122
Indeed, there is strong evidence that in vivo fluorescence per unit chI a is highly variable
(Loftus and Seliger, 1975; Falkowski and Kiefer, 1985; Keller, 1987). Even if the samples
are carefully adapted to the state where qp = 1 and
Owens and CS Ting, unpublished results). In vivo fluorescence should therefore be regarded
only as a qualitative indicator of algal biomass.
Fluorescence as an indicator of ''physiological state ". The number of factors that affect the
yield of chI fluorescence in vivo outlined above might suggest that we should expect a poor
quantitative relationship between the yield of fluorescence and photosynthesis under varying
environmental conditions. However, Walker and co-workers have elegantly demonstrated (in
higher plants) that relative changes in fluorescence yield are tightly coupled to photosynthetic
rate under conditions where photosynthetic rates are changing rapidly (changes in light
intensity or CO 2 availability) (Sivak and Walker, 1985). Subsequent work by a number of
investigators in higher plants and algae suggests that the measurement of photochemical and
non-photochemical fluorescence quenching can provide at least qualitative information on the
effects of environmental conditions on the basic reactions of photosynthesis (ie: physiological
state). For example, Holmes et al. (1989) have used the relationship between fluorescence
quenching and photosynthetic rate described by Weis and Berry (1987) to correctly predict
the diversion of reductant from CO2 to nitrogen assimilation under conditions of transient
nitrogen uptake. During nitrogen uptake, changes in the fluorescence quenching are due to
direct effects of altered demand for A TP and NADPH as well as to changes in functional
antenna size of PS II (light state transitions)(Turpin and Bruce, 1990). Miller et al. (1988)
have similarly demonstrated diversion of ATP from CO 2 fixation to inorganic carbon uptake.
Under controlled laboratory conditions, a number of studies have shown that the quantum
yield of photosynthesis is affected by both light and nutrient availability (Welsch meyer and
Lorenzen, 1981; Bjorkman and Demmig, 1987; Cleveland and Perry, 1989; Kolber et al.,
1988; Herzig and Falkowski, 1989). Equations 6 and 7 suggest that decreases in the yield of
photochemistry must be accompanied by decreased fluorescence yield and are likely to be
attributed to increased non-photochemical quenching in PS II. In all laboratory studies,
changes in fluorescence yield in response to growth conditions are determined in a controlled
Indeed, there is strong evidence that in vivo fluorescence per unit chI a is highly variable
(Loftus and Seliger, 1975; Falkowski and Kiefer, 1985; Keller, 1987). Even if the samples
are carefully adapted to the state where qp = 1 and
Owens and CS Ting, unpublished results). In vivo fluorescence should therefore be regarded
only as a qualitative indicator of algal biomass.
Fluorescence as an indicator of ''physiological state ". The number of factors that affect the
yield of chI fluorescence in vivo outlined above might suggest that we should expect a poor
quantitative relationship between the yield of fluorescence and photosynthesis under varying
environmental conditions. However, Walker and co-workers have elegantly demonstrated (in
higher plants) that relative changes in fluorescence yield are tightly coupled to photosynthetic
rate under conditions where photosynthetic rates are changing rapidly (changes in light
intensity or CO 2 availability) (Sivak and Walker, 1985). Subsequent work by a number of
investigators in higher plants and algae suggests that the measurement of photochemical and
non-photochemical fluorescence quenching can provide at least qualitative information on the
effects of environmental conditions on the basic reactions of photosynthesis (ie: physiological
state). For example, Holmes et al. (1989) have used the relationship between fluorescence
quenching and photosynthetic rate described by Weis and Berry (1987) to correctly predict
the diversion of reductant from CO2 to nitrogen assimilation under conditions of transient
nitrogen uptake. During nitrogen uptake, changes in the fluorescence quenching are due to
direct effects of altered demand for A TP and NADPH as well as to changes in functional
antenna size of PS II (light state transitions)(Turpin and Bruce, 1990). Miller et al. (1988)
have similarly demonstrated diversion of ATP from CO 2 fixation to inorganic carbon uptake.
Under controlled laboratory conditions, a number of studies have shown that the quantum
yield of photosynthesis is affected by both light and nutrient availability (Welsch meyer and
Lorenzen, 1981; Bjorkman and Demmig, 1987; Cleveland and Perry, 1989; Kolber et al.,
1988; Herzig and Falkowski, 1989). Equations 6 and 7 suggest that decreases in the yield of
photochemistry must be accompanied by decreased fluorescence yield and are likely to be
attributed to increased non-photochemical quenching in PS II. In all laboratory studies,
changes in fluorescence yield in response to growth conditions are determined in a controlled