The effect of 2,4-dinitrophenol on the absorption and translocation of calcium by pumpkin plants

The study deals with the absorption and translocation of Ca²⁺ by án intact plant of pumpkinCucurbita pepo L. and with the effect of various concentrations of 2,4-dinitrophenol (DNP) on these processes. The absorption of Ca²⁺ was reduced by the application of this inhibitor, but not completely stopped. The translocation of Ca²⁺ into shoots was affected more expressively: it was almost completely inhibited by higher DNP concentrations. The uptake of Rb⁺ is aubstantially more influeneed than that of Ca²⁺: higher inhibitor concentrations caused its complete cessation.     

The effect of nitrate assimilation deficiency on the carbon and nitrogen status of Arabidopsis thaliana plants

Carbon (C) and nitrogen (N) metabolism are integrated processes that modulate many aspects of plant growth, development, and defense. Although plants with deficient N metabolism have been largely used for the elucidation of the complex network that coordinates the C and N status in leaves, studies at the whole-plant level are still lacking. Here, the content of amino acids, organic acids, total soluble sugars, starch, and phenylpropanoids in the leaves, roots, and floral buds of a nitrate reductase (NR) double-deficient mutant of Arabidopsis thaliana (nia1 nia2) were compared to those of wild-type plants. Foliar C and N primary metabolism was affected by NR deficiency, as evidenced by decreased levels of most amino acids and organic acids and total soluble sugars and starch in the nia1 nia2 leaves. However, no difference was detected in the content of the analyzed metabolites in the nia1 nia2 roots and floral buds in comparison to wild type. Similarly, phenylpropanoid metabolism was affected in the nia1 nia2 leaves; however, the high content of flavonol glycosides in the floral buds was not altered in the NR-deficient plants. Altogether, these results suggest that, even under conditions of deficient nitrate assimilation, A. thaliana plants are capable of remobilizing their metabolites from source leaves and maintaining the C–N status in roots and developing flowers.     

Light and dark assimilation of nitrate in plants

Abstract. Heterotrophic assimilation of nitrate in roots and leaves in darkness is closely linked with the oxidative pentose phosphate pathway. The supply of glucose-6-phosphate to roots and chloroplasts in leaves in darkness is essential for assimilation of nitrite into amino acids. When green leaves are exposed to light, the key enzyme, glucoses-phosphate dehydrogenase, is inhibited by reduction with thioredoxin. Hence the dark nitrate assimilatory pathway is inhibited under photoautotrophic conditions and replaced by regulatory reactions functioning in light. On account of direct photo-synthetic reduction of nitrite in chloroplasts and availability of excess NADH for nitrate reduclase, the rate of nitrate assimilation is extremely rapid in light. Under dark anaerobic conditions also nitrate is equally rapidly reduced to nitrite on account of abolition of competition for NADH between nitrate reductase and mitochondrial oxidation.     

Nitrate reductase and its role in nitrate assimilation in plants

Nitrate reductase (EC 1.6.6.1) is an enzyme found in most higher plants and appears to be a key regulator of nitrate assimilation as a result of enzyme induction by nitrate. The biochemistry of nitrate reductase has been elucidated to a great extent and the role that nitrate reductase plays in regulation of nitrate assimilation is becoming understood.     

Effect of nodulation on the nitrate assimilation in vegetative soybean plants

Summary Nitrate assimilation in the first trifoliate leaf of vegetative soybean plants (Glycine max L. Merr, cv Hodgson) was studied in relation to nodulation. Nodulated and non-nodulated plants were grown in a nitrate medium (4 mM). As a control nodulated plants were grown in a nutrient medium without combined nitrogen. This study included measurements of the acetylene reduction activity of the whole plant and of thein vitro nitrate reductase, glutamine synthetase and glutamate dehydrogenase activities in the first leaf and of the nitrate concentration. Nitrate accumulation and nitrate reductase activity were depressed in nodulated plants; root growth was decreased in the presence of nitrate. The relationships between nitrate assimilation and nodulation are discussed.     

The partitioning of nitrate assimilation between root and shoot of higher plants

Abstract The partitioning of nitrate assimilation between root and shoot of higher plant species is indicated by the relative proportions of total plant nitrate reductase activity (NRA) in the two plant parts and the relative concentrations of nitrate and reduced N in the xylem sap. These have been collated here from the literature and temperate and tropical species compared. Both the distribution of NRA and xylem sap nitrate: reduced N indicate that the following four generalizations can be made.

