18
NOTE: It is possible to misinterpret results if PAR and temperature changes are not
taken into account. One leaf may appear to be stressed compared to another when the
only difference is light irradiation level. PPFD or PAR must be measured very close to
the sample or errors can result.
In addition, it is important not to change the orientation of a leaf and to avoid shading the
sample measuring area with the PAR clip or by other means. Extraneous reflections and
breathing on the sample should also be avoided (Rosenqvist and van Kooten 2006).
PAR Clips also allow measurement of relative ETR or relative Electron Transport Rate. ETR
is a parameter designed to measure the electron transport of PSII. It has also been found to
correlate well with CO2 assimilation. More advanced fluorometers provide built-in
illuminators for greater experimental control of light irradiation intensity. This allows pre-
illumination with a controlled predetermined intensity value for sample comparison.
For reliable Yield and ETR measurements, photosynthesis must be at steady state and with
illumination on the same side of the leaf that is being measured (see number eight under
correlation to carbon assimilation). Steady state photosynthesis is an equilibrium condition
reached after a several minutes of exposure to existing light radiation conditions. Maxwell
and Johnson (2000) tested 22 different species of British plant and found that steady state
occurred in fifteen to twenty minutes in the plants measured. Measurements taken under
variable
lighting conditions may not provide reliable Yield results (Rascher 2000). No dark adaptation
is required for Yield measurements.
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