Regulation of Photosynthetic Light Energy Capture
29
of intact leaves. A brief outline of the fluorescence measurements used
in the present context is given below. For a recent comprehensive review
of the foundation for such analyses, see Krause and Weis (1991; see also
Chap. 1, this Vol.).
2.4.2.1 Basic Chlorophyll Fluorescence Measurements
At ambient temperatures nearly all of the fluorescence emitted from a leaf
emanates from PS II. The basic fluorescence parameters measured are F o ,
Fm, and F. Fo is the minimal fluorescence yield, (i.e., the intensity of
fluorescence divided by the intensity of the light used to excite fluorescence),
which occurs when all reaction centers are in the oxidized, or open, state. Fo
is measured under conditions where all the reaction centers are rendered
open by removing all light that might be captured by the chlorophyll except
for a very weak excitation beam which does not cause an accumulation of
reduced electron acceptors in PS II. (In some cases it may be necessary to
briefly expose the leaf to far red light to insure that all PS II centers are
open; the far red light excites PS I only, thereby removing any electrons
from the PS II centers). Fm is the maximum fluorescence yield, measured
during exposure to a brief light flash of an intensity and duration just
sufficient to temporarily close all centers. F is the yield measured under the
actual actinic PFD and the actual degree of center closure. When all centers
are open then F = Fo; conversely, when all centers are closed then F = Fm.
These "control" Fo and Fm values are determined in the absence of any
lowering (quenching) of fluorescence caused by nonradiative dissipation
(NRD). When the actinic PFD is sufficiently high to cause excess light and
NRD to develop, then the fluorescence yield declines. This mainly affects
Fm, which in this quenched state is termed Fm', but may to a lesser degree
also affect Fo which is then termed Fo'. Accordingly, the variable fluorescence, Fv and Fv', is the difference between Fm and Fo and between Fm'
and Fo', respectively.
The actual efficiency of energy conversion in PS II is estimated from the
expressionu' = (Fm' - F)/Fm'. When all centers are open then F = Fo'
andI1' = (Fm' - Fo')/Fm' = Fv' /Fm'. Furthermore, the maximum, or
intrinsic, efficiency,u, is obtained when all centers are open and NRD =
0; hencen = F)Fm. The fraction of centers that remain in the open
(oxidized) state, Oox/Ot is given by (Fm' - F)/(Fm' - Fo'); hence the
fraction of centers that are in the closed (reduced) state Or/Ot is equal to
1 - OoxlOt and to (F - Fo')/(Fm' - Fo').
Finally, the decline in Fm to Fm' caused by NRD is termed nonphotochemical quenching (NPO). In this chapter, we have chosen to calculate
NPO according to the Stern-Volmer equation, NPO = (Fm - Fm')/Fm' =
Fm/Fm' - 1. On theoretical grounds, if the only cause of the quenching of
Fm is an increase in NRD, then NPO calculated in this way should be
29
of intact leaves. A brief outline of the fluorescence measurements used
in the present context is given below. For a recent comprehensive review
of the foundation for such analyses, see Krause and Weis (1991; see also
Chap. 1, this Vol.).
2.4.2.1 Basic Chlorophyll Fluorescence Measurements
At ambient temperatures nearly all of the fluorescence emitted from a leaf
emanates from PS II. The basic fluorescence parameters measured are F o ,
Fm, and F. Fo is the minimal fluorescence yield, (i.e., the intensity of
fluorescence divided by the intensity of the light used to excite fluorescence),
which occurs when all reaction centers are in the oxidized, or open, state. Fo
is measured under conditions where all the reaction centers are rendered
open by removing all light that might be captured by the chlorophyll except
for a very weak excitation beam which does not cause an accumulation of
reduced electron acceptors in PS II. (In some cases it may be necessary to
briefly expose the leaf to far red light to insure that all PS II centers are
open; the far red light excites PS I only, thereby removing any electrons
from the PS II centers). Fm is the maximum fluorescence yield, measured
during exposure to a brief light flash of an intensity and duration just
sufficient to temporarily close all centers. F is the yield measured under the
actual actinic PFD and the actual degree of center closure. When all centers
are open then F = Fo; conversely, when all centers are closed then F = Fm.
These "control" Fo and Fm values are determined in the absence of any
lowering (quenching) of fluorescence caused by nonradiative dissipation
(NRD). When the actinic PFD is sufficiently high to cause excess light and
NRD to develop, then the fluorescence yield declines. This mainly affects
Fm, which in this quenched state is termed Fm', but may to a lesser degree
also affect Fo which is then termed Fo'. Accordingly, the variable fluorescence, Fv and Fv', is the difference between Fm and Fo and between Fm'
and Fo', respectively.
The actual efficiency of energy conversion in PS II is estimated from the
expression
and
intrinsic, efficiency,
0; hence
(oxidized) state, Oox/Ot is given by (Fm' - F)/(Fm' - Fo'); hence the
fraction of centers that are in the closed (reduced) state Or/Ot is equal to
1 - OoxlOt and to (F - Fo')/(Fm' - Fo').
Finally, the decline in Fm to Fm' caused by NRD is termed nonphotochemical quenching (NPO). In this chapter, we have chosen to calculate
NPO according to the Stern-Volmer equation, NPO = (Fm - Fm')/Fm' =
Fm/Fm' - 1. On theoretical grounds, if the only cause of the quenching of
Fm is an increase in NRD, then NPO calculated in this way should be