Regulation of Photosynthetic Light Energy Capture
27
leaf; the latter may be especially significant during salinity stress. Some
NADPH and ATP may also be consumed in nitrogen reduction and in
"repair" processes. (A small amount of NADPH is also consumed during
the operation of the xanthophyll cycle but this can probably be neglected; it
is not the mechanism by which this cycle regulates energy dissipation; see
Sect. 2.4.3).
Another potentially important process of metabolic energy dissipation
which may have a regulatory role is the Mehler-ascorbate peroxidase reaction (Neubauer and Schreiber 1989; Neubauer and Yamamoto 1992). This
reaction involves the reduction of O2 by photosystem I (PS I), leading to the
formation of H20 2 which in turn reacts with ascorbate to form H20 and O 2
(see also Chap. 1, this Vol). This reaction does not result in any net gas
exchange and, to our knowledge, quantitative estimates of the amount of
energy dissipated by this mechanism in leaves are lacking at this time.
However, in addition to providing any direct energy dissipation, this process
may have an important role in supporting the electron transport needed for
development of nonradiative dissipation in the pigment bed (Sect. 2.4.3; see
also Chap. 1, this Vol.).
Photo respiration is the only major metabolic process for which good
estimates of energy dissipation may presently be obtained. It is well established that instead of reacting with CO2 to form 2 mol phosphoglycerate (a
net gain of 1 mol C), a considerable fraction of the ribulosebisphosphate
pool reacts with O2, leading to the formation of phosphoglycolate, part of
which is converted to CO2 (a net loss of 0.5 C). Both the carboxylation
and the oxygenation are catalyzed by the enzyme Rubisco and, since O 2
competes with COl> the rate of oxygenation depends on the relative concentration of these gases. The rate of photorespiration relative to CO 2 fixation
also increases with increasing leaf temperature.
Values for energy dissipation by photosynthetic CO2 fixation, P[C0 2 ],
and photorespiration, PR, at midday in fully exposed leaves of a cotton crop
growing in the San Joaquin Valley, California, are shown in Table 2.1. The
plants were grown under normal agricultural irrigation practices as well
as under two levels of long-term water stress, obtained by terminating
irrigation early in the growing season. In irrigated plants which had high
rates of CO2 fixation, dissipation via this process was estimated to account
for 26% of the total excitation energy. Because of the high leaf temperatures, photorespiration was also relatively high and accounted for the
dissipation of some 20% of the total excitation energy. Hence the remaining
excess would be some 55% of the total. Increased water stress resulted in a
progressive decrease in photosynthetic rate and, in severely stressed plants,
energy dissipation via P[C02] fell to 10% or less of the total energy absorbed. Since water stress caused partial stomatal closure, the leaf temperature rose and the intercellular CO2 pressure declined. As a result, the
ratio of PR to P[C02] increased. However, the decline in CO 2 pressure
was relatively modest, as the intrinsic (i.e., COTsaturated) photosynthetic
27
leaf; the latter may be especially significant during salinity stress. Some
NADPH and ATP may also be consumed in nitrogen reduction and in
"repair" processes. (A small amount of NADPH is also consumed during
the operation of the xanthophyll cycle but this can probably be neglected; it
is not the mechanism by which this cycle regulates energy dissipation; see
Sect. 2.4.3).
Another potentially important process of metabolic energy dissipation
which may have a regulatory role is the Mehler-ascorbate peroxidase reaction (Neubauer and Schreiber 1989; Neubauer and Yamamoto 1992). This
reaction involves the reduction of O2 by photosystem I (PS I), leading to the
formation of H20 2 which in turn reacts with ascorbate to form H20 and O 2
(see also Chap. 1, this Vol). This reaction does not result in any net gas
exchange and, to our knowledge, quantitative estimates of the amount of
energy dissipated by this mechanism in leaves are lacking at this time.
However, in addition to providing any direct energy dissipation, this process
may have an important role in supporting the electron transport needed for
development of nonradiative dissipation in the pigment bed (Sect. 2.4.3; see
also Chap. 1, this Vol.).
Photo respiration is the only major metabolic process for which good
estimates of energy dissipation may presently be obtained. It is well established that instead of reacting with CO2 to form 2 mol phosphoglycerate (a
net gain of 1 mol C), a considerable fraction of the ribulosebisphosphate
pool reacts with O2, leading to the formation of phosphoglycolate, part of
which is converted to CO2 (a net loss of 0.5 C). Both the carboxylation
and the oxygenation are catalyzed by the enzyme Rubisco and, since O 2
competes with COl> the rate of oxygenation depends on the relative concentration of these gases. The rate of photorespiration relative to CO 2 fixation
also increases with increasing leaf temperature.
Values for energy dissipation by photosynthetic CO2 fixation, P[C0 2 ],
and photorespiration, PR, at midday in fully exposed leaves of a cotton crop
growing in the San Joaquin Valley, California, are shown in Table 2.1. The
plants were grown under normal agricultural irrigation practices as well
as under two levels of long-term water stress, obtained by terminating
irrigation early in the growing season. In irrigated plants which had high
rates of CO2 fixation, dissipation via this process was estimated to account
for 26% of the total excitation energy. Because of the high leaf temperatures, photorespiration was also relatively high and accounted for the
dissipation of some 20% of the total excitation energy. Hence the remaining
excess would be some 55% of the total. Increased water stress resulted in a
progressive decrease in photosynthetic rate and, in severely stressed plants,
energy dissipation via P[C02] fell to 10% or less of the total energy absorbed. Since water stress caused partial stomatal closure, the leaf temperature rose and the intercellular CO2 pressure declined. As a result, the
ratio of PR to P[C02] increased. However, the decline in CO 2 pressure
was relatively modest, as the intrinsic (i.e., COTsaturated) photosynthetic