When chlorophyllous tissue is irradiated with photosynthetically active radiation (400-700 nm) or wavelengths shorter than 400 nm, it emits radiation of longer wavelengths (approx. 680-760 nm). This fluorescence originates mainly from chlorophyll a associated with photosystem II (PS II). The measurement of the kinetics of chlorophyll fluorescence has been developed into a sensitive tool for probing state variables of the photosynthetic apparatus in vivo. In an ecophysiological context, this is a useful technique to quantify effects of stress on photosynthetic performance that is also applicable under field conditions.
Photons absorbed by chlorophyll give rise to (1) an excited state of the pigment which is channelled to the reaction center and may give rise to photochemical charge separation. The quantum yield of this process is given by fP. Alternative routes for the excitation energy are (2) dissipation as heat (fD) and (3) fluorescent emission (fF). These three processes are competitive. This leads to the assumption that the sum of the quantum yields of the three processes is unity:
Since only the first two processes are subject to regulation, the magnitude of fluorescence depends on the added rates of photochemistry and heat dissipation. Measurement of fluorescence, therefore, provides a tool for quantification of these processes.
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