A series of experiments was conducted to characterize alterations in carbohydrate utilization in leaves of nitrogen stressed plants. Two-weekold, nonnodulated soybean plants (Glycine max [L.] Merrill, 'Ransom'), grown previously on complete nutrient solutions with 1.0 millimolar N03-, were transferred to solutions without a nitrogen source at the beginning of a dark period. Daily changes in starch and sucrose levels of leaves were monitored over the following 5 to 8 days in three experiments. Starch accumulation increased relative to controls throughout the leaf canopy during the initial two light periods after plant exposure to N-free solutions, but not after that time as photosynthesis declined. The additional increments of carbon incorporated into starch appeared to be quantitatively similar to the amounts of carbon diverted from amino acid synthesis in the same tissues. Since additional accumuhted starch was not degraded in darkness, starch levels at the beginning of light periods also were elevated. In contrast to the starch effects, leaf sucrose concentration was markedly higher than controls at the beginning of the first light period after the N-limitation was imposed. In the days which followed, diurnal turnover patterns were similar to controls. In source leaves, the activity of sucrose-P synthase did not decrease until after day 3 of the N-limitation treatment, whereas the concentration of fructose-2,6-bisphosphate was decreased on day 2. Restricted growth of sink leaves was evident with N-limited plants within 2 days, having been preceeded by a sharp decline in levels of fructose-2,6 bisphosphate on the first day of treatment. The results suggest that changes in photosynthate partitioning in source leaves of N-stressed plants resulted largely from a stable but limited capacity for sucrose formation, and that decreased sucrose utilization in sink leaves contributed to the whole-plant diversion of carbohydrate from the shoot to the root.The activities of carbon and nitrogen assimilatory processes are closely related to rates of plant growth and development. The constancy of this association has led modelers of physiological responses to explain whole-plant growth and coordinated growth between the shoot and root predominantly in terms of carbon and nitrogen interactions (31, 33 utilization are commonly observed: (a) starch accumulates in leaves (1,19,21, 34) and (b) a larger portion of available carbohydrate is translocated from leaves into the root system, resulting in a decline in the shoot to root weight ratio (3, 1 1). The responses imply a general decline in carbohydrate utilization efficiency within the leaf canopy; however, little is known about the regulatory processes involved.Accumulation ofstarch in leaves ofplants undergoing nitrogen stress could result from increased net synthesis in the light and/ or decreased degradation in darkness. An increase in starch formation could reflect a decline in the rate of sucrose biosynthesis. The rate of sucrose synthesis in the cytosol of leaf m...
The use of the relative ureide content of xylem sap l(ureide-N/total N) x 1001 as an indicator of N2 fixation in soybeans (Merr.) was examined under greenhouse conditions. Acetylene treatments to inhibit N2 fixation were imposed upon the root systems of plants totally dependent upon N2 fixation as their source of N and of plants dependent upon both N2 fixation and uptake of exogenous nitrate. Significant decreases in the total N concentration of xylem sap from plants of the former type were observed, but no significant decrease was observed in the total N concentration of sap from the latter type of plants. In both types of plants, acetylene treatment caused significant decreases in the relative ureide content of xylem sap. The results provided further support for a link between the presence of ureides in the xylem and the occurrence of N2 fixation in soybeans. The relative ureide content of xylem sap from plants totally dependent upon N2 fixation was shown to be insensitive to changes in the exudation rate and total N concentration of xylem sap brought about by diurnal changes in environmental factors. There was little evidence of soybean cultivars or nodulating strains affecting the relative ureide content of xylem sap. 'Ransom' soybeans nodulated with Rhizobium japonicum strain USDA 110 were grown under conditions to obtain plants exhibiting a wide range of dependency upon N2 fixation. The relative ureide content of xylem sap was shown to indicate reliably the N2 fixation of these plants during vegetative growth using a 1"N method to measure N2 fixation activity. The use of the relative ureide content of xylem sap for quantification of N2 fixation in soybeans should be evaluated further.Soybean plants acquire N from their environment by the uptake of nitrogenous compounds from the soil solution and by the symbiotic fixation of atmospheric N2 within their root nodules.Although several methods have been developed to estimate the contribution made by N2 fixation to the total N accumulation of soybean and other leguminous plants, a need still exists for a rapid, inexpensive, and quantitative method that will separate the contributions made by the uptake of soil and fertilizer N and the fixation of N2 in field-grown soybeans. The C2H2 reduction assay (6, 10, 22) provides a sensitive, relatively inexpensive, and simple method for measuring instantaneous nitrogenase activity. However, application of this method to the quantitative measurement of seasonal profiles of N2 fixation in field situations has a number of shortcomings. The procedure underestimates N2 fixation due to the negative effect of root excision on C2H2 reduction (7, 18) and the difficulty in extracting all nodules from the soil, especially at later developmental stages when nodules may exist on lateral roots at some distance from the root crown. Overestimations arise when roots are extracted from the soil due to decreased resistance to gaseous diffusion (7) and when legume root nodules which evolve H2 under ambient conditions are assayed (23)...
