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Spinach Leaf Chloroplast CO₂ and NO₂⁻ Photoassimilations Do Not Compete for Photogenerated Reductant: Manipulation of Reductant Levels by Quantum Flux Density Titrations

J. Michael Robinson
Plant Physiology
Vol. 88, No. 4 (Dec., 1988), pp. 1373-1380
Stable URL: http://www.jstor.org/stable/4271762
Page Count: 8
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Since scans are not currently available to screen readers, please contact JSTOR User Support for access. We'll provide a PDF copy for your screen reader.
Spinach Leaf Chloroplast CO₂ and NO₂⁻ Photoassimilations Do Not Compete for Photogenerated Reductant: Manipulation of Reductant Levels by Quantum Flux Density Titrations
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Abstract

Potential competition between CO2 and $\text{NO}_{2}{}^{-}$ 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 CO2 and/or $\text{NO}_{2}{}^{-}$ to produce the highest demand for reductant by CO2 and/or $\text{NO}_{2}{}^{-}$ assimilatory processes (enzymes). Even in the presence of CO2 fixation, $\text{NO}_{2}{}^{-}$ 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 CO2 assimilation. Regardless of the magnitude of the incident light energy, CO2 assimilation did not repress $\text{NO}_{2}{}^{-}$ 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 CO2 as well as with inorganic nitrogen photoassimilation.

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