The effects of aminoacetonitrile (a competitive inhibitor of glycine oxidation) on net photosynthesis, glycolate pathway intermediates, and ribulose-1,5-bisphosphate (RuBP) levels have been investigated at different 02 and CO2 concentrations with soybean (Glycine max)ILI Merr. cv Pioneer 1677) leaf discs floated on 25 milUmolar aminoacetonitrile (AAN) for 50 minutes prior to assay.At 2% 02 and 200 or 330 microliters per liter C02, the inhibitor had no effect on the rate of net photosynthesis and RuBP levels when compared with the control levels. At 11% to 60% 02, AAN caused a decrease in net photosynthesis in addition to the inhibition by 02. This extra inhibition ranged from 22% to 59% depending on the 02 and CO2 concentrations. The levels of RuBP, however, were 13 to 2.7 times higher than in the control plants at the same 02 concentrations. At 40% 02 and 200 microUters per Uter CO2, the inhibitor caused a 6-fold increase in glycine and more than 2-fold increase in glyoxylate levels, whereas those of glycolate decreased by approximately one-half.The decrease in net photosynthesis observed with AAN is not the result of the depletion of the RuBP pool due to the lack of recycling of carbon from the glycolate pathway to the Calvin cycle. The higher levels of RuBP caused byAAN in photorespiratory conditions, suggest that RuBP carboxylase was inhibited. Glyoxylate could be a possible candidate for the inhibition of the enzyme but what is known so far about its inhibitory properties in vitro may not fit the existing in vivo conditions. An alternative explanation for the inhibition is proposed.Due to the catalytic properties of the enzyme RuBP2 carboxylase, photorespiration appears to be an unavoidable process in an atmosphere containing high 02 and low CO2 (2). The oxygenation of RuBP leads to the formation of P-glycolate (1) and its subsequent metabolism through the photosynthetic carbon oxidative cycle is viewed as a means of recovering part of that carbon diverted from the Calvin cycle. Under physiological conditions, only 25% of the carbon entering P-glycolate is lost as CO2 (19) and the remaining is available for the synthesis of amino acids and sucrose or reentry into Calvin cycle. In the past, it seemed feasible to increase photosynthetic productivity by inhibiting some steps of the glycolate pathway. This idea, however, has been supported by little experimental data (29, 30). It is now well substantiated that once P-glycolate has been synthesized, it must be metabolized through the complete photorespiratory pathway in order to avoid detrimental effects of photosynthesis. This has been shown through the use of inhibitors ofglycolate oxidase such as BHB (7,14,24) and of glycine decarboxylase such as INH (9,14,24) and AAN (27). Also the requirement of the complete functioning of the glycolate pathway has been demonstrated in studies involving mutants deficient in specific enzymes of the glycolate pathway (25,26). In photorespiratory conditions, these mutants are simply not viable. Earlier studies on 14...

When dark 14CO2 fixation in maize leaves was carried out under anaerobic conditions after prelliumination in the absence of 02, the 14C incorporation in aspartic acid was transient; its maximum level was very low compared with that of malic acid. The addition of 5% O2 during the dark fixation period increased the total uptake of 14C02 and the 14C incorporation into aspartic acid.A study of the intramolecular distribution of radioactivity showed that 71 to 76% of the 14C was located in the C4 (8-carboxyl)

By placing leaf segments first in CO2 in the dark, then in pure nitrogen either in the dark and afterwards in the light or immediately in the light, the existence of internal CO2 pools which can be used for photosynthesis had been demonstrated. In Zea mays L. there are two such pools: one which in the absence of any energy source is short-lived (t1/2 ca. 2 min), and another which is relatively long-lived (t1/2 ca. 50 min).Under different oxygen concentrations the level of the short-lived CO2 pool exibited a parallel variation with the level of aspartic acid. Only a fraction of the total aspartic acid (60%) constituted the active pool, the quantity of which was equal to the short-lived CO2. In the absence of O2 but under far-red irradiation (maximum 740 nm), a net synthesis of aspartic acid was observed; its extent depended on the intensity of the light.The similarity in the response to O2 and to long-wavelength irradiation suggests that aspartate synthesis is regulated by ATP, the high-energy compound common to both oxidative and cyclic phosphorylations. The formation of aspartic acid observed in the dark under N2+1% CO2 immediately following illumination under pure N2 suggests use of ATP accumulated in the preceding light period, in aspartate synthesis.Even though Zea mays is predominantly a "malate former", it appears that aspartate must also be considered as a readily available donor of CO2 since, when aspartate is present, O2 release is always immediate while, when it is not, O2 release is delayed.

The enhanced dark CO2 uptake after a preillumination period under varying 02 concentrations has been measured with maize, a C4 plant. For comparison the same study has been conducted with tomato, a C3 plant. Increasing the 02 concentration during preiHumination inhibits by 70% the subsequent dark CO2 uptake in tomato but stimulates 2-fold this CO2 uptake in maize. The 02 enhancement of CO2 uptake in maize is due to the enhancement of malate and aspartate synthesis. In a previous study (3) we reported that when "CO2 was given to maize leaves in the dark immediately after a preillumination period without CO2 and 02, 73.5% of the radioactivity in the resulting malate and aspartate molecules was located in the C-4 carboxyl and the remainder in the C-1. We showed that this intramolecular labeling pattern was unchanged regardless of whether 14CO2 was given in the absence or presence of 5% 02-It seemed unlikely that this distribution was due to randomization reactions during the CO2 pulse and could be explained as follows. In the 1st min of darkness part of the CO2 entering the leaf was directly fixed by RuBP' carboxylase, the resulting PGA was rapidly converted into PEP Experimental Procedure. Two maize leaf segments (about 400 mg fresh weight) or one tomato leaflet was placed in an air-tight cell (30 cm3) inserted in an open circuit (16) and subjected to the following successive treatments. First they were swept for 5 min in the light (20,000 lux) with C02-free N2 (15 1/h) containing various proportions of 02 (0, 0.1, 1.5, 10, 21, 50, and 100%o). Then after a 6-s vacuum period they were put in darkness and were swept for 15 s with a mixture of N2 + 0.5% 14CO2 delivered by a gas cylinder. The flow rate was 30 1/h and the specific radioactivity of the "4CO2 was 8.22 x 106 dpm/,umol. At the end of this treatment the leaf was immediately dipped into liquid N2. For each set of experiments all of the selected 02 concentrations were assayed on segments or leaflets cut from the same leaf. The data presented represent the average of two or four sets of experiments.Determination of Total 14CO2 Uptake. In order to measure the total radioactivity incorporated into the samples after the respective treatments, small pieces of tomato leaflets or maize leaves were burned up in an Oxymat (Intertechnique). The resulting CO2 was quantitatively trapped in 2-phenylethylamine and its radioactivity counted with a liquid scintillation counter (SL: 4221 Intertechnique) in a mixture of the following compounds: 2-phenylethylamine, toluol, methanol, water, butyl PBD (respectively 350, 400, 220, 50 ml, 7 g). A part of each sample was lyophilized in order to determine dry weight. An additional method according to Galmiche (6) was also used with maize. The leaf segments were ground in liquid N2 with a VirTis homogenizer. The leaf powder was then lyophilized, extracted for 10 min with 2.5 ml formamide,