2 Regulation of Photosynthetic Light Energy
Capture, Conversion, and Dissipation in Leaves
of Higher Plants
o. Bjorkman and B. Demmig-Adams
2.1 Introduction
In nature, the intensity of light, or photon flux density (PFD), shows great
variation, both temporally and spatially. For example, a leaf in the understory can experience changes in the incident PFD up to lOO-fold within a few
seconds (Chazdon and Pearcy 1991). Large changes in PFD are also experienced by exposed leaves when intermittent clouds obscure the sun. In
addition, the total daily integrated photon flux varies greatly among habitats
as well as within the canopy of a given plant stand. Plants on the floor
of a tropical rainforest (Bjorkman and Ludlow 1972) or redwood forest
(Bjorkman and Powles 1981) may receive as little as 1 % of the daily photon
flux above the plant canopy.
This great variation in the light environment imposes a great demand on
the responsiveness of the photosynthetic system. At PFDs that are limiting
to photosynthesis, light must be captured and utilized with the highest
possible efficiency. On the other hand, it is imperative that overexcitation of
the photosynthetic reaction centers be avoided when the light becomes
excessive, as such overexcitation can result in severe damage to these centers
(photoinhibitory damage). This problem is further exacerbated in the presence of environmental stresses that lower the capacity for light-saturated
photosynthesis as this results in an increased level of excessive light energy.
Examples of such stresses are unfavorably low temperatures, which directly
decrease the rates of photosynthetic electron transport and the turnover
of the carbon reduction-oxidation cycles, and drought, which leads to a
decrease in the CO2 available for carbon fixation because of its effect on
stomatal conductance.
On these grounds it is clear that plants must possess the ability to regulate
the level of excitation energy, in both the long and the short term. In this
chapter we will illustrate the various ways in which this may be achieved.
Regulation can occur by changes in light interception, by changes in photosynthetic capacity, and by dissipation of excess excitation energy through
various pathways within the chloroplasts. Wherever possible, we will use
examples relevant to actual field conditions and for the most part these
examples are taken from published and unpublished studies in which the
authors have been active participants.
Capture, Conversion, and Dissipation in Leaves
of Higher Plants
o. Bjorkman and B. Demmig-Adams
2.1 Introduction
In nature, the intensity of light, or photon flux density (PFD), shows great
variation, both temporally and spatially. For example, a leaf in the understory can experience changes in the incident PFD up to lOO-fold within a few
seconds (Chazdon and Pearcy 1991). Large changes in PFD are also experienced by exposed leaves when intermittent clouds obscure the sun. In
addition, the total daily integrated photon flux varies greatly among habitats
as well as within the canopy of a given plant stand. Plants on the floor
of a tropical rainforest (Bjorkman and Ludlow 1972) or redwood forest
(Bjorkman and Powles 1981) may receive as little as 1 % of the daily photon
flux above the plant canopy.
This great variation in the light environment imposes a great demand on
the responsiveness of the photosynthetic system. At PFDs that are limiting
to photosynthesis, light must be captured and utilized with the highest
possible efficiency. On the other hand, it is imperative that overexcitation of
the photosynthetic reaction centers be avoided when the light becomes
excessive, as such overexcitation can result in severe damage to these centers
(photoinhibitory damage). This problem is further exacerbated in the presence of environmental stresses that lower the capacity for light-saturated
photosynthesis as this results in an increased level of excessive light energy.
Examples of such stresses are unfavorably low temperatures, which directly
decrease the rates of photosynthetic electron transport and the turnover
of the carbon reduction-oxidation cycles, and drought, which leads to a
decrease in the CO2 available for carbon fixation because of its effect on
stomatal conductance.
On these grounds it is clear that plants must possess the ability to regulate
the level of excitation energy, in both the long and the short term. In this
chapter we will illustrate the various ways in which this may be achieved.
Regulation can occur by changes in light interception, by changes in photosynthetic capacity, and by dissipation of excess excitation energy through
various pathways within the chloroplasts. Wherever possible, we will use
examples relevant to actual field conditions and for the most part these
examples are taken from published and unpublished studies in which the
authors have been active participants.