Partitioning of previously-accumulated nitrate to translocation,reduction, and efflux in corn roots

The effect of nitrate uptake, or its absence, on the utilization of nitrate previously accumulated by dark-grown, decpitated maize (Zea mays L., cv. DeKalb XL-45) seedlings was examined. Five-d-old plants that had been pretreated with 50 mM ¹⁴NO ₃ ^? for 20 h were exposed for 8 h to nutrient solutions containing either no nitrate or 50 mM ¹⁵NO ₃ ^? , 98.7 atom % ¹⁵N. The ambient solution, xylem exudate, and plant tissue were analyzed to determine the quantities of previously-accumulated (endogenous) ¹⁴NO ₃ ^? that were translocated to the xylem, lost to the solution, or reduced within the tissue during the 8-h period. Energy was continuously available to the roots from the attached endosperm. In the absence of incoming nitrate, appreciable reduction and translocation of the endogenous ¹⁴NO ₃ ^? occurred, but efflux of ¹⁴NO ₃ ^? to the external solution was minimal. In contrast, during ¹⁵NO ₃ ^? uptake, there was considerable efflux of ¹⁴NO ₃ ^? as well as translocation of ¹⁴NO ₃ ^? to the xylem, but little ¹⁴NO ₃ ^? was reduced. Thus there appeared to be an inverse relationship between ¹⁴NO ₃ ^? efflux and reduction. The data are tentatively interpreted on the basis of a model which envisages (a) two storage locations within roots, one of which primarily supplies nitrate for translocation and the other of which primarily supplies nitrate for outward passage through plasmalemma, and (b) the majority of nitrate reduction as occurring during or immediately following influx across the plasmalemma, with endogenous ¹⁴NO ₃ ^? initially moving outward being recycled inward and thereby being reduced.     

Potassium influx into maize root systems : influence of root potassium concentration and ambient ammonium

Potassium influx into roots of dark-grown decapitated maize seedling (Zea mays L., cv Pioneer 3369A) was examined in presence and absence of ambient ammonium and at various root potassium concentrations. Six-day old seedlings which were dependent on the endosperm reserves for their energy source were exposed to KCl (labeled with ⁸⁶Rb) ranging from 5 to 200 micromolar. At both low (13 micromoles per gram fresh weight) and high (100 micromoles per gram fresh weight) root potassium concentration, isotherms indicated two potassium influx systems, one approaching saturation at 50 to 100 micromolar potassium and an additional one tentatively considered to be linear. A mixed-type inhibition by ammonium for the low-concentration saturable system was indicated by a concomitant decrease in V_max and increase in K_m. High root potassium concentration decreased V_max but had little effect on K_m of this system. The rate constant for the second quasilinear system was decreased by ambient ammonium and by high root potassium status. Transfer of high potassium roots to potassium-free solutions resulted in an increase in influx within 2 hours; by 24 hours influx significantly exceeded that of roots not previously exposed to potassium. In roots of both low and high root potassium concentrations, potassium influx was restricted progressively as ambient ammonium increased to about 100 micromolar, but there was little further inhibition as ammonium concentrations increased beyond that to 500 micromolar. The data imply that potassium influx has two components, one subject to inhibition by ambient ammonium and one relatively resistant.     

Effect of tyrosine administration on dopa accumulation in light- and dark-adapted retinas from normal and diabetic rats

The interaction of tyrosine concentration and lighting on in vivo dihydroxyphenylalanine (dopa) accumulation rate was studied in retinas of normal and diabetic rats. In both groups of rats, dopa accumulation and in vitro hydroxylase activity were higher in retinas exposed to light than in those adapted to darkness. In light-adapted diabetic rats, though, retinal tyrosine level, dopa accumulation, and in vitro tyrosine hydroxylase activity were all below normal. In both normal and diabetic rats exposed to light, tyrosine injection raised retinal tyrosine concentrations and stimulated dopa accumulation. Injection of tyrosine into dark-adapted rats raised retinal tyrosine level but did not enhance dopa accumulation. Together, these results suggest that in vivo retinal amacrine cells will vary their dopa accumulation rate as a function of substrate supply, but only in the light, when tyrosine hydroxylase is activated. They further indicate that dopa accumulation rate remains sensitive to tyrosine supply in the light-activated diabetic retina.     

Photorespiratory phenomena in maize: oxygen uptake, isotope discrimination, and carbon dioxide efflux

Concurrent O₂ evolution, O₂ uptake, and CO₂ uptake by illuminated maize (Zea mays) leaves were measured using ¹³CO₂ and ¹⁸O₂. Considerable O₂ uptake occurred during active photosynthesis. At CO₂ compensation, O₂ uptake increased. Associated with this increase was a decrease in O₂ release such that a stoichiometric exchange of O₂ occurred. The rate of O₂ exchange at CO₂ compensation was directly related to O₂ concentration in the atmosphere at least up to 8% (v/v).     

Quantitative Assay of Polysaccharide Components Obtained from Cell Wall Lipopolysaccharides of Xanthomonas Species

The cell wall lipopolysaccharides from 17 species belonging to the genus Xanthomonas were extracted from the cells with hot 45% phenol. After purification, the components of the polysaccharide were obtained by acid hydrolysis of the lipopolysaccharide and were quantitatively assayed. Data obtained show that all preparations contained uronic acid, phosphate, and mannose in molar ratios of approximately 1:2:1, and glucose and rhamnose in more variable concentrations. Most lipopolysaccharides contained either xylose or fucose, but those extracted from X. sinensis and X. campestris contained neither xylose nor fucose.     

