynthetic process, light energy is absorbed by a large array of light-harvesting chlorophyllprotein
chlorophyllprotein complexes and transferred to the reaction centers of photosystem I and photosystem II, where charge
separation and stabilization take place. During this energy transfer the excited state may undergo a fluorescence
emission in the region from 660 nm to 800 nm. In a full functioning photosynthetic system the major part of the
absorbed light energy is used for photosynthesis. The remainder is lost as heat and re-emitted as fluorescence
The quantum yield of chlorophyll fluorescence exhibits an inverse dependence on the capability of the
photosynthetic apparatus to photochemically convert the absorbed light energy [2]. Owing to this inverse
relationship, the measurement of the chlorophyll fluorescence can be used to describe the physiological state of
vegetation. However, the study of fluorescence from intact plant leaves at room temperature, has shown that
photosynthesis together with fluorescence yield, can be also lowered by processes not directly related to the
efficiency of photochemistry and mainly due to excess of light. These mechanisms, denoted as non
photochemical quenching, are of general occurrence in outside conditions [3]. They could be understood as
regulatory or protective deactivation pathways, developed by plants to prevent gross destruction of the
photosynthetic apparatus.