Enhanced Dark Carbon Dioxide Fixation in Maize

Abstract

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,