119
of a proton gradient across the thylakoid membrane, the light-dependent conversion of
violaxanthin to zeaxanthin, and proper redox-poising of an unidentified, antimycin-sensitive
electron transport component (Demmig-Adams, 1990). Interconversion of violaxanthin and
zeaxanthin via the xanthophyll cycle has not been identified in any other algal class.
Violaxanthin and zeaxanthin are absent in several algal classes, particularly the blue-green
algae. It is certainly possible that other carotenoids such as diadinoxanthin and diatoxanthin
may be capable of functioning in quenching of chI excited states. Clearly, a broader
understanding of non-photochemical quenching processes is a prerequisite for using
fluorescence as a diagnostic tool in most algal classes.
There is a more general problem in interpreting fluorescence measurements from any
photosynthetic sample. Our present knowledge of processes regulating fluorescence yield is
based on relatively few experiments involving a limited number of species grown under a
particular set of conditions. One cannot neglect the possibility that the quantitative relationship
between a measured photosynthetic parameter and fluorescence yield may vary with growth
conditions, or that variation in growth conditions may produce a range of effects between
species. For example, the relationship between .dpH and
especially with stress conditions during growth (Laasch, 1987). Under these conditions, even
if all other factors remain constant, it would be difficult to unambiguously relate fluorescence
changes to the size of the pH gradient across the thylakoid membrane.
Fluorescence as a predictor of photosynthetic rates. There is a widespread interest among
the biological oceanographic community in the possibility that in vivo fluorescence
measurements can be used to predict in situ rates of photosynthesis. This hypothesis is largely
based on the assumption that parameters quantifying the rise in variable fluorescence from
F 0 to F max, measured in the presence of the inhibitor DCMU, are proportional to the amount
of PS II present and are therefore related to photosynthetic capacity (Malkin et a1., 1981).
Similar measurements have utilized enhancement of steady-state in vivo fluorescence upon
addition of DCMU as a measure of photosynthetic rate (Roy and Legendre, 1979; Vincent,
1980). Laboratory experiments have generally demonstrated a reasonable correlation between
variable fluorescence parameters and photosynthetic capacity measured by 14C incorporation
of a proton gradient across the thylakoid membrane, the light-dependent conversion of
violaxanthin to zeaxanthin, and proper redox-poising of an unidentified, antimycin-sensitive
electron transport component (Demmig-Adams, 1990). Interconversion of violaxanthin and
zeaxanthin via the xanthophyll cycle has not been identified in any other algal class.
Violaxanthin and zeaxanthin are absent in several algal classes, particularly the blue-green
algae. It is certainly possible that other carotenoids such as diadinoxanthin and diatoxanthin
may be capable of functioning in quenching of chI excited states. Clearly, a broader
understanding of non-photochemical quenching processes is a prerequisite for using
fluorescence as a diagnostic tool in most algal classes.
There is a more general problem in interpreting fluorescence measurements from any
photosynthetic sample. Our present knowledge of processes regulating fluorescence yield is
based on relatively few experiments involving a limited number of species grown under a
particular set of conditions. One cannot neglect the possibility that the quantitative relationship
between a measured photosynthetic parameter and fluorescence yield may vary with growth
conditions, or that variation in growth conditions may produce a range of effects between
species. For example, the relationship between .dpH and
especially with stress conditions during growth (Laasch, 1987). Under these conditions, even
if all other factors remain constant, it would be difficult to unambiguously relate fluorescence
changes to the size of the pH gradient across the thylakoid membrane.
Fluorescence as a predictor of photosynthetic rates. There is a widespread interest among
the biological oceanographic community in the possibility that in vivo fluorescence
measurements can be used to predict in situ rates of photosynthesis. This hypothesis is largely
based on the assumption that parameters quantifying the rise in variable fluorescence from
F 0 to F max, measured in the presence of the inhibitor DCMU, are proportional to the amount
of PS II present and are therefore related to photosynthetic capacity (Malkin et a1., 1981).
Similar measurements have utilized enhancement of steady-state in vivo fluorescence upon
addition of DCMU as a measure of photosynthetic rate (Roy and Legendre, 1979; Vincent,
1980). Laboratory experiments have generally demonstrated a reasonable correlation between
variable fluorescence parameters and photosynthetic capacity measured by 14C incorporation