Spinach Leaf Chloroplast CO(2) and NO(2) Photoassimilations Do Not Compete for Photogenerated Reductant: Manipulation of Reductant Levels by Quantum Flux Density Titrations.

Potential competition between CO(2) and NO(2) (-) 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 CO(2) and/or NO(2) (-) to produce the highest demand for reductant by CO(2) and/or NO(2) (-) assimilatory processes (enzymes). Even in the presence of CO(2) fixation, NO(2) (-) 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 CO(2) assimilation. Regardless of the magnitude of the incident light energy, CO(2) assimilation did not repress NO(2) (-) 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 CO(2) as well as with inorganic nitrogen photoassimilation.