Seasonal and Canopy Variation in Nitrate Reductase Activity of Soybean (Glycine max L. Merr.) Varieties¹

Abstract

An in vivo assay was used to determine if differences existed in nitrate reductase activity among 16 soybean (Glycine max L. Merr.) varieties within four maturity groups. Canopy and seasonal profiles of nitrate reductase activity were determined. All varieties exhibited highest nitrate reductase activity per leaf ✕ nr in upper leaves of the plant canopy during growth stages up to mid pod fill. Activity levels were similar throughout the plant leal canopy following mid pod fill. Nitrate reductase activity per leaf × hr at a given node was generally highest the initial date at which the leaf was sampled and activity declined with leaf age. Mean nitrate reductase activity (µmoles NO‐2 formed per g fresh wt × hr) for the entire leaf canopy was highest in the seedling stage for all maturity groups and declined as the plants matured. Activity of the total plant (µmoles NO‐2 formed per plant × hr) was maximal at approximately the full bloom stage with all maturity groups, irrespective of the calendar date at which the maturity groups attained full bloom. Thus, enzyme activity appeared to be closely associated with physiological growth stage. Genetic variation (up to 1.8 fold) in enzyme activity (seasonal means of µmoles NO‐2 formed per plant × hr) was found within maturity groups II, III, and IV. However, enzyme activity did not correlate with either seed yield or seed protein content.

Factors Influencing Nitrate Reduction in Soybean Foliage

“…Recent estimates suggest that the nitrate reduction pathway contributes 50-75% of the reduced nitrogen in soybean (Harper, Nicholas and Hageman, 1972). In several species the reduction of nitrate by nitrate reductase (NR) (reduced NAD(P): nitrate oxidoreductase, E.C.…”

Comparison ofIn VivoandIn Vitroassays for nitrate reductase activity determination in soybean tissues

“…NR activity levels were much lower than those generally reported for soybeans (7,8,10). This may have occurred because 'of low light intensities in the growth room which can limit NR activity (2).…”

“…several modifications have been made (5,7,8,10,14,15) to find a suitable assay method to measure the rate of nitrate reduction. At present, two types of assays.…”

“…The in vitro techniques employ the extraction of the enzyme by grinding plant tissue in suitable extracting media and measuring NR activity in a reaction mixture (5,6). In vivo techniques involve the incubation of the original plant tissue in a reaction mixture (7,8,10,13). HAGEMAN and FLESHER (5) and HARPER and COOPER (6) used a composite sample of corn shoot and soybean leaf tissue respectively for measuring NR activity.…”

“…Instead of a composite sample and extracting the enzyme, JAWORSKI (10) used two soybean leaf discs for NR activity determination .. HARPER and HAGEMAN (7) in an in vivo assay estimated NR activity which agreed closely with the actual N content of soybean grown in gravel culture and nutrient solution. Using the same technique, HARPER et al (8) overestimated the nitrate reduced by soil grown soybean.…”