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at its current orientation and changing the angle of the leaf will cause plant mechanisms
to start to adjust to the new actinic light level. Yield measurements are always done at
steady state photosynthesis or errors will result. Reoriented leafs are not at steady state.
According to Maxwell and Johnson (2000), it take between fifteen and twenty minutes
for most leaves to reach steady state. (See the section on Yield in depth discussion)
In addition, the PAR clip should be oriented on the leaf to minimize sample measuring
area shading by the PAR clip and the operator.
What is the value of a PAR clip in photosynthesis measurement?
PAR clips should be used for all field applications and where ambient light and
temperature fluctuate. Since Y(II) measurements and other parameters, change with light
levels and temperature, the ability to study measurements as a function of PAR value or
temperature allow for reliable plant stress measurement.
PAR clips have also been used for fixed level illumination studies and predetermined
variable illumination studies as well. In such cases, quenching studies, quenching
relaxation studies, and plant performance at different light levels can be evaluated as a
function of PAR and temperature. In order to measure ETR a PAR clip is necessary.
PAR Clips are also used for development of rapid light curves.
The Term PAR means photosynthetically active radiation in the wave band between 400-
700 nm. PAR can be measured in different dimensions such as Watts per meter or in
micro- Einsteins (:E)or micro-moles (:mols). When using a PAR Clip, dimensions will
always be in the equivalent terms, micro-Einsteins, or micro-moles.
PPFD, or “photosynthetic photon flux density”, is the number of PAR photons incident
on a surface in time and area dimensions. These terms are equivalent for PAR Clip leaf
radiation measurements. Furthermore, both can be presented in either of the equivalent
dimensions, micro-moles or micro-Einsteins.
Both the PAR and PPFD terms have used extensively in biology. Micro-Einsteins, and
micro-moles have also been used extensively in biology.
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