20
6. Measurements not taken at steady state photosynthesis can lead to non-linearity caused by
state transitions. This error can be in the range of 10% to 30% depending on the organism
(Allen and Mullineau 2004). The error can be avoided by allowing plant samples to reach
steady state photosynthesis, a process that takes between fifteen and twenty minutes (Maxwell
and Johnson 2000).
7. At very high light stress levels, the correlation between ETR and CO
2
assimilation breaks
down. It is thought by some to be caused by the inability of the most intense saturation light
sources to completely close all PSII reaction centers under high light stress conditions. To
compensate for this issue, Earl (2004) uses saturation pulses at various levels and extrapolates
the saturation pulse fluorescence intensity at infinity using linear regression analysis. This
method restores the correlation of ETR and CO2 assimilation and it is an option that is
offered on the Opti-Sciences OS1p and the OS5p.
8. While linear correlation and curvilinear correlation are possible (Genty 1989), (Genty
1990), (Baker Oxborough 2004), exact correlation between fluorescence ETR and gas
exchange ETR is not possible due to the fact that fluorescence comes from only the upper
most layers of the leaf while gas exchange measurements measure lower layers as well
(Schreiber 2004).
9. In CAM plants, gas exchange measurements are not possible during daylight hours so
Yield measurements can provide insights into daytime light reactions (Rosenqvist and van
Kooten 2006).
As illustrated by the exceptions listed above, in some cases the relationship between light
reactions and dark reactions is not straightforward. The energy molecules ATP and NADPH
can be used for carbon fixation and for photorespiration (Rosenqvist and van Kooten 2006),
or light reaction electrons may flow to other electron sinks (Ohashi 2005), (Baker Oxborough
2004), (Fryer M. J., Andrews J.R., Oxborough K., Blowers D. A., Baker N.E. 1998). For this
reason, it is not uncommon for authors to differentiate between work done under non-
photorespiratory conditions and under photorespiratory conditions (e.g. Earl 2004), (e.g.
Genty B, Harbinson J., Baker N.R. 1990).
The Opti-Sciences chlorophyll fluorometer models OS5p and OS1p can be used to make
Quantum Photosynthetic Yield Y(II) measurements. Both units accommodate optional digital
PAR Clips.
Yield,Y(II), is the more versatile fluorescence measuring parameter, but it is best to use a
system that offers multiple test parameters for diverse stress applications. While systems that
provide true yield measurements tend to cost more than ones that provide just Fv/Fm
measurements, they offer greater capability.