• 1 Temperate, perennial species growing in low external nitrate concentrations (about 1 mol m^?3) carry out most of their nitrate assimilation in the root. As external nitrate concentration increases (in the range found in agricultural soils, 1–20 mol m^?3), shoot nitrate assimilation becomes increasingly important.
• 2 Temperate, annual legume species growing in low external nitrate concentrations carry out most of their nitrate assimilation in the root. Shoot nitrate assimilation increases in importance as external nitrate concentration is increased.
• 3 Temperate, annual non-legume species vary greatly in their partitioning of nitrate assimilation between root and shoot when growing in low external nitrate concentrations. Regardless of the proportion carried out in the root at low external nitrate concentrations, nitrate assimilation in the shoot becomes increasingly important as external nitrate concentration is increased.
• 4 Tropical and subtropical species, annual and perennial, carry out a substantial proportion of their nitrate assimilation in the shoot when growing in low external nitrate concentrations. The partitioning of nitrate assimilation between root and shoot remains constant as external nitrate concentration increases.

It is proposed that a greater proportion of nitrate assimilation occurs in the shoot when an increase in the rate of nitrate uptake does not induce an increase in NR level in the root. Thus, a greater proportion of the nitrate taken up remains unassimilated and is passed into the xylem. A constant partitioning of nitrate assimilation between root and shoot is achieved by balancing NR levels in the root with rates of nitrate uptake. The advantages and disadvantages of assimilating nitrate in either the root or shoot are discussed in relation to temperate and tropical habitats.     

The effect of light on nitrate and nitrite assimilation by chlorella and ankistrodesmus

Light stimulates the assimilation of nitrate and nitrite by two green algae, Chlorella pyrenoidosa and Ankistrodesmus braunii. Assimilation can be observed when the algae are illuminated in the absence of carbon dioxide under both aerobic and anaerobic conditions. The rates of assimilation by Chlorella do not depend on the presence of carbon dioxide, but Ankistrodesmus assimilates nitrate and nitrite more rapidly when cultures are illuminated in the presence of carbon dioxide than in its absence. The ratios of O(2) : NO(3') and O(2) : NO(2') vary from one experiment to the other and, with the exception of Chlorella cultures reducing nitrite they are higher than the 'expected' values of 2.0 and 1.5 respectively. Oxygen evolution accompanying nitrate and nitrite by algae illuminated in the absence of carbon dioxide is completely inhibited by DCMU at concentrations of 4 × 10(-6) M. However, nitrite assimilation by both Ankistrodesmus and Chlorella and nitrate assimilation by Ankistrodesmus are less sensitive to the inhibitor.     

Sulphur deficiency inhibits nitrogen assimilation and recycling in barley plants

Sulphur (S) is incorporated into diverse primary and secondary metabolites that play important roles in proper growth and development of plants. In cereals, a fraction of the nitrogen (N) accumulated in developing grains is guaranteed by amino acid remobilization from vegetative tissues, a contribution that becomes critical when soil nutrients are deficient. Glutamine synthetase (GS) and amino acid transporters (AAT) are key components involved in N assimilation and recycling. The aim of the present study was to evaluate the effect of S availability on the expressions of HvGS and several selected HvAAT genes in barley plants and on the phloem exudation rate of amino acids. To this end, two independent experiments were designed to impose low S availability conditions to barley plants. Low S availability caused a decrease in the phloem exudation rate of amino acids as well as in the gene expression of all the HvGS genes and five of the six HvAAT genes analyzed. The strong correlation found between the phloem amino acid exudation rate and HvGS1-1, HvGS1-2, HvAAP7, and HvProT1 gene expression may indicate the participation of these genes in the regulation of amino acid remobilization through the phloem.     

Effects of P deficiency on assimilation and transport of nitrate and phosphate in intact plants of castor bean (Ricinus communis L.)

