864 
Over the past decades the study of several parameters of the chlorophyll fluorescence emission has 
become a rapid, sensitive and non-destructive laboratory method to study numerous aspects of the 
photosynthetic function [4,5]. As the possibility of airborne detection of laser-induced fluorescence from trees, 
bushes, and grasses has been demonstrated [ 6 ], the application of one of these fluorescence techniques in remote 
sensing is under discussion [7]. In addition to chlorophyll fluorescence, recent developments take into account 
the additional emission bands at 440 nm and 520 nm, which appear upon excitation with UV lasers [ 8 ]. Within 
the frame of the EUREKA project LASFLEUR (EU 380), several institutes and companies are investigating the 
use of different fluorescence parameters for the remote detection of vegetation stress. 
However the intensity of the fluorescence signal depends on several factors including ambient light, 
chlorophyll content, fluorescence re-absorption and fluorescence quantum yield. The later parameter is essential 
in order to define plant status, as it can be related to photochemistry and carbon assimilation [9]. This paper 
describes a new method to determine the chlorophyll fluorescence quantum yield, based on the measurement of 
the lifetime. In addition recent progress in our knowledge of blue fluorescence are presented. 
3. CHLOROPHYLL FLUORESCENCE LIFETIME IN-VIVO 
The absorption of a light by a chlorophyll molecule leads to an excited state. The excess of energy of this state 
can be dissipated by a number of competing reactions. One of these ways is the re-emission of a fluorescence 
photon. The fluorescence lifetime is the mean time of duration of the excited state. It depends of all the 
competing processes including radiationless transitions and has only a statistical meaning (see Fig. 1). 
Laser 
pulse 
CHL* 
Mean lifetime X = 
I 
t. F(t) dt 
I 
F(t) dt 
FLUORESCENCE 
FIG. 1 . Definition of the mean lifetime 
1600 
ÌL 
; 
— 
— 
- 
Control ! 
. 
t : 
« 
*. 1 
•. Tv* 
Q« qu 
enching 
V 
1 
• • 
• 
• • 
! 
— 
— 
0.4 0.6 0.8 
Relative fluorescence yield 
FIG. 2. Relationship between mean fluorescence lifetime of chl_a and fluorescence yield during Qe quenching 
in an attached leaf of barley. The fluorescence yield is changed by varying the pre-illumination (After Y. Goulas 
I20JI 
It is well documented from a large number of reports that chlorophyll fluorescence in vivo is a heterogeneous 
emission [10-17], containing at least 3 lifetime components, the origin of which is still a matter of discussion. 
However it has also been shown in previous works [12, 18, 20] that the mean chlorophyll fluorescence lifetime