Limitations on Leaf Nitrate Reductase Activity during Flowering and Podfill in Soybean

Abstract

The objective of this study was to identify factors which limit leaf nitrate reductase (NR) activity as decline occurs during flowering and beginning seed development in soybean (Glycine max [L.] Meff. cv Clark). Level of NR enzyme activity, level of reductant, and availability of NO3-as substrate were evaluated for field-rown soybean from flowering through leaf senescence. Timing of reproductive development was altered within one genotype by (a) exposure of Clark to an artificially short photoperiod to hasten flowering and podfill, and (b) the use of an early flowering isoline. Nitrogen (N) was soil-applied to selected plots at 500 kilograms per hectare as an additional variable. Stem NO3-concentration and in vivo leaf NR activity were significantly correlated (R2 = 0.69 with nitrate in the assay medium and 0.74 without nitrate in the medium at P = 0.001) across six combinations of reproductive and soil N-treatment. The supply of NO3-from the root to the leaf tissue was the primary limitation to leaf NR activity during flowering and podfill. Levels of NR enzyme and reductant were not limiting to leaf NR activity during this period.The relative contribution of NO3-reduction and symbiotic dinitrogen fixation to the N nutrition of soybean (Glycine max [L.] Merr.) has been investigated by a number of workers (10,28,29). Earlier work has established that NO3-reduction in the leaves is chiefly responsible for N accumulation during vegetative development, reaching a maximum at beginning to full flower (12, 28). A sharp decline in leaf NR2 activity has been reported to follow within the 2 to 3 weeks after flowering (10,29 of N03-resulted in no increase in activity. Shaner and Boyer (24,25) reported that a variety of factors which decreased the movement of nitrate from roots to leaves resulted in a parallel decrease in NR activity in soybean. The transport of NO3-to the leaves from the roots was found to play a much larger role than leaf NO3-level in regulating leaf NR activity (24). Studies monitoring the flow of NO3-from the root to the leaf by direct xylem exudate analysis have demonstrated decreasing NO3-levels over the growing season (14, 26). Entire stem segment NO3-analysis has been used to indicate the supply of NO3-available to the leaves via the xylem, since levels of NO3-are near zero in the phloem (20). Decreasing leaf NO3-concentration has also been reported in association with declining leaf NR activity in soybean (16, 28). However, total leaf NO3-measurements overestimate NO3-readily available for reduction, by measuring NO3-held in a relatively unavailable storage pool in the vacuole (8,24). High levels of soil-applied N have been shown to increase leaf NR activity and to delay its late-season decline in soybean (23).There remains uncertainty as to the precise relationship between reproductive development and seasonal change in the N nutrition of soybean. Investigations comparing the seasonal N nutrition patterns of cultivars representing different relative maturities have unavoidably compared ...

Regulation of NO3− Assimilation by Anion Availability in Excised Soybean Leaves

“…In accordance with this hypothesis, the NO3-accumulated in the storage pool was shown to be nearly unavailable for reduction (8). From these results and others (19,30), the conclusion has emerged that NO3-reduction in illuminated leaves is regulated mostly by the xylem flux of NO3-. The objective of this work was to test this hypothesis with soybean (Glycine max L. Merr.…”

Decreased Rates of Nitrate Uptake During Pod Fill by Cowpea, Green Gram, and Soybean

“…The effectiveness of the soybean plant in assimilating exogenous nitrate diminishes during pod fill (Harper, 1971;Streeter, 1972;Imsande, 1986) and is associated with a dramatic decline in foliar nitrate reductase activity during reproductive growth (Harper and Hageman, 1972;Thibodeau and Jaworski, 1975;Nelson-Schreiber and Schweitzer, 1986). These observations led to the suggestion that during reproductive growth an inadequately nodulated soybean plant may be deficient in N regardless of the availability of exogenous nitrate (lmsande, 1986).…”

Nitrogen Pulse and Competition Affects Nitrogen Metabolism in Invasive Weed (Amaranthus retroflexus) and Native Crop (Glycine max)

“…However, G. max is an abstemious consumer of N, high nitrogen fertilizer may have negative effects on G. max growth [36]; when the nitrogen was supplied once on the planting date or before the flowering period of G. max, the nitrogen would act as a higher "priming N" than the continuous supplying nitrogen in G. max growth at the seedling stage, whereas after the flowering stage, the non-continuous N supplying would suppress the nodule development, decrease the nitrogenase activity, and leghemoglobin content in G. max more significantly than the continuous N supplying, and the suppress effect would weaken in the pod fill period [37]. Thus, in SS treatment, the highest NR activities in G. max roots and stems occurred in August, whereas NR activities of G. max leaves and reproductive organs were increasing from June to August, due to the increasing movement of NO3 -from roots to leaves during the flowering and podding period [38].…”