An experimentally-based modelling technique was applied to describequantitatively the uptake, translocation, storage, and assimilationof     

Characterization of ammonium and nitrate uptake and assimilation in roots of tea plants

It has been pointed out that tea (Camellia sinensis (L.) O. Kuntze) prefers ammonium (NH ₄ ⁺ ) over nitrate (NO ₃ ^? ) as an inorganic nitrogen (N) source. ¹⁵N studies were conducted using hydroponically grown tea plants to clarify the characteristics of uptake and assimilation of NH ₄ ⁺ and NO ₃ ^? by tea roots. The total ¹⁵N was detected, and kinetic parameters were calculated after feeding ¹⁵NH ₄ ⁺ or ¹⁵NO ₃ ^? to tea plants. The process of N assimilation was studied by monitoring the dynamic ¹⁵N abundance in the free amino acids of tea plant roots by GC-MS. Tea plants supplied with ¹⁵NH ₄ ⁺ absorbed significantly more ¹⁵N than those supplied with ¹⁵NO ₃ ^? . The kinetics of ¹⁵NH ₄ ⁺ and ¹⁵NO ₃ ^? influx into tea plants followed a classic biphasic pattern, demonstrating the action of a high affinity transport system (HATS) and a low affinity transport system (LATS). The V _max value for NH ₄ ⁺ uptake was 54.5 nmol/(g dry wt min), which was higher than that observed for NO ₃ ^? (39.3 nmol/(g dry wt min)). K_M estimates were approximately 0.06 mM for NH ₄ ⁺ and 0.16 mM for NO ₃ ^? , indicating a higher rate of NH ₄ ⁺ absorption by tea plant roots. Tea plants fed with ¹⁵NH ₄ ⁺ accumulated larger amounts of assimilated N, especially glutamine (Gln), compared with those fed with ¹⁵NO ₃ ^? . Gln, Glu, theanine (Thea), Ser, and Asp were the main free amino acids that were labeled with ¹⁵N under both conditions. The rate of N assimilation into Thea in the roots of NO ₃ ^? -supplied tea plants was quicker than in NH ₄ ⁺ -supplied tea plants. NO ₃ ^? uptake by roots, rather than reduction or transport within the plant, seems to be the main factor limiting the growth of tea plants supplied with NO ₃ ^? as the sole N source. The NH ₄ ⁺ absorbed by tea plants directly, as well as that produced by NO ₃ ^? reduction, was assimilated through the glutamine synthetase-glutamine oxoglutarate aminotransferase pathway in tea plant roots. The ¹⁵N labeling experiments showed that there was no direct relationship between the Thea synthesis and the preference of tea plants for NH ₄ ⁺ .     

Evidence for a role of calcium in nitrate assimilation in wheat seedlings

Severely Ca-deficient Triticum aestivum L. seedlings accumulated high levels of nitrite and moderate levels of nitrate and organic nitrogen, but contained unaltered levels of hydroxylamine. Nitrite accumulation was not related to molybdenum deficiency, or altered cellular pH. Nitrate reductase was decreased by Ca deficiency, apparently by repression of enzyme synthesis from accumulated nitrite and not by inhibition of enzyme activity. Nitrite reductase and NADP diaphorase activities were not affected by Ca deficiency, and Ca did not restore activity to nitrite reductase inactivated by cyanide. The results indicated that the role of Ca is in intracellular transport of nitrite and not in induction or activity of enzymes.     

Root respiration associated with ammonium and nitrate absorption and assimilation by barley

We examined nitrate assimilation and root gas fluxes in a wild-type barley (Hordeum vulgare L. cv Steptoe), a mutant (nar1a) deficient in NADH nitrate reductase, and a mutant (nar1a;nar7w) deficient in both NADH and NAD(P)H nitrate reductases. Estimates of in vivo nitrate assimilation from excised roots and whole plants indicated that the nar1a mutation influences assimilation only in the shoot and that exposure to NO₃⁻ induced shoot nitrate reduction more slowly than root nitrate reduction in all three genotypes. When plants that had been deprived of nitrogen for several days were exposed to ammonium, root carbon dioxide evolution and oxygen consumption increased markedly, but respiratory quotient—the ratio of carbon dioxide evolved to oxygen consumed—did not change. A shift from ammonium to nitrate nutrition stimulated root carbon dioxide evolution slightly and inhibited oxygen consumption in the wild type and nar1a mutant, but had negligible effects on root gas fluxes in the nar1a;nar7w mutant. These results indicate that, under NH₄⁺ nutrition, 14% of root carbon catabolism is coupled to NH₄⁺ absorption and assimilation and that, under NO₃⁻ nutrition, 5% of root carbon catabolism is coupled to NO₃⁻ absorption, 15% to NO₃⁻ assimilation, and 3% to NH₄⁺ assimilation. The additional energy requirements of NO₃⁻ assimilation appear to diminish root mitochondrial electron transport. Thus, the energy requirements of NH₄⁺ and NO₃⁻ absorption and assimilation constitute a significant portion of root respiration.     

