area was increased by 20–30% in plants exposed to doubling of the ambient CO 2 concentration [60,63].
Moreover, elevated CO 2 treatment prevented the ozone-induced suppression of net photosynthesis and
photorespiration of soybean, which amounted to 30 and 41%, respectively, in the ambient CO 2 level [57].
Such photosynthetic increase is frequently attributed to inhibition of the oxygenase activity of Rubisco,
i.e., inhibition of photorespiration.
Electron transport rates of photorespiratory systems are expected to be different from nonphotores-
piratory ones because of the exclusion of the consumption of reducing equivalents in the carbon reduc-
tion pathway. Thus, suppression of the photorespiratory pathway will in turn affect electron transport;
e.g., the resulting net oxygen evolution under nonphotorespiratory conditions is expressed as percent
stimulation of the oxygen gas exchange under photorespiratory conditions and taken as a measure of the
photorespiration rate of the respective organism (see Warburg effect in plant physiology textbooks). The
light-dependent linear electron transport was decreased more than 90% at a photon flux density of 800
mol m^2 sec^1 under conditions of inhibited photosynthesis and photorespiration (by either HCN or
glycolaldehyde) in intact leaves of spinach (Spinacia oleraceaL.) and sunflower (Helianthus annuusL.)
[64]. Concomitantly, nonphotochemical quenching of chlorophyll fluorescence was increased after inhi-
bition of CO 2 assimilation and photorespiration to dissipate excess excitation energy. Despite the effec-
tive nonphotochemical energy quenching, appreciable oxygen-dependent photoinactivation was observed
not only of photosystem II but also of photosystem I; it was significantly reduced or even completely ab-
sent when the oxygen concentration of the atmosphere was reduced from 21% to 1%. This observation il-
lustrates the importance of Mehler reactions in trapping excess electrons under these conditions [64].
The photosynthetic electron transport rates usually exceed the capacity of carbon reduction and usu-
ally there is an “excess” of electrons that might be used, e.g., for NO^3 and NO^2 reduction or even for
a reduction of the quinone pool. Laisk and Edwards [65] evaluated the photosynthetic linear electron
transport rate in excess of that used for CO 2 reduction in Sorghum bicolorMoench. [NADP–malic en-
zyme (ME)–type C4 plant), Amaranthus cruentusL. (NAD-ME–type C4 plant), and Helianthus annuus
L (C3 plant) leaves at different CO 2 and O 2 concentrations. Under high light intensities there was a large
excess of electron transport at 10–100% O 2 in the C3 plant because of photorespiration but very little in
Sorghumand somewhat more in Amaranthus, showing that photorespiration is suppressed more in the
NADP-ME– and less in the NAD-ME–type species. In C4 plants, such excess was very sensitive to the
presence of O 2 in the gas phase, rapidly increasing between 0.01 and 0.1% O 2 ; at 2% O 2 it was about two
thirds of that at 21% O 2. This shows the importance of the Mehler-type O 2 reduction as an electron sink
compared with photorespiration in C4 plants [65]. However, the rate of the Mehler reaction is still too low
to account fully for the extra ATP that is needed in C4 photosynthesis. In a mutant of Festuca pratensis,
the calculated electron flux through the photosystem was substantially higher than in the wild type and
more electrons were directed into the photorespiratory chain [66]. Treatment of the plants with the pho-
torespiratory inhibitors phosphinothricin (PPT) and aminooxyacetic acid (AOA) for more than 1 hr in-
duced a depletion in the ratios of Fv/Fm, Fv/Fo, and Fm/Fo—in spite of the existence of a good linear cor-
relation between the photochemical efficiency of PSII and the quantum yield of CO 2 assimilation [67].
C4 photosynthesis has long been known to be virtually O 2 insensitive. However, a dual inhibitory ef-
fect of O 2 , below or above the optimum partial pressure (5 kPa), on the net rate of CO 2 assimilation among
species representing all three C4 subtypes from both monocots and dicots was found and described [68].
Apparently, inhibition of net CO 2 assimilation with increasing O 2 partial pressure above the optimum has
to be associated with photorespiration, whereas inhibition at suboptimal O 2 concentrations may be caused
by a reduced supply of ATP to the C4 mechanism. In C4 plants, inhibition of photochemical reactions
such as PSII quantum yield, increased state of reduction of QA, and decreased efficiency of open PSII cen-
ters could account for photosynthesis inhibition under low O 2 partial pressure [68]. Photorespiration ap-
pears to buffer the quantum efficiency of CO 2 assimilation from changes associated with decreases in the
rate of CO 2 fixation resulting from imbalances in photosynthetic photon Feux density (PPFD) absorption
by PSI and PSII [69]. A photorespiratory response to oxygen has also been reported for the leaves of
maize plants [69]. However, in this case the authors argued that the possible occurrence of photorespira-
tion in maize leaves, which could result from an inhibition of the CO 2 concentrating mechanism, cannot
account for the decrease in the quantum efficiency of CO 2 assimilation. Atmospheric levels of O 2 (20
kPa) caused increased inhibition of photosynthesis as a result of higher levels of photorespiration in the
C4 cycle–limited mutant of Amaranthus edulis(a phosphoenolpyruvate carboxylase–deficient mutant).
Thus, the optimal O 2 partial pressure for photosynthesis was reduced from approximately 5 to 1–2 kPa
312 BADER AND ABDEL-BASSET