Nonphotochemical Quenching of Chlorophyll Fluorescence

Abstract

Measurement of chlorophyll fluorescence is an invaluable method for examining the interaction of photosynthetic organisms with their environment and for the non-invasive probing of the metabolism of the chloroplast (Krause & Weis, 1991; Horton & Bowyer, 1990). Fluorescence, most of which is emitted from PSII chlorophylls at room temperature, can be quenched from its maximum value, F_m, as found in a healthy dark adapted sample, either photochemically or nonphotochemically. Whereas photochemical quenching occurs due to the process of energy consumption in photosynthetic electron transport, the quenching of fluorescence nonphotochemically in general terms reflects a change in “state” of the photosynthetic apparatus — thus, its measurement has provided evidence of the presence of, on the one hand, regulatory mechanisms, and on the other, inhibition and damage to the thylakoid membrane under stress conditions. Nonphotochemical quenching is measured most often as the decrease in the F_m value to some new value F_m’, and this can be recorded in any given steady state using modulated fluorimetry, in the laboratory or in the field, by application of a brief saturating light pulse to close PSII centres (Quick & Horton, 1984; Schreiber et al., 1986). If the change in F_m is normalised on F_m’, the value of qN then approaches direct proportionality to the rate of energy dissipation (Demmig-Adams, 1990; Bilger & Björkman, 1990); however, this is only strictly correct if the quenching process is of the Stern-Volmer type. Alternatively, qN can be calculated by normalising the change in F_m on the F ^’_v , and in this case has a range of 0 to 1 (Schreiber et al., 1986; van Kooten & Snel, 1990); the disadvantage of this calculation is that F_o’ needs to be measured to determine F_v’, something quite difficult in the field, and furthermore, the qN value becomes very insensitive to increases in energy dissipation rate at high values.