918 
The construction of optical system provides coincidence of pump and probe 
beams in the sounding water volume and collection of backscattered optical 
response to the input aperture of OMA polychromator. The folding mirror is 
mounted at falseboard of a vessel. The OMA is intended for spectral analysis 
of optical response from the water. The OMA detector is gated by 50 ns pulses 
synchronized with probe laser pulses, so the system is insensitive both to 
ambient solar irradiation and to Chl-a fluorescence emission induced by pump 
pulses. This technique allows to conduct day-and-night continuous measurements 
at the routes of more than 1000 km long. 
Recently the described system has been adapted to measure the variable 
Chl-a fluorescence from plants (Fig.l, and Gorbunov and Chekalyuk, 1994a). 
5. - OPTIMALIZATION OF LASER EXCITATION 
The laboratory experiments on laser implementation of pump-and-probe technique 
have been carried out for various algal species and leaves of green plants. As 
the first ones are described in (Chekalyuk and Gorbunov, 1994), we will 
briefly discuss the experiments with leaves on optimalization of laser 
excitation parameters (intensities of pump and probe pulses, delay between 
them). More information one could find in (Gorbunov and Chekalyuk, 1994a). 
The spectra of Chl-a fluorescence from a maize leaf induced the by weak 
probing pulses (I = 10 22 quanta*cm' 2 s _1 ) are shown in Fig.2 for various 
intensities of the pumping pulses 1^ (from 0 to 500 kW/cm" 2 ). Fig.3 represents 
the calculated dependence of fluorescence intensity at 690 nm ( F690) and 735 
nm (F735) on I . 
550 600 650 700 750 800 
Wavelength (nm) 
PUMP PULSE INTENSITY (relative) 
Figure 2 (left). Spectra of chlorophyll-a fluorescence from the maize 
leaf measured in response to weak probe pulse for various intensities 
of the preceding pump pulse. 
Figure 3 (right). Dependencies of chlorophyll-a fluorescence at 690 nm 
(F690) and 735 nm (F735) excited by weak probe pulse on the intensity 
of the preceding pumping pulse. 
The increase in I caused the rise in amount of the PS II RCs closed by 
1 
the pump pulse and, as a result, the corresponding increase (Fig.2,3) in 
intensity of fluorescence induced by the following probe pulse. The remarkable 
feature of that rise were observed changes in Chl-a fluorescence spectra with 
the variation in pumping intensity, and corresponding difference between 
fluorescence increase observed at 690 nm (4 times) and 735 nm (about 2.5 
times). The possible origin of this phenomenon is discussed in (Gorbunov and 
Chekalyuk, 1994a).