66
U. Schreiber et al.
restriction in photosynthetic performance of a sample in comparison with a
"model leaf":
L(PFD) = 1 - ~F/(Fm' x 0.83)
(12)
or
L(PFD) = 1 - (qp x Fv'/Fm')/0.83.
(13)
Equation (13) makes clear that limitation with respect to the optimal
quantum yield (0.83) is caused by the combination of a lowering of qp (from
optimal value of 1) and of Fv'/Fm' (from optimal value of 0.83). Hence
L(PFD) becomes zero only, if qp = 1 and Fv'/Fm' = 0.83. In the example of
Figs. 3.9 and 3.10, L(1000) amounts to 0.46 in the case of the control and to
0.63 with the heat-treated sample (see Table 3.1).
Any deviation from the optimal quantum yield line, whether it is expressed
in terms of "limitation", "excessive PFD", or simply "quantum yield lowering", may be considered an integrative measure of the stress to which a
sample is exposed in a given situtation. The immediate response revealed by
fluorescence is that of "light stress" in a broad sense. Actinic light is used as
a diagnostic tool to assess the efficiency of the photosynthetic apparatus
"under light pressure". In this way, certain deficiencies, e.g., at the sink
or Calvin cycle levels, are expressed which would remain unnoticed at
moderate light intensity. The extent to which a given light intensity is
excessive depends on a large number of factors, including light and temperature acclimation, water status and stomatal opening, the developmental
state, and previous stress treatments. Hence, in practice, in order to draw
reliable conclusions on the photosynthetic performance of a plant from
fluorescence measurements, knowledge and control of environmental factors
is essential.
3.10 Conclusions
In conclusion, it may be stated that chlorophyll fluorescence analysis, which
in the past has primarily served as a pioneering tool in basic photosynthesis
research, has now reached a point of sophistication and reliability to be
applied for the assessment of in vivo photosynthesis under field conditions.
Instrumentation and methods are available with which it is possible to judge
various aspects of photosynthetic performance of plants in their natural
environment. Relevant information which can be obtained within seconds
includes the maximal quantum yield of PS II (Fv/Fm), the effective quantum
yield (~FlFm'), the relative electron transport rate (PFD x ~FIFm'), the
excitation capture efficiency (Fv'/Fm'), photochemical and nonphotochemical
quenching coefficients (qp and qN), as measures of PS II openness and
. downregulation of PS II, and eventually L(PFD) = 1 - ~F/(Fm' x 0.83),
representative for the limitation that a sample is experiencing at a given light
U. Schreiber et al.
restriction in photosynthetic performance of a sample in comparison with a
"model leaf":
L(PFD) = 1 - ~F/(Fm' x 0.83)
(12)
or
L(PFD) = 1 - (qp x Fv'/Fm')/0.83.
(13)
Equation (13) makes clear that limitation with respect to the optimal
quantum yield (0.83) is caused by the combination of a lowering of qp (from
optimal value of 1) and of Fv'/Fm' (from optimal value of 0.83). Hence
L(PFD) becomes zero only, if qp = 1 and Fv'/Fm' = 0.83. In the example of
Figs. 3.9 and 3.10, L(1000) amounts to 0.46 in the case of the control and to
0.63 with the heat-treated sample (see Table 3.1).
Any deviation from the optimal quantum yield line, whether it is expressed
in terms of "limitation", "excessive PFD", or simply "quantum yield lowering", may be considered an integrative measure of the stress to which a
sample is exposed in a given situtation. The immediate response revealed by
fluorescence is that of "light stress" in a broad sense. Actinic light is used as
a diagnostic tool to assess the efficiency of the photosynthetic apparatus
"under light pressure". In this way, certain deficiencies, e.g., at the sink
or Calvin cycle levels, are expressed which would remain unnoticed at
moderate light intensity. The extent to which a given light intensity is
excessive depends on a large number of factors, including light and temperature acclimation, water status and stomatal opening, the developmental
state, and previous stress treatments. Hence, in practice, in order to draw
reliable conclusions on the photosynthetic performance of a plant from
fluorescence measurements, knowledge and control of environmental factors
is essential.
3.10 Conclusions
In conclusion, it may be stated that chlorophyll fluorescence analysis, which
in the past has primarily served as a pioneering tool in basic photosynthesis
research, has now reached a point of sophistication and reliability to be
applied for the assessment of in vivo photosynthesis under field conditions.
Instrumentation and methods are available with which it is possible to judge
various aspects of photosynthetic performance of plants in their natural
environment. Relevant information which can be obtained within seconds
includes the maximal quantum yield of PS II (Fv/Fm), the effective quantum
yield (~FlFm'), the relative electron transport rate (PFD x ~FIFm'), the
excitation capture efficiency (Fv'/Fm'), photochemical and nonphotochemical
quenching coefficients (qp and qN), as measures of PS II openness and
. downregulation of PS II, and eventually L(PFD) = 1 - ~F/(Fm' x 0.83),
representative for the limitation that a sample is experiencing at a given light