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CC >2 concentration and a concomitant decrease of photosynthetic assimilation. In C3 plants photosynthetic 
electron flow and its control by non photochemical quenching is not sensitively affected by moderate 
dehydration. The main reason for this seems to be the increase of the photorespiration electron pathway [33] 
As a consequence large variations of the fluorescence yield are hardly seen in ¿3 plants. In contrast it has been 
shown for C4 plants, like maize, in which the photorespiration mechanism does not exists, that the parameter 
(Fm-Fs)/Fs is changed by water stress and can be considered as a good measurement of the relative quantum 
yield of CO 2 assimilation [5]. 
Two months old maize plants were submitted to water-stress (W.S.) by withholding water during one 
week. The water deficit was respectively 40% and 10% for W.S. and control plants. This treatment was 
reversible as the recovery was complete in 24 h after watering. The experimental conditions were the following. 
After dark adaptation of the plants for 1 h, measurements at Fo (no pre-illumination) or Fm (5000 fiFnr^-l 
saturating intensity for =1 s) conditions were done. The same attached leaf was pre-illuminated with intensities 
of 200 pEm' 2 s"l (low intensity) or 1000 pEm'^s'l (strong intensity) in order to obtain a different stationnaiy 
level. Simultaneous measurements with a PAM 2000 fluorimeter (Walz, Germany) on the same part the leaf 
allowed to check that the stationary fluorescence level (Fs) was reached within =30 mn. Then the measurement 
was repeated for the Fs level, like for Fo. Due to some ringing 4 decays separated by a few seconds were 
accumulated. Fig 5 A shows the variation of x(Fs) versus the light intensity for both W.S. and control plants. 
After dark adaptation a longer lifetime is found in the case of W.S., however the difference vanishes on going to 
higher light intensities. Fig 5 C and D shows a good agreement between Ax/x m and AF/F m parameters, 
excepted at 1000 pEm'^s'l where the increases of x(Fm) is not seen in the measurement with the PAM. This 
difference may be explained by the contribution of a slow component having a low relative intensity. 
We conclude from this study that the LEDAR system is able to characterize water-stress at distance like 
does the PAM in contact. Further works are in progress at LURE to better characterize water-stress on the 
fluorescence lifetime and yield parameters. 
6 . TIME-RESOLVED MEASUREMENTS ON COMPLEX TARGETS. 
It was mentioned above that the time response of the instrument (0.35 ns) is of the same order of magnitude as 
the mean lifetime. Therefore a deconvolution of the fluorescence signal by the back-scattered signal is needed to 
retrieve the actual fluorescence decay. In addition, measurements in this time domain for complex target may 
introduce new difficulties.The major one is the "field depth" of the illuminated area. 
FIG. 6. The partially specular character of the back-scattered reflectance from leaves 
produces an amplitude decorrelation when compared with the fluorescence signal 
When trees or bushes are illuminated by a laser spot of =7 cm diameter, as it is the case with our 
instrument, fluorescence is usually emitted by several layers of leaves (Fig. 6). Let us suppose that all leaves 
fluoresce with the same decay law F(t). One can suppose, that the fluorescence signal is still expressed as the