880
It is known [8] that the in-vivo Chl-a fluorescence includes two components: a constant F 0 and a
variable F v . The first is associated with the transfer of energy to the open Reaction Centers (RC), while the sec
ond is produced by the charge recombination following the photon capture by every closed RC. For a dark
adapted alga or plant in ideal physiological conditions, the response to a light excitation not capable of inducing
saturation (probe pulse) is a mimimum and corresponds to the constant component F 0 . When a saturating laser
pulse (pump pulse) closes all the RC’s, after a time corresponding to the decay of quinone quenchers (about 50
ps), the fluorescence induced by a further probe pulse gets its maximum value F m = F 0 + F v . The quantity
F m- F 0
Y- (1)
F m
should be the analogous with pulsed laser sources of the photosynthetic activity quantum yield as measured with
a conventional PAM instrument [12]. Its evaluation has been perforated on a great number of different experi
mental conditions for the algae under test.
For measuring Y we used the excimer laser as pump pulse and the triplicated NdrYAG laser as
probe pulse. The energies of probe and pump pulses, as well as their durations, are critical in order to obtain
reliable measurements and have been initially set according to the indications of ref [11], and then carefully
checked [13].
Fig. 4 shows probe and pump-probe spectra for Spirulina and Tetraselmis. It should be noted the
increase in F690 due to the pump effect Finally Fig. 5 shows Y versus time ona sample of Spirulina during an
experiment of photoinhibition. The sample was illuminated with a 1500 pmol/m2/s lamp and the strong de
crease of Y with time, indicating photoinhibition, is well evident.
Fig. 5 - Yield Y vs time in an experiment of photoinhibition.
CONCLUSION
The lidar fluorosensor built at the ENEA Research Center in Frascati has been proved to be an effective tool in
algal species recognition, and in remote measuring the concentration of some phytoplankton species of remaik-
