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Metabolism and Enzymology

Spinach Leaf Chloroplast CO₂ and NO₂^– Photoassimilations Do Not Compete for Photogenerated Reductant

Manipulation of Reductant Levels by Quantum Flux Density Titrations

United States Department of Agriculture, Agricultural Research Service, Beltsville Agricultural Research Center-West, Plant Science Institute, Beltsville, Maryland 20705, Plant Photobiology Laboratory, Beltsville, Maryland 20705

Potential competition between CO₂ and NO₂^– photoassimilation for photogenerated reductant (e.g. reduced ferredoxin and NADPH) was examined employing isolates of mesophyll cells and intact chloroplasts derived from mature `source' spinach leaves. Variations in the magnitude of incident light energy were used to manipulate the supply of reductant in situ within chloroplasts. Leaf cell and plastid isolates were fed with saturating CO₂ and/or NO₂^– to produce the highest demand for reductant by CO₂ and/or NO₂^– assimilatory processes (enzymes). Even in the presence of CO₂ fixation, NO₂^– reduction in intact leaf cell isolates as well as plastid isolates was maximal at light energies as low as 50 to 200 microeinsteins per second per square meter. Simultaneously, 500 to 800 microeinsteins per second per square meter were required to support maximal CO₂ assimilation. Regardless of the magnitude of the incident light energy, CO₂ assimilation did not repress NO₂^– reduction, nor were these two processes mutually repressed. These observations have been interpreted to mean that reduced ferredoxin levels in situ in the plastids of mature source leaf mesophyll cells were adequate to supply the concurrent maximal demands exerted by enzymes associated with CO₂ as well as with inorganic nitrogen photoassimilation.

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