Canopy and Seasonal Profiles of Nitrate Reductase in Soybeans (Glycine max L. Merr.)

Abstract

Nitrate reductase activity of soybeans (Glycine max L. Merr.) was evaluated in soil plots and outdoor hydroponic gravel culture systems throughout the growing season. Nitrate reductase profiles within the plant canopy were also established. Mean activity per gram fresh weight per hour of the entire plant canopy was highest in the seedling stage while total activity (activity per gram fresh weight per hour times the total leaf weight) reached a maximum when plants were in the full bloom to midpod fill stage. Nitrate reductase activity per gram fresh weight per hour was highest in the uppermost leaf just prior to full expansion and declined with leaf positions lower in the canopy. Total nitrate reductase activity per leaf was also highest in the uppermost fully expanded leaf during early growth stages. Maximum total activity shifted to leaf positions lower in the plant canopy with later growth stages.Nitrate reductase activity of soybeans grown in hydroponic systems was significantly higher than activity of adjacent soil grown plants at later growth stages, which suggested that under normal field conditions the potential for nitrate utilization may not be realized. Nitrate reductase activity per gram fresh weight per hour and nitrate content were positively correlated over the growing season with plants grown in either soil or solution culture. Computations based upon the nitrate reductase assay of plants grown in hydroponics indicated that from 1.7 to 1.8 grams N could have been supplied to the plant via the nitrate reductase process. The harvested seed contained 1.1 to 1.2 grams N per plant. Thus, based on previous estimates of approximately 32% of the final N distribution being in the vegetative plant parts, the estimated input of reduced nitrogen via the enzyme assay was in agreement with the actual N accumulation.The amount of calculated N2-fixation by nodules per season with plants grown in hydroponics was less than 2 % of the computed nitrate reduced via leaf nitrate reductase. Thus, the level of nitrate in the nutrient solution appeared to be quite inhibitory to N2-fixation.The response of the soybean plant to nitrogen is confounded by the ability of the plant to utilize both nitrate and N2. Nitrate is considered the primary source of nitrogen available from the soil. The uptake of nitrate and subsequent reduction by nitrate reductase is the primary pathway of soil nitrogen utilization. The utilization of N2 through the symbiotic relationship with Rhizobium japonicum (Kirchner) Buchanan affords a second major pathway of nitrogen input to soybeans.Nitrogen fixation is normally initiated in soybeans 20 to 30 days after planting (3). Thus, initial nitrogen requirements must be met through utilization of nitrogen from the seed and nitrogen from the soil. Recent estimates indicate that some 25 to 30% (80-110 kg N per hectare per season) of the total plant nitrogen was supplied through the N2-fixation process in soybeans as measured with the acetylene-reduction technique (3, 4). It has also been estim...

Partitioning of nitrate reductase activity in Coffea arabica L. and its relation to carbon assimilation under different irradiance regimes

“…Roots were carefully detached, immediately blotted dry on paper towels and weighed. The in vivo NRA was measured according to Jaworski (1971), modified by Harper and Hageman (1972). Root and leaf samples (0.2 g) were placed in glass vials with 5mL of incubation mixture containing 0.1 mol L -1 potassium buffer (pH 8.0), 0.05 mol L -1 KNO 3 and 1% n-propanol.…”

Characterization of Amino Acid Efflux from Isolated Soybean Cells

“…Approximately half of this N is remobilized (translocated) from vegetative tissue when the plant has a reduced capacity for assimilating atmospheric and soil N during the time of rapid seed growth (9). This syndrome of satisfying the N demand by degradation of leaf protein and the subsequent re-mobilization of N to the seeds has been referred to as the "self destruction" hypothesis by Sinclair and de Wit (21).…”

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

“…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.…”

“…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.…”