Carbon dioxide and nitrite photoassimilatory processes do not intercompete for reducing equivalents in spinach and soybean leaf chloroplasts.

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 NO(2) (-) photoreduction for reduced ferredoxin. This resulted in a repression of NO(2) (-) 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 CO(2) and NO(2) (-) photoassimilation, in situ. In isolated, intact plastid preparations, regardless of whether the demand for reductant by CO(2) photoassimilation was high (5 millimolar ;CO(2)') with rates of CO(2) fixation in the range 40 to 90 micromoles CO(2) fixed per hour per milligram chlorophyll, low (0.5 millimolar ;CO(2)') with rates in the range 5 to 8 micromoles CO(2) per hour per milligram chlorophyll, or zero (no ;CO(2)'), NO(2) (-) photoreduction displayed equal rates in the range of 8 to 22 micromoles per hour per milligram chlorophyll. In the absence of ;CO(2)', 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 NO(2) (-) reduction which ranged from 23.4 to 38.5. Conversely, in plastid preparations, NO(2) (-) at levels of 50 to 100 micromolar, stimulated plastid CO(2) fixation when ;CO(2)' was saturating with respect to carboxylation. Further, levels of NO(2) (-) in the range 250 to 2500 micromolar, stimulated soybean leaf mesophyll cell net CO(2) fixation as much as 1.5-fold if ;CO(2)' was saturating with respect to CO(2) fixation. It appeared likely that, in high light in vivo, CO(2) and NO(2) (-) photoassimilatory processes are not forced to intercompete for reduced ferredoxin in the intact chloroplast.