Wheat (Triticum vulgare L., cv. Blueboy) seedlings, grown with 0.25, 1.0 and 15 mM nitrate in complete nutrient solutions, were transferred 10 days after germination to 1.0 mM K(15)NO3 (∼99 A% (15)N) plus 0.1 mM CaSO4 at pH 6.0. The solutions were replaced periodically over a 6-h period (5 mW cm(-2); 23°). Changes in the [(15)N]- and [(14)N]nitrate in the solution were determined by nitrate reductase and mass-spectrometric procedures and potassium by flame photometry. Influx of [(15)N]nitrate was depressed in plants grown at 1.0 mM nitrate relative to those grown at 0.25 mM, but there was no appreciably difference in [(14)N]nitrate efflux. Prior growth at 15 mM further restricted [(15)N]nitrate influx which, together with a substantial increase in [(14)N]nitrate efflux, resulted in no net nitrate uptake during the course of the experiment. Efflux of [(14)N]nitrate occurred to solutions containing no nitrate but it was significantly enhanced upon exposure to [(15)N]nitrate in the external solution. Influx of [(15)N]nitrate was more restricted at 5°, relative to 23°, than was [(14)N]nitrate efflux. The nitrate concentrations of the root tissue immediately before exposure to the K(15)NO3 solutions did not give a precise indication of the subsequent [(15)N]nitrate influx rates nor of the [(14)N]nitrate efflux rates. Net K(+) uptake was related to the magnitude of the net nitrate uptake, not to the initial K(+) concentration in the roots. The data are interpreted as indicating that [(15)N]nitrate influx and [(14)N]nitrate efflux are largely independent processes, subject to different controls, and that net nitrate uptake provides the driving force for net potassium uptake.
Previous experiments have revealed a much greater efficiency of ammonium utilization by bean plants (Phaseolus vulgaris L.) when the acidity of the ambient medium was maintained at near-neutral conditions with carbonates or hydroxides. The present investigation, in which (15)N-labeled ammonium was used, permitted an assessment of the origin of nitrogen in tissue nitrogen pools with and without acidity control (CaCO(3) treated and untreated, respectively) in the root environment. Control of acidity resulted in greater ammonium uptake and greater incorporation into the amino fraction, amide, and ethanol-insoluble nitrogen by the root tissue. These differences were clearly evident by the fifth day after ammonium nitrogen had been applied.Shoots of the untreated plants rapidly accumulated free ammonium and amino nitrogen. A substantial portion of both fractions came from pre-existing nitrogen in the plants, indicating significant protein degradation. No evidence was found for such degradation in the roots of the untreated plants or in either roots or shoots of CaCO(3) treated plants. The data indicate that control of ambient acidity in the root environment during ammonium absorption enhanced the conversion of entering ammonium to organic nitrogen compounds in the root tissue thereby restricting movement of free ammonium to shoots. Consequently, the detrimental effects of high ammonium concentrations in the leaves were largely prevented.
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