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CHAPTER 12
MEASUREMENTS WITH THE MINI-PAM
the overall yield of assimilation is equivalent to the yield of energy
conversion at PS II. For a limitation to become apparent, the system
must be 'put under light pressure'. For example, if some stress factor
has caused a decrease in Calvin cycle activity, this will be only
expressed in YIELD or ETR, if a sufficiently high PAR is applied to
make dark enzymic steps of the Calvin cycle limiting. The maximal
YIELD of a dark-adapted sample, as measured by Fv/Fm (see 12.3.1)
and by YIELD-values at low PAR, will be affected only, if the stress
treatment has caused a limitation at the level of the primary reactions
of energy conversion (excitation energy capture efficiency and
charge separation efficiency at the reaction centers). This is, for
example, the case after photoinhibitory treatment. Photoinhibition
occurs, if a sample is exposed for longer time periods to excessive
light intensities. To what extent a given light intensity is excessive
depends on the physiological state of the sample and can be judged
by YIELD-measurements (see following section 12.3.9 on LIGHT
CURVES). A suppression of YIELD upon exposure of a sample to
excessive light does not necessarily reflect permanent damage, but
can also reflect a high potential for photoprotection by non-radiative
energy dissipation. The latter is associated with high values of qN
and NPQ (see 12.3.6).
12.3.9 LIGHT CURVE (menu point 17) and LIGHT-CURVE+REC
(menu point 18)
The recording of a LIGHT CURVE involves 9 consecutive
YIELD-measurements. The illumination series may start at
ACTINIC INTENSITY 1, 2, 3, 4 or 5 (set via the LC-INT function,
MODE-menu point 20). This feature allows to adjust the range of
applied intensities to the light adaptation properties of the sample
(sun or shade plant). When choosing the start-intensity, it should be
also considered that a lower intensity range reduces the danger of
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