Inhibition of inorganic pyrophosphatase of animal mitochondria by calcium

Calcium ion is an uncompetitive inhibitor of the inorganic pyrophosphatases of bovine heart and rat liver mitochondria with respect to substrate MgPPi at pH 8.5 and a non-competitive inhibitor of the former enzyme at pH 7.2. The concentration of Ca2+ required to decrease the maximal velocities for both enzymes at pH 8.5, 0.4 mM Mg2+ was about 10 microM. The inhibition results from the binding of two Ca2+ ions to both free enzymes and their complexes with the substrate. The results suggest that Ca2+ regulates pyrophosphatase activity and hence PPi level in mammalian mitochondria.     

Resumption of feeding in vitro by hookworm third-stage larvae: a comparative study.

Third-stage infective larvae of the canine hookworm Ancylostoma caninum resume feeding in vitro in response to several stimuli. Experiments were conducted to characterize the in vitro feeding behavior of several hookworm species. Reduced glutathione and, to a lesser extent, canine and human serum stimulated third-stage larvae of Ancylostoma duodenale to resume feeding. Glutathione-induced feeding reached a maximum by 16 hr and was concentration-dependent between 0- and 15-mM glutathione. Oxidized glutathione and the reducing agents dithiothreitol and L-cysteine failed to induce feeding, suggesting that reducing conditions alone were not stimulatory. Serum incubated with glutathione was the most efficient stimulus for Ancylostoma ceylanicum, Ancylostoma braziliense, and Ancylostoma tubaeforme larvae, whereas Uncinaria stenocephala larvae responded best to canine serum alone. Necator americanus larvae did not resume feeding in response to glutathione, serum, glutathione plus serum, or linoleic acid (0.1-10 mM). These differences in feeding behavior suggest that generalizations concerning hookworm biology must be interpreted cautiously.     

Uptake of nitrogen by flue-cured tobacco during maturation and senescence

A field experiment with flue-cured tobacco,Nicotiana tabacum L., was conducted to estimate the uptake and partitioning of nitrogen during maturation and senescence. On the 83rd day after transplanting (crop day 83), nitrate which had been leached from the plow layer was replaced with an equivalent amount of¹⁵N-labeled nitrate. Plants were harvested at crop day 83, 90, 96, 106, 113, and 127, and each of 11 plant parts was analyzed for nitrogen derived from the soil (NDS) and from the applied¹⁵N-labeled fertilizer (NDF). Equivalent quantities of NDF and NDS were taken up during the initial week after¹⁵N-fertilizer application; in the subsequent 5 weeks, ten times more NDS than NDF were taken up. It appears likely that the leached nitrate (NDS) accumulated below the hard pan where it became available to plants as their roots penetrated this layer via fractures originating from prior deep chiseling. Of the NDF taken up during the initial week, 20% was partitioned to the root and 42%, 24%, 14% respectively, to the upper, middle and bottom node positions (leaves plus stems). The partitioning reflected the respective growth rates of the tissues. Little change in partitioning was evident during the subsequent 5-week period, indicating that little remobilization of NDF from older to younger tissues occurred. In contrast, some remobilization of NDS was apparent between crop day 96 and 106 when the uptake of both NDF and NDS was negligible. During this period root growth was sustained by the apparent transfer of NDS from the root stump and from the adjacent lower leaf and stem tissues. These responses occurred in tobacco grown under higher nitrogen fertility levels than those usually considered optimal for the growth of flue-cured tobacco, but under conditions which are sometimes encountered. Paper no. 11640 of the Journal series of the North Carolina Agricultural Research Service, Raleigh, NC 27695-7601. Part of a thesis submitted by the senior author in partial fulfilment of the requirements of the Ph.D degree.     

Cleavage of A-CAM by endogenous proteinases in cultured lens cells and in developing chick embryos

We describe two truncated forms of A-CAM (N-cadherin) and present evidence suggesting that both forms are proteolytically derived from the intact A-CAM molecule. The first is a membrane-bound fragment of A-CAM displaying an apparent molecular weight of 78 kDa. This polypeptide, containing the C-terminal portion of the protein, may be generated in cultured chicken lens cells, either by a short treatment with trypsin-EGTA, or by endogenous proteinase(s) during incubation in low Ca2+ medium. Immunofluorescent labeling of normal and EGTA-treated cells indicated that the 78-kDa fragment is uniformly distributed over the cell surface. Moreover, staining of developing chick embryos with pairs of antibodies which distinguish the 78-kDa fragment from intact A-CAM indicated that, at early stages of sclerotome dissociation in developing somites, a truncated derivative of the molecule is generated. The second truncated form of A-CAM is a 97-kDa polypeptide which is constitutively released by cultured lens cells into the culture medium in the presence of normal medium. We present evidence that the 97-kDa molecule is proteolytically derived from A-CAM by the action of an endogenous proteinase. We discuss possible mechanisms leading to the formation of these two truncated derivatives and their possible involvement in the physiological modulation of A-CAM-mediated interactions.