CHAPTER 12 MEASUREMENTS WITH THE MINI-PAM
intensities at sample site being exceptionally low or high) or for
differences in light saturation properties of plants. The latter aspect is
particularly relevant in conjunction with the automatic recording of
light response curves (see 12.3.9). It is important to note that the
relationship between AL-FACT and PAR is non-linear. It depends on
the setting of ACT-INT (menu point 15) and also on battery voltage.
For example, at setting 10 and with a freshly charged battery the
PAR is increased by a factor of ca. 1.7 when AL-FACT is increased
from 1.0 to 1.5, and PAR is decreased by a factor of ca. 0.4 when
AL-FACT is decreased from 1.0 to 0.5. In practice, it is
recommended to measure the effective PAR with the Leaf-Clip
Holder 2030-B.
The duration of the actinic illumination periods is set via menu
point 14: ACT-WIDTH, with an upper limit of 5:00. For longer
illumination times an external actinic light source, like the External
Halogen Lamp 2050-HB is recommended. When several actinic
illumination periods are consecutively triggered, as with CLOCK-
operation or LIGHT CURVE recordings (see 12.3.9), the ACT-
WIDTH should be small, in order to avoid excessive internal heating
of the MINI-PAM. In these applications it is limited to 3 min.
The ACT + YIELD function provides very essential information
on the state of the photosynthetic apparatus of a sample. At a given
photon flux density of photosynthetically active radiation (PAR),
which can be monitored by the micro-quantum-sensor incorporated
in the Leaf-Clip Holder 2030-B, the measured values of YIELD and
ETR of different samples can be directly compared and interpreted in
terms of relative electron transport rates. The efficiency of
photosynthetic electron transport can be limited by numerous steps in
the long sequence of reactions between the primary process of
photochemical energy conversion at the reaction centers and the
export of the assimilates out of the chloroplasts. In the steady state,
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