Aspartic acid as an internal CO2 reservoir in Zea mays: Effect of oxygen concentration and of far-red illumination

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Abstract
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.