62
U. Schreiber et at.
feasible in systems which display high ratios of PS I vs. PS II activity,
as, e.g., in cyanobacteria (Melis 1991; Mi et al. 1992) and bundle-sheath
chloroplasts. In systems with balanced PS I vs. PS II, any backcycling of
electrons from the PS I acceptor side into the plastoquinone pool will
inevitably lead to "overreduction" of the intersystem chain (As ada et al.
1990). Hence, although there can be no doubt about the existence of cyclic
PS I electron flow per se (Arnon and Chain 1975; Moss and Bendall 1984),
its contribution to membrane energization under conditions of excess radiation in vivo is still uncertain. In any case, even cyclic flow requires "poising"
by O2 (Arnon and Chain 1977) and the Oz-dependent flux involved in the
poising reaction may not be insignificant when light is applied which drives
both photoreactions.
Although important questions, like the role of cyclic PS I in membrane
energization and the actual molecular mechanism of nonphotochemical
quenching, still await final clarification, the practicability of the fluorescence
quenching analysis, as outlined in Sections 3.5 and 3.6, has been well
established. Hence, the point has been reached for the fluorescence method
to be applied in ecophysiology to study photosynthesis in situ. An example
of in situ measurements is given in the following section.
3.8 In Situ Measurements of .-1F/Fm '
and of Relative Electron Transport Rate
Since it was realized that reliable information on the quantum yield of
photosynthesis can be obtained by the saturation pulse method, particularly
in conjunction with the approach of Genty et al. (1989), great efforts have
been made to develop portable systems for in situ on-line determinations of
.-1FIFm ' and of the relative electron transport rate PFD x .-1FlFm '. Currently
at least one suitable system is commercially available (PAM-2000, Walz) ,
which is an extremely miniaturized computer-controlled version of the PAM
Fluorometer (Schreiber et al. 1986). With this instrument, all relevant fluorescence parameters (Fo, Fo', Fm , Fm ', F) are measured and the resulting
values of FvfFm , .-1FIFm ', NPQ, qp, and qN are calculated. The incident light
intensity (PFD) can be continuously monitored in the leaf plane at the same
site where fluorescence is assessed. The resulting value of PFD x .-1FIFm '
closely reflects relative electron transport rate. In this way, assessment of in
situ photosynthesis has become rather simple and efficient.
Figure 3.9 shows an example of in situ measurements of light saturation
curves of the relative electron transport rate (PFD x .-1FIFm ') obtained with
two leaves of Phaseolus coccineus, one of which had been previously exposed
for 5 min to 43°C. Heat pretreatment was in the dark (2 h after sunset) and
the actual measurements were at noon of the following day. Natural daylight
U. Schreiber et at.
feasible in systems which display high ratios of PS I vs. PS II activity,
as, e.g., in cyanobacteria (Melis 1991; Mi et al. 1992) and bundle-sheath
chloroplasts. In systems with balanced PS I vs. PS II, any backcycling of
electrons from the PS I acceptor side into the plastoquinone pool will
inevitably lead to "overreduction" of the intersystem chain (As ada et al.
1990). Hence, although there can be no doubt about the existence of cyclic
PS I electron flow per se (Arnon and Chain 1975; Moss and Bendall 1984),
its contribution to membrane energization under conditions of excess radiation in vivo is still uncertain. In any case, even cyclic flow requires "poising"
by O2 (Arnon and Chain 1977) and the Oz-dependent flux involved in the
poising reaction may not be insignificant when light is applied which drives
both photoreactions.
Although important questions, like the role of cyclic PS I in membrane
energization and the actual molecular mechanism of nonphotochemical
quenching, still await final clarification, the practicability of the fluorescence
quenching analysis, as outlined in Sections 3.5 and 3.6, has been well
established. Hence, the point has been reached for the fluorescence method
to be applied in ecophysiology to study photosynthesis in situ. An example
of in situ measurements is given in the following section.
3.8 In Situ Measurements of .-1F/Fm '
and of Relative Electron Transport Rate
Since it was realized that reliable information on the quantum yield of
photosynthesis can be obtained by the saturation pulse method, particularly
in conjunction with the approach of Genty et al. (1989), great efforts have
been made to develop portable systems for in situ on-line determinations of
.-1FIFm ' and of the relative electron transport rate PFD x .-1FlFm '. Currently
at least one suitable system is commercially available (PAM-2000, Walz) ,
which is an extremely miniaturized computer-controlled version of the PAM
Fluorometer (Schreiber et al. 1986). With this instrument, all relevant fluorescence parameters (Fo, Fo', Fm , Fm ', F) are measured and the resulting
values of FvfFm , .-1FIFm ', NPQ, qp, and qN are calculated. The incident light
intensity (PFD) can be continuously monitored in the leaf plane at the same
site where fluorescence is assessed. The resulting value of PFD x .-1FIFm '
closely reflects relative electron transport rate. In this way, assessment of in
situ photosynthesis has become rather simple and efficient.
Figure 3.9 shows an example of in situ measurements of light saturation
curves of the relative electron transport rate (PFD x .-1FIFm ') obtained with
two leaves of Phaseolus coccineus, one of which had been previously exposed
for 5 min to 43°C. Heat pretreatment was in the dark (2 h after sunset) and
the actual measurements were at noon of the following day. Natural daylight