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Relative Electron Transport Rate
Relative Electron Transport Rate - ETR
:
mols = (Y) (PAR) (.84) (.5)
ETR or Relative Electron Transport Rate is a parameter that is measured with a PAR Clip.
ETR is a relative measurement that provides comparative electron transport rates for PSII at
different light or radiation levels. It is considered relative because chlorophyll fluorescence
does not correlate exactly with absolute gas exchange measurements. While most
fluorescence occurs in the upper most layers of the leaf, gas exchange measures the effects of
photosynthesis in deeper layers as well. (Schreiber 2004).
Even with this in mind, relative ETR can be extremely valuable. While exact correlation to
gas exchange carbon assimilation is not possible, linear correlation is possible with C
4
plants
(Genty 1989) and a curvilinear correlation is possible with C
3
plants (Genty 1990). While four
electrons must be transported for every CO
2
molecule assimilated, or O
2
molecule evolved,
differences from gas exchange measurements can occur under conditions that promote of
photorespiration, cyclic electron transport, and nitrate reduction (Schreiber 2004) (Baker,
Oxborough 2004). For more detailed information concerning the relationship between
fluorescence and gas exchange measurements again refer to Opti-Sciences application note
#0509 on Yield measurements.
The equation for Relative ETR is ETR = (Yield or
F/Fm’) (0.84) (0.50) (PPFD or PAR)
In this equation, Yield represents overall PSI and PSII yield. It assumes an average leaf light
absorbance to be 84%, and the portion of light provided to PSII to be 50%. PPFD is PAR
irradiation measured very near the leaf in micromoles or micro-einsteins (equivalent units).
The end result is a close approximation of PSII ETR that can be used for relative evaluation
of different samples. With the OS1p, the actual values for leaf light absorbance and the
portion used by PSII can be input into the actual formula to provide more accurate results.
ETR absorption setting screen
This screen can be reached by touching the box with ETR in it on the Y(II) measuring
screen.
The values for leaf absorption and PSII absorption can be changed by
touching the green box surrounding the value. For recommended measured
values found for different types of plants, refer to the papers listed below.
The PAR Correction factor allows for PAR measurement correction of
different types of light sources other than sun light, and the internal actinic
light source. Under most conditions, it should be set at 1.00.
It also allows for distance and location error correction capability. When
using an artificial light source, the location of the PAR sensor may cause an
error relative to the location of the leaf. This correction factor allows
following the correction procedure in a paper by Rascher (2000) listed
below.
The default values of ETR= Y (0.50) (0.84) (PAR) are average plant values
used for relative comparisons.
