ENEA, Dip. Innovazione, Settore Fisica Applicata
CR Frascati, C. P. 65 - 00044 Frascati, ITALY
A milestone in vegetation fluorescence research is the so called Kautsky curve, which actually represents the characteristic time behaviour (kinetics) of induced fluorescence and is measured by exposing a previously dark adapted plant to an intense photosynthetically active light. Pulse Amplitude Modulated (PAM) fluorometers are currently used to measure locally fluorescence yields at the onset of an actinic lamp.
In order to investigate the feasibility of remotely monitoring physiological parameters of living vegetation tissues a lidar fluorosensor instrumentation has been assembled at ENEA-Frascati in different laboratory configurations. Laser Induced Fluorescence (LIF) in the red chlorophyll band @ 690 nm has been detected, under probe laser excitations either @ 355 nm and @ 532 nm, to seek for stress conditions in the investigated samples due to nutrients depletion, the presence of herbicides, the occurrence of photo-inhibition or parasite attacks. The chlorophyll LIF Kautsky profile has been remotely measured from dark-adapted vegetation samples (pine-tree, maize) by superimposing laser probe pulses upon an actinic light illuminating the targets. From the induced fluorescence jump occurring at the lamp onset, the fluorescence yield has been determined, which is correlated to the quantum yield of net photosynthesis occurring in intact leaves.
In addition, a dual laser pump-and-probe methodology has been experimentally tested upon the same targets. By this technique, we directly measured the fluorescence yield Y based on the detection of the red channel induced by a probe alone and probe following pump pulse from the same laser transmitter. The system being able to operate remotely also in the absence of the actinic lamp appears as the most suited for remote sensing of vegetation in its actual environmental radiation conditions. Experimental results have been interpreted in terms of an ad-hoc developed theoretical computer model, which takes into account the different time constants of the deexcitation channels, regulating the photosynthetic process after absorption of the laser energy by the chloroplasts Light Harvesting Antennae. The relationship between the remote sensed laser yield Y and locally detected PAM yield has been clarified, both experimentally and from a theoretical point of view, so as to allow for a correct use of this parameter in field campaigns.