9
It is in PSII that oxygen evolution and the splitting of water occurs. In the presence of light
energy, an electron is pulled from a water-splitting complex and is used to reduce a PSII
reaction center.
Charge separation occurs as Q
A
, the primary plastoquinone receptor, is reduced to Q
A-
.
Electrons flow from there to other nearby plastoquinone molecules in the thylakoid membrane
by oxidation-reduction reactions. They act as energy transfer molecules in an electron
transport chain.
The next stop is a cytochrome b
6
f complex where a proton is supplied to the thylakoid lumen.
These protons and those supplied by the splitting of water are used by an ATP pump in the
thylakoid membrane, in the presence of ATP synthase, to create ATP from ADP. Eventually
the cytochrome b
6
f complex also supplies an electron to PSI.
PSI then goes through a somewhat similar process to PSII, and eventually produces NADPH
at the end of PSI. Both ATP and NADPH are used as energy sources to drive the dark
reaction, called the Calvin Benson Cycle of carbon fixation.
Factors such as light levels, light quality, water availability, nutrient availability, heat, cold,
herbicides, pesticides, pollution, disease, and genetic make up can all have an impact on CO
2
assimilation, plant health and condition. When these factors are not provided at optimal levels
plant stress occurs, and most of these types of plant stress are also reflected in the
fluorescence signal from PSII (For more detailed information about the best test for specific
types of plant stress, consult the Plant Stress guide supplied on this disc or visit
www.optisci.com
under stress testing).
In 1989, Bernard Genty found that there is a linear correlation between fluorescent quantum
photosynthetic yield measurements in C
4
plants and CO
2
assimilation. In 1990, Genty found a
curve linear correlation between fluorescent quantum photosynthetic yield and CO
2
assimilation in C
3
plants, where photorespiration can also use significant products of electron
transport. Psydo-cyclic electron transport and other electron sinks may also be involved.
Different types of plant stress affect PSII differently, therefore one should consult the Plant
Stress Guide on this disc or contact Opti-Sciences at
www.optisci.com
to determine the best
measuring protocol or special assay before working with a specific type of plant stress.
Research referenced in the Plant Stress guide shows that while some types of plant stress
affect chlorophyll fluorescence of a plant in a dark adapted state (Fv/Fm), measuring some
types of plant stress, at a sensitive useful level, requires the light adapted Y(II) or
)
F/Fm’.
The Plant Stress guide is updates on a regular basis. For an updated version contact Opt-
Sciences Inc at
www.optisci.com
.
The most prominent antenna pigment that absorbs energy is usually chlorophyll a. Other
accessory pigments can also be involved, such as chlorophyll b., carotenoids, or phycobilins
in cyanobacteria, or bacteriochlorophyll in some bacteria. We are primarily concerned with
chlorophyll a fluorescence.
Light energy absorbed initially by the antenna and transferred to the reaction centers is
channeled to competitive, different plant processes that include photochemistry, photo-
protective heat dissipation, and other heat dissipation. Normally a healthy plant will channel
