Being a byproduct of primary photosynthesis stages, Chl-a fluorescence 
emission depends to a large extent on the physiological state of 
photosynthetic apparatus and, in particular, on the state of PS II RCs. If the 
photosynthetic apparatus of PS II is in good functional state, under weak 
light the majority of PS II RCs are in open state (P680 Pheo Qa, where P680, 
Pheo and Qa are respectively the primary donor, the intermediate pheophytin 
acceptor, and the first quinone acceptor in PS II RC), providing maximum 
efficiency of light energy conversion. 
In this case Chl-a fluorescence yield is minimal (F ). The influence of 
min 
saturated light (or addition of herbicide DCMU) leads to transfer of RCs to 
the so-called closed state with reduced Qa ( P680PheoQK ). The resulting 
fluorescence yield reaches its maximum value $> due to addition of variable 
m 
fluorescent component originating from charge recombination in the closed PS 
II RCs. In common case, the actual fluorescence yield (<!») lies between its 
minimum and maximum values, depending on the states of PS II RCs. The 
difference $ = $ - $> is called variable fluorescence. 
V Ifl 
3. - LIDAR IMPLEMENTATION OF PUMP-AND-PROBE TECHNIQUE 
The basic idea of pump-and-probe technique (Falkowski and Kiefer, 1985) 
is the use of special actinic (pump) flash of light which is sufficiently 
intense to cause the transition of all initially open PS II RCs to the closed 
state (P680PheoQA ). The maximum Chl-a fluorescence $ is measured in response 
m 
to excitation by the weak probe flash of light, following the pump flash with 
a certain delay. The actual Chl-a fluorescence $> is measured in response to 
the probe flash preceding the pump flash. 
Relying on these measurements, corresponding relative increase in Chl-a 
fluorescence yield (i.e. the relative yield of variable fluorescence) n = (i> 
m 
- $)/<!> = Ai>/<I> can be calculated. The possibility to estimate this parameter 
n> m 
with pump-and-probe technique is of particular importance, as its value is a 
quantitative measure of the efficiency of light energy conversion in 
photochemical reactions in PS II (e.g. see Falkowski and Kiefer, 1985). 
Recently it has been shown (Genty et al., 1989) that this parameter determines 
the quantum yield of non-cyclic electron transport due to photochemistry. 
Since the pump-and-probe technique allows to conduct relatively simple 
measurements of in a wide range of variations of background irradiance, 
the use of this technique provides the direct way to monitor the on-going 
photosynthesis under ambient light in natural conditions. 
The implementation of this approach has conceptually increased the 
potential of fluorometric techniques in the field, in particular - in sea 
research (Falkowski and Kolber, 1990; Falkowski et al., 1991). The use of 
submersible pump-and-probe fluorometer provides information about vertical 
profiles of the on-going photosynthesis in real-time, in situ, nondestruc 
tive^. Recent field studies indicated a good agreement between these 
estimates and conventional radiocarbon-based measurements of primary 
production (Falkowski and Kolber, 1990; Falkowski et al., 1991). Nevertheless, 
as in the case of any conventional shipboard observations, the common problems 
are relatively low space/time resolution and corresponding difficulties in 
investigations of fast-developing processes distributed over large areas. 
The next essential step is therefore the development of lidar implementa 
tion of pump-and-probe technique, capable of remote monitoring both photosyn 
thetic efficiency and phytoplankton abundance in the sea from onboard a moving 
carrier (ship, aircraft, helicopter). Our idea (Chekalyuk and Gorbunov, 1992) 
was to use pulsed lasers as sources of pump and probe light flashes in the 
water column. Due to low beam divergence and high power of laser pulses, in 
principle it is possible to ensure required modes of excitation remotely.