Fresh perspectives on an established technique: Pulsed Amplitude
Modulation (PAM) chlorophyll a fluorescence
Abstract
Pulsed amplitude modulation (PAM) chlorophyll a fluorescence provides
information about photosynthetic energy transduction. When reliably
measured, chlorophyll a fluorescence provides detailed information about
critical in vivo photosynthetic processes. Such information has recently
provided novel and critical insights into how yield potential of crops
could be improved. While PAM chlorophyll a fluorometers measure
fluorescence intensity per se, herein we articulate the criteria by
which instrumentally detected intensities can be assumed to assess
fluorescence yield, a phenomenon quite different than fluorescence
intensity and one that provides critical insight about how solar energy
is variably partitioned into the biosphere. An integrated mathematical,
phenomenological, and practical discussion of many useful chlorophyll a
fluorescence parameters is presented. We draw attention to, and provide
examples of, potential uncertainties that can result from incorrect
methodological practices and potentially problematic instrumental design
features. Fundamentals of fluorescence measurements are discussed,
including the major assumptions underlying the signals and the
methodological caveats about taking measurements during both dark- and
light-adapted conditions. Key fluorescence parameters are discussed in
the context of recent applications under environmental stress. Nuanced
information that can be gleaned from intracomparisons of
fluorescence-derived parameters and intercomparisons of
fluorescence-derived parameters with those based on other techniques is
elucidated.