PBIO*3110 - CROP PHYSIOLOGY
Lecture #7
The equations for economic and biological yield in Lecture #1 contain a component that
represents the conversion of absorbed energy by the crop canopy into crop dry matter (ε).
Radiation use efficiency (RUE) is used in agronomic research to quantify the conversion of
intercepted solar radiation into above-ground dry matter and approximate maximum values for
RUE were presented for a number of crop species in Lecture #2. RUE is not a constant, it varies
with phase of development, abiotic factors such as PPFD level and temperature, and abiotic
stresses. An understanding of how and why RUE varies among crop species and environmental
conditions can be acquired by analyzing the processes that underlie RUE: leaf photosynthesis,
canopy photosynthesis and crop respiration, or the crop carbon balance, during periods ranging
from a 24-hour day to several weeks. The crop carbon balance will be discussed in Section II at
the leaf-level of organization and in Section III at the crop-canopy level of organization.
Leaf Photosynthesis: Light reactions
The overall process of photosynthesis constitutes the oxidation of water, using light energy to
drive electrons away from H2O (i.e., the light reactions) and the reduction of CO2 to form
organic compounds that make up dry matter (i.e., the dark reactions). This process can be
summarized as:
CO2 + 2H2O + PPFD → (CH2O) + O2 + H2O
In this lecture we will review where the light reactions occur, how the energy of an absorbed
photon in PAR is transferred into ATP and NADPH in photochemistry (i.e., the "Z-scheme"),
and other possible fates of photons absorbed by chlorophyll. Subsequently, we will introduce
you to the use of chlorophyll fluorescence to measure quantum efficiency of linear electron
transport and discuss some factors that may reduce photosynthetic efficiency.
CHLOROPLAST STRUCTURE
A chloroplast consists of (a) stroma, which is a gel-like, enzyme rich material, and (b)
thylakoids that are embedded in the stroma (Fig. 1). Dark reactions occur in the stroma and
light reactions occur in thylakoids. Thylakoids can be found in single strands and in stacks: the
stacks are grana and the single strands extend through the stroma, connecting one granum to
another (i.e., stroma thylakoid). The cavity within the thylakoid membrane is called the lumen.
Reactions in Photosystem I (PS I) occur in the stroma thylakoids and outer regions of the grana.
Reactions in Photosystem II (PS II) occur where one granum thylakoid contacts another (i.e.,
the appressed region).
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FUNCTION AND COMPONENTS OF THYLAKOID MEMBRANE
Thylakoids have four major components (Fig. 2): Photosystem II (PS II), the cytochrome b6cytochrome f (cyt b6-f) complex, Photosystem I (PS I), and ATPase synthase. The functions of
the four components in photosynthesis can be summarized as follows. (1) Photons absorbed by
pigments in the PS II core complex, which absorb at about 680 nm, are used to reduce oxidized
plastiquinone (PQ) using electrons from water, thereby splitting water in H+ and O2. (2) The
major function of the cyt b6-f complex is to pass electrons from PS II to PS I and, in the process,
2 H+ are released in the lumen per re-oxidized PQ. (3) Photons absorbed by pigments in PS I
core complex, which absorb at about 700 nm, are used to oxidize plastocyanin (PC) and reduce
ferrodoxin (Fd). Subsequently, 4 electrons from Fd and 2 H+ (per 4 photons) are used to form 2
NADPH in the stroma. Electrons from Fd can also be used to reduce cyt b6 and from there
move to PQ. Reduction/oxidation of PQ causes the movement of H+ from the stroma to the
lumen (see above). This process is called cyclic electron transport. (4) The fourth complex (ATP
synthase) uses the large H+ concentration gradient between the lumen and the stroma (due to the
oxidation of H2O and PQH2) to drive photophosphorylation (i.e., formation of ATP from ADP
and Pi).
