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FV/Fm measuring Screen
Fv/Fm
= (Fm – Fo) / Fm This is a dark adapted test used to determine Maximum quantum
yield. This ratio is an estimate of the maximum portion of absorbed quanta used in PSII
reaction centers (Kitajima and Butler, 1975). Another way to look at Fv/Fm is a measurement
ratio that represents the maximum potential quantum efficiency of Photosystem II if all
capable reaction centers were open. 0.79 to 0.84 is the approximate optimal value range for
most land plant species with lowered values indicating plant stress. It is important to dark-
adapt samples properly for reliable test results. Since dark adaption requirement can vary with
species and light history, testing should be done to ensure proper dark adaption, (See the
section on dark adaptation). This test is a normalized ratio.
Y(II) or Yield Measuring Screen
Yield or quantum photosynthetic yield of PSII= (Fm’ – Fs) / Fm’ -This test is also
known as
Δ
F/Fm’ or Y(II).
Yield of PSII is a fast light adapted test taken at steady state
photosynthesis levels. It provides a measure of actual or effective quantum yield. This ratio is
an estimate of the effective portion of absorbed quanta used in PSII reaction centers. (Genty,
1989) It is affected by closure of reaction centers and heat dissipation caused by non-
photochemical quenching. Y(II) allows investigation of the photosynthetic process while it is
happening. No dark adaption is required. According to Maxwell and Johnson (2000) it takes
between fifteen to twenty minutes for a plant to reach steady state photosynthesis at a specific
light level. To obtain a reliable Y(II) measurement, photosynthesis must reach steady state.
Fast
-
Light Adapted Y(II)
Yield Test -
Usually less than 2 seconds
Fast
-Dark Adapted Fv/Fm
Usually less than 2 seconds
