ammonia, which cannot easily be taken as a measure for photorespiratory activity as NH 3 (for reasons of
both bioenergetic economy and toxicology) is rapidly refixed via the GOGAT system (GOGAT glu-
tamine-2-oxoglutarate aminotransferase).
Many crops have been investigated with respect to their photorespiration rates—not least with the
background idea of the search for increased photosynthetic yields under conditions of low photorespira-
tion. This theory, however, could not be substantiated and in many cases even the contrary turned out to
be correct. Nonphotorespiratory conditions did not result in higher yields of plants and crops; they were in
general detrimental for the plants and sometimes even lethal. In Dunaliella tertiolecta, photorespiratory
metabolism was quantified by determining the concentrations of extracellular dissolved glycolate or in-
tracellular free pools of serine and glycine as the parameter in field studies [56]. In this case, the amount
of glycolate was light dependent and reached 100 nmol (10^6 cells)^1 for a cell concentration of around 1.5
108 cells L^1 which “disappeared” from the dissolved phase in the dark. Under photorespiratory con-
ditions, i.e., elevated oxygen partial pressure, the activities of glycolate oxidase, hydroxypyruvate reduc-
tase, and catalase were decreased 10–25% by elevated CO 2 in late reproductive growth of soybean. Ser-
ine concentrations were concomitantly decreased at elevated CO 2 concentrations [57]. In spinach leaves,
the required reducing equivalents for serine reduction to glycerate in the peroxisomes were provided by
mitochondria via the malate-oxaloacetate (OAA) shuttle, in which OAA was reduced in the mitochondrial
matrix by NADH generated during glycine oxidation [58]. Redox equivalents can be transferred from the
mitochondria to peroxisomes for glycerate formation in the photorespiratory cycle because a very low re-
ductive state of the NADH/NAD system prevails in the cytosol of mesophyll cells during photosynthesis.
The rate of peroxisomal glycerate formation and the malate/OAA ratio were similar in both a reconstituted
system of spinach and the cytosol of mesophyll cells of intact illuminated spinach leaves. The malate/OAA
ratio was in equilibrium with an NADH/NAD ratio equivalent to 8.8 10 ^3 [58].
In C3 plants, the competition between CO 2 and O 2 on the active site of Rubisco is limiting for the
carbon dioxide assimilation rates in the sense that elevated CO 2 /O 2 ratios enhance photosynthesis with si-
multaneously inhibited photorespiration and vice versa. Lowering the O 2 partial pressure or elevating that
of CO 2 (2% O 2 or 1000 ppm CO 2 , respectively) is conventionally used to inhibit photorespiration signif-
icantly. When CO 2 fixation by Rubisco is limited in C4 plants, an increase in the CO 2 concentration in
bundle sheath cells via the C4 pathway may further reduce the oxygenase activity of Rubisco. Decreased
oxygenase activity of Rubisco decreases the inhibition of photosynthesis under high partial pressures of
O 2 while it increases CO 2 leakage and overcycling of the C4 pathway [59].
Generally, an increasing external CO 2 concentration leads to an immediate increase in the internal
CO 2 concentration in the leaf, accelerated photosynthetic activity, and—at least transiently—repressed
photorespiration. Under these conditions, net photosynthesis of soybean increased 56% on average and
photorespiration decreased 36% in terminal mainstem leaves [57]. Under permanently elevated concen-
trations, however, the down-regulation of photosynthesis counteracts this effect and photosynthesis may
come down to normal levels. A decrease in the activity and quantity of Rubisco and a decrease of mes-
senger RNA (mRNA) encoding Rubisco activase and chlorophyll-binding proteins contribute to accli-
mation to elevated CO 2 values [60–62]. However, in some cases the rate of photosynthesis per unit leafPHOTOSYNTHETIC GAS EXCHANGE AND RESPIRATION 311Figure 11 Schematic diagram summarizing the counteracting effects of photosynthesis and photorespiration
with respect to the oxygen and carbon dioxide gas exchange. (The photorespiratory NH 3 liberation is omitted.)