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ABSORPTION OF PHOTONS BY CHLOROPHYLL
Chlorophyll molecules absorb photons in the 400- to 700-nm wavelength interval. The
absorption of a photon causes excitation of an electron (Fig. 3). Electrons in a stable ground
state can be driven away from their ground state in the positively charged nucleus, at a distance
proportional to the energy of the absorbed photon. Chlorophyll molecules can remain in an
excited state for only a very short time period (in the order of nanoseconds or 10-9 seconds). The
excitation energy can have three possible fates: (1) The energy can migrate through inductive
resonance to an energy-collecting pigment. In the PS II light harvesting complex these pigments
are chlorophyll a molecules that absorb at 680 nm (P680). In the PS I light harvesting complex,
the energy-collecting pigments absorb at 700 nm (P700). (2) The excitation energy can also be
totally lost as the electron moves back to its ground state and the energy is lost by heat release.
(3) The third way that excitation energy can be lost is by a combination of heat loss and
fluorescence. The wavelength of a re-release of photon of fluorescent light is approximately 685
nm and the energy difference between the absorbed photons and low wavelengths and the
fluorescent photons at higher wavelengths are released as heat. Only about 2 to 3% of excitants
are released as fluorescence under "normal" conditions.
CHLOROPHYLL FLUORESCENCE
Chlorophyll fluorescence can be used to quantify the quantum efficiency of linear electron
transport. In the dark, the PS II reaction centers are "open": P680 is reduced, exciton can be used
to drive photochemistry, and fluorescence is low. In the light, some of the PS II reaction centers
are "closed": P680 is oxidized and P680 must be reduced by H2O splitting before reaction
centers will open and, consequently, the relative rate of fluorescence is high. The quantum
efficiency can be quantified by chlorophyll fluorescence "in the dark" and in the light:
Fv/Fm = (Fm - Fo) / Fm
where Fo is fluorescence when a short light flash is given to a dark adapted leaf (i.e., all P680
reactions centers are open) and Fm is fluorescence at saturating light conditions (i.e., many P680
reaction centers are closed, stimulating other means of dissipation the excitation energy). A
typical value of Fv/Fm for a healthy leaf is about 0.83, which indicates that 83% of absorbed
photons are used in photochemistry.
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PHOTOSYNTHETIC EFFICIENCY
In photochemistry, eight photons are absorbed (four photons by PS II and four photons by PS I)
in order to move four electrons from H2O to NADPH. In this process, two H2O molecules are
oxidized, resulting in the release of one molecule of O2, formation of two molecules of
NADPH, and eight H+ are released into the lumen. The eight H+ are nearly enough to produce
three molecules of ATP in non-cyclic photophosphorylation (i.e., the number of H+ that must be
transported to form ATP is three). The reduction of CO2 requires a minimum of 2 NAPH and
more than 3 ATP, hence the quantum efficiency will always be higher than 8 photons per mole
of CO2 reduced.
Quantum efficiency will be smaller (i.e., the number of photons absorbed per mol of CO2 fixed
will be greater) under the following conditions:
•
•
•
When the products of photosynthesis will be high-energy compounds (such as lipids and
protein) that will require more energy per mol of CO2 fixed (we will discuss this in the
respiration lectures).
When very high PPFD is applied to a leaf or when moderate levels of PPFD are applied in
combination with some other stress that affects photosynthesis (e.g., low temperature),
photoinhibition may reduce the quantum efficiency (i.e., when PPFD absorbed exceeds that
which can be utilized, oxidative damage to the thylakoid membrane may occur).
When herbicides are applied that affect PS II (e.g., atrazine, bromoxynil), PS II reaction
centers will appear to be in a "closed" state, which may result in oxidative damage (see (b)
above). Uncouplers of photosphosphorylation (i.e., no ATP is produced when H+ is
transported from the lumen to the stroma) such as dinitrophenols (e.g., "Prowl") also reduce
quantum efficiency.
A detailed description of the light reactions can be found in the slide set Light Reactions in
Photosynthesis.
SUGGESTED READING
Salisbury & Ross, pp. 207-224.
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