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Y(II) Protocol: Quantum Photosynthetic Yield of PSII
Y(II) or
)
F/Fm’
Yield (or 
)
F/Fm’, or Fm’-Fs /Fm’) (Genty 1989) is a fast (it normally takes about two
seconds) light adapted, steady-state photosynthetic test, that provides a measure of the
proportion of the amount of light used in photochemistry in PSII to the amount of light
absorbed by chlorophyll associated with PSII. (Maxwell and Johnson 2000). It can also
be viewed as achieved efficiency of photosystem II under current steady-state
photosynthetic lighting conditions with quenching mechanisms at steady-state
photosynthesis. It is the easiest test to take because the test sample does not have to be
dark adapted. Modulated fluorometer design allows this measurement.
This is different from the Fv/Fm measurement that measures maximum photosynthetic
efficiency in a dark adapted state. Fv/Fm does not measure photosynthetic efficiency
while photosynthesis is taking place.
Y(II) provides a ratio that is a related to electron transport and achieved photosynthesis
efficiency. Under laboratory conditions, and most stress conditions, Y(II) provides a
measure directly related to linear electron transport. It has been found to correlate to
carbon assimilation in a linear manner for C
4
plants, and in a curve linear manner for C
3
plants. (Genty 1989, 1990). Results are different for C
3
plants than for C
4
plants due to
photorespiration, pseudo-cyclic electron transport, and possibly other electron sinks. For
more information, review the Plant Stress Guide provided at
www.optsci.com .
In the Y(II) test mode, it is assumed that the sample is being illuminated by an actinic
light source to drive photosynthesis. Either the sun or an artificial light can be used. The
OS1p has a white light diode that can be used as an actinic light source for Y(II)
measurements.
Yield Y(II) has been shown to measure plant stress more successfully than Fv/Fm for
some types of plant stress. For example it is more effective than Fv/Fm for heat stress and
for water stress. For details about other types of plant stress, please review the Opti-
Sciences Plant stress guide.
www.optsci.com
Y(II) varies with light level and temperature. For that reason, it is imperative that the
light level is measured and controlled. Only samples with very similar PAR values
should be compared, and only sun leaves should be compared with sun leaves. The
photosynthetic make up of sun leaves and shade leaves are different. With this in mind, a
PAR Clip is highly recommended for all field measurements.
Steady State photosynthesis is an equilibrium of various plant mechanisms at a specific
light level. Factors that determine the time required for steady state photosynthesis to be
reached include the xanthophyll cycle,

)
ph of the thylakoid lumen, and state transitions.
The xanthophyll cycle, 
)
ph of the thylakoid lumen typically reach steady state in a few
minutes, and state transitions can take fifteen to twenty minutes. Maxwell and Johnson
(2000) found that it took fifteen to twenty minutes for 22 British terrestrial plants to reach
steady state photosynthesis.