Regulation of Light Harvesting in Green Plants (Indication by Nonphotochemical Quenching of Chlorophyll Fluorescence).

ce has become one of the most powerful methods for assessing photosynthetic performance in plant physiological experiments (Horton and Bowyer, 1990; Krause and Weis, 1991). This has resulted almost entirely from the development of methods to distinguish photochemical and nonphotochemical quenching of fluorescence. Moreover, it is now clear that the process of nonphotochemical quenching itself indicates important regulatory adjustments in the photosynthetic membrane in response to altered external and internal conditions (Demmig-Adams and Adams, 1992; Horton and Ruban, 1992). In particular, the dissipation of excess absorbed excitation that is monitored by the main component of nonphotochemical quenching is a process that is necessary if plants are to avoid photoinhibition and photodestruction under conditions of light stress. When light is absorbed by the Chl molecules in the thylakoid membrane, the excited state has several alternative and competing fates: a small proportion is emitted as fluorescence, but, under light-limiting conditions, the major pathway of de-excitation is through photosynthetic electron transfer. The effect of photochemical utilization of energy is to quench fluorescence, and it is well known that when photosynthetic electron flow is saturated the yield of fluorescence rises. This photochemical quenching has been termed qP and, using the light-doubling principle as applied with modulated fluorimetry, it is easy to calculate it in leaves, chloroplasts, and cells (Schreiber et al., 1986; Horton and Bowyer, 1990; van Kooten and Snel, 1990). However, qP does not account for all of the quenching observed. Indeed, in light saturating for electron transport, qP tends to zero, yet there can be large amounts of quenching. Such quenching is therefore called nonphotochemical quenching and refers to the difference between the initial, dark-adapted maximum level of fluorescence and that recorded after a period of illumination. This quenching can be calculated in a number of ways, leading to it being termed variously as qN (Schreiber et al., 1986; van Kooten and Snel, 1990), NPQ (Bilger and Bjorkman, 1994), or SV, (Gilmore and Bjorkman, 1994); these all refer to the