26
O. Bjorkman and B. Demmig-Adams
compensated for by a greater number of chloroplasts across the leaf section,
and there is no consistent difference in the amount of chlorophyll per unit
leaf area between the two types of leaves. Moreover, the principal effect
of an increased chlorophyll content per leaf area is a broadening of the
absorption bands, resulting in an increased absorptance in the green and the
far-red regions, with only a small effect at wavelengths at which the lightharvesting pigments have a high absorption coefficient. Therefore, the increase in u, integrated over the photosynthetically active spectral range, is
not linearly dependent on ChI content per leaf area. For example, in Hedera
canariensis, leaves having ChI contents of 175, 350, and 700mgm- 2 , the
corresponding U values were 0.70, 0.80, and 0.90 (Bjorkman and Demmig
1987), i.e., a 59% reduction in chlorophyll content resulted in only a 10-13%
reduction in u. We therefore conclude that changes in chlorophyll content
per unit leaf area are probably not a major factor in regulating light interception in natural situations.
2.4 Regulation of Energy Dissipation
There are four different ways in which the excitation energy resulting from
the absorption of light by chlorophyll can be dissipated. The first is by
conversion into chemically bound energy contained in the final products of
photosynthesis. The second is by energy consumption in metabolic processes
that do not result in energy storage. The third is by reemission of photons as
fluorescence (radiative dissipation). The fourth is by conversion of light
energy into heat in the pigment bed (thermal or nonradiative dissipation).
Strictly speaking, energy dissipation via metabolic processes is nonradiative
as well but we will reserve this term for nonmetabolic energy dissipation
occurring within the pigment bed itself.
The amount of excitation energy that can be dissipated by fluorescence is
quite small, at most 3-4% of the total, and can be neglected for most
practical purposes. However, the chlorophyll fluorescence emitted from a
leaf can serve as a very powerful intrinsic probe to assess the efficiency of
energy conversion or the degree of reaction center closure in photosystem II
as well as the extent of nonradiative energy dissipation (Sect. 2.4.2).
2.4.1 Dissipation in Metabolic Processes
Only part of the reduced NADPH and ATP generated in the photoacts of
photosynthesis is stored in final stable photosynthetic products. A considerable fraction is also consumed in processes that do not result in carbon
fixation. Among these are photorespiration and transport processes involved
in the maintenance of ionic balance in the different compartments of the
O. Bjorkman and B. Demmig-Adams
compensated for by a greater number of chloroplasts across the leaf section,
and there is no consistent difference in the amount of chlorophyll per unit
leaf area between the two types of leaves. Moreover, the principal effect
of an increased chlorophyll content per leaf area is a broadening of the
absorption bands, resulting in an increased absorptance in the green and the
far-red regions, with only a small effect at wavelengths at which the lightharvesting pigments have a high absorption coefficient. Therefore, the increase in u, integrated over the photosynthetically active spectral range, is
not linearly dependent on ChI content per leaf area. For example, in Hedera
canariensis, leaves having ChI contents of 175, 350, and 700mgm- 2 , the
corresponding U values were 0.70, 0.80, and 0.90 (Bjorkman and Demmig
1987), i.e., a 59% reduction in chlorophyll content resulted in only a 10-13%
reduction in u. We therefore conclude that changes in chlorophyll content
per unit leaf area are probably not a major factor in regulating light interception in natural situations.
2.4 Regulation of Energy Dissipation
There are four different ways in which the excitation energy resulting from
the absorption of light by chlorophyll can be dissipated. The first is by
conversion into chemically bound energy contained in the final products of
photosynthesis. The second is by energy consumption in metabolic processes
that do not result in energy storage. The third is by reemission of photons as
fluorescence (radiative dissipation). The fourth is by conversion of light
energy into heat in the pigment bed (thermal or nonradiative dissipation).
Strictly speaking, energy dissipation via metabolic processes is nonradiative
as well but we will reserve this term for nonmetabolic energy dissipation
occurring within the pigment bed itself.
The amount of excitation energy that can be dissipated by fluorescence is
quite small, at most 3-4% of the total, and can be neglected for most
practical purposes. However, the chlorophyll fluorescence emitted from a
leaf can serve as a very powerful intrinsic probe to assess the efficiency of
energy conversion or the degree of reaction center closure in photosystem II
as well as the extent of nonradiative energy dissipation (Sect. 2.4.2).
2.4.1 Dissipation in Metabolic Processes
Only part of the reduced NADPH and ATP generated in the photoacts of
photosynthesis is stored in final stable photosynthetic products. A considerable fraction is also consumed in processes that do not result in carbon
fixation. Among these are photorespiration and transport processes involved
in the maintenance of ionic balance in the different compartments of the