19
Correlation to Carbon assimilation:
In 1989, Genty developed the yield measurement and provided strong evidence of a linear
correlation between Yield measurements, Electron Transport Rate, and CO2 assimilation for
C4 plants (Baker and Oxborough 2004) and many others have confirmed the relationship
(Edwards and Baker 1993), (Krall and Edwards 1990, 1991), (Siebke 1997). It was found that
a curve-linear correlation between Yield and CO2 assimilation exists for C3 species where
photorespiration can also use significant products of electron transport (Genty 1990),
(Harbinson 1990), (Baker and Oxborough 2004). Psydo-cyclic electron transport and other
electron sinks may also be involved.
Limitations of Y(II) or )F/Fm’
The strong relationship between Yield and CO2 assimilation correlation has been reaffirmed
repeatedly by many researchers with the following caveats:
1. There is a small percentage of chlorophyll fluorescence that comes from photosystem I that
does not change with light intensity (PPFD) or plant stress. Therefore, the error is greatest at
very high light levels when yield is minimized and PSI fluorescence remaining constant. This
error is not large (Baker Oxborough 2004).
2. “Super-saturating flash” error is produced by using a very intense saturation light source
that is longer that 2 milliseconds causing multiple turnovers of primary PSII receptor Q
A
and
the reduction of plasotoquinone to plastoquinol. This raises Fms (or Fm’) and can cause an
overestimate of Yield by less than 10% (Baker and Oxborough 2004), (Schreiber 2004). Use
of a super-saturation flash is by far the most common method of measuring yield in higher
plants.
3. Cold stress can produce a non-linear correlation with CO
2
assimilation. Electron transport
of PSII in cold stressed corn far exceeds the requirements for CO
2
assimilation by more than
three to one. This indicates, that under these conditions, other electron sinks are at work. The
ratio of ETR to CO
2
assimilation, under cold stress, can be diagnostic for cold stress. (Fryer
M. J., Andrews J.R., Oxborough K., Blowers D. A., Baker N.E. 1998)
4. The ratio of ETR to CO
2
assimilation can be diagnostic for water stress in C
3
plants. C
3
plants exhibit strong electron transport rates for early and moderate levels of water stress even
when CO
2
assimilation has decreased due to water stress. This indicates that there are other
electron sinks for electron transport. (Ohashi 2005). This problem of early water stress
measurement and detection may be overcome by using the Burke assay (Burke 2010). Yield
can be used to measure very early water stress (Burke 2007 and Burke 2010).
5. Mangrove leaves growing in the tropics. Here again electron transport rate is more that
three times that of CO
2
assimilation. It is believed that this is mostly due to reactive oxygen
species as an electron sink. (Baker Oxborough 2004), (Cheeseman 1997)
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