Carbon Dioxide and Nitrite Photoassimilatory Processes Do Not Intercompete for Reducing Equivalents in Spinach and Soybean Leaf Chloroplasts

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Abstract
Previously, C Baysdorfer and JM Robinson (1985 Plant Physiol 77: 318-320) demonstrated that, in a reconstituted spinach chloroplast system, NADP photoreduction functioning at most maximal rate and reductant demand, was the successful competitor with NO2- photoreduction for reduced ferredoxin. This resulted in a repression of NO2- reduction until all NADP available had been almost totally reduced. Further experiments, employing isolated, intact spinach leaf plastids and soybean leaf mesophyll cells, were conducted to examine competition for reductant between CO2 and NO2- photoassimilation, in situ. In isolated, intact plastid preparations, regardless of whether the demand for reductant by CO2 photoassimilation was high (5 millimolar ''CO2'') with rates of CO2 fixation in the range 40 to 90 micromoles CO2 fixed per hour per milligram chlorophyll, low (0.5 millimolar ''CO2'') with rates in the range 5 to 8 micromoles CO2 per hour per milligram chlorophyll, or zero (no ''CO2''), NO2- photoreduction displayed equal rates in the range of 8 to 22 micromoles per hour per milligram chlorophyll. In the absence of ''CO2'', but in the presence of saturating white light, 3-phosphoglycerate photoreduction at rates of 82 to 127 micromoles per hour per milligram chlorophyll did not repress, and occasionally stimulated concomitant rates of NO2- reduction which ranged from 23.4 to 38.5. Conversely, in plastid preparations, NO2- at levels of 50 to 100 micromolar, stimulated plastid CO2 fixation when ''CO2'' was saturating with respect to carboxylation. Further, levels of NO2- in the range 250 to 2500 micromolar, stimulated soybean leaf mesophyll cell net CO2 fixation as much as 1.5-fold if ''CO2'' was saturating with respect to CO2 fixation. It appeared likely that, in high light in vivo, CO2 and NO2- photoassimilatory processes are not forced to intercompete for reduced ferredoxin in the intact chloroplast.