The effect of calcium on lead absorption in rats.

The effects of Ca2+ on lead absorption as PbCl2 and 203PbCl2 were studied in rats. 1. Doubling of dietary calcium with Ca3(PO4)2 significantly decreased lead absorption as assessed by 203Pb retention, tissue lead concentration, urinary excretion of delta-aminolaevulinate and increased activities of delta-aminolaevulinate dehydratase and ferrochelatase. 2. Similar effects on lead absorption were shown by the Ca2+ salts, Cl-, CO32-, PO43-, SO42-, gluconate and glycerophosphate. 3. CaCl2 and calcium glycerophosphate were found to be most effective in decreasing lead absorption when administered immediately before lead dosage. 4. A negative exponential relationship was found between CaCl2 concentration and 203Pb absorption at 120h. The result suggest that, above 4 mmol of administered calcium, residual lead absorption is unaffected by increasing gastrointestinal calcium concentrations. 5. Increased systemic calcium had no effect on lead retention. 6. Calcium in the concentrations found in domestic hard-water supplies significantly decreased absorption of a solution of 203Pb dissolved in it compared with absorption of 203Pb dissolved in soft or distilled water. 7. Milk and skimmed milk were found to have no effect on 203Pb absorption in rats.     

The regulation of activity of the enzymes involved in the assimilation of nitrate by higher plants

1. Possible mechanisms regulating the activities of three enzymes involved in nitrate assimilation, nitrate reductase, nitrite reductase and glutamate dehydrogenase, were studied in radish cotyledons. 2. Nitrate-reductase and nitrite-reductase activities are low in nitrogen-deficient cotyledons, and are induced by their substrates. 3. Glutamate dehydrogenase is present regardless of the nitrogen status, and the enzyme can be increased only slightly by long-term growth on ammonia. 4. Although nitrate is the best inducer of nitrate reductase, lower levels of induction are also obtained with nitrite and ammonia. The experiments did not distinguish between direct or indirect induction by these two molecules. 5. Nitrite reductase is induced by nitrite and only indirectly by nitrate. 6. The induction of both nitrate reductase and nitrite reductase is prevented by the inhibitors actinomycin D, puromycin and cycloheximide, indicating a requirement for the synthesis of RNA and protein. 7. The decay of nitrate reductase, determined after inhibition of protein synthesis, is slower than the synthesis of the enzyme. Nitrite reductase is much more stable than nitrate reductase. 8. The synthesis of nitrate reductase is not repressed by ammonia, but is repressed by growth on a nitrite medium. 9. There is no inhibition of nitrate reductase, nitrite reductase or glutamate dehydrogenase by the normal end products of assimilation, but cyanate is a fairly specific inhibitor of nitrate reductase.     

Nitrate absorption and assimilation in ryegrass as influenced by calcium and magnesium

The absorption and assimilation patterns of ¹⁵NO₃⁻ supplied as the Ca²⁺ and Mg²⁺ salts to intact ryegrass (Lolium perenne) seedlings were compared. No statistically significant effect of ambient cation on the amounts of ¹⁵NO₃⁻ absorbed was observed in the initial six hours, but during the subsequent six hours, absorption from Ca(¹⁵NO₃)₂ exceeded that from Mg (¹⁵NO₃)₂.     

The effect of nitrate and nitrite on oxygen evolution and carbon-dioxide assimilation and the reduction of nitrate and nitrite by intact chloroplasts

Summary Intact chloroplasts isolated from spinach reduced NO₃ ^- and NO₂ ^- in the light without the addition of either co-factors or added enzymes. The maximum rate observed, however, for the reduction of NO₃ ^- was approximately 3Moles hr^-1 mg^-1 (chlorophyll) and for NO₂ ^- 6 Moles hr^-1 mg^-1 (chlorophyll). These rates were consistent with the enzyme content of whole chloroplasts, but much lower than those found in whole leaf extracts.The addition of both NO₃ ^- and NO₂ ^- in low concentrations resulted in transient increases in both O₂ evolution and CO₂ fixation. The increases in oxygen evolution were not consistent in amount and bore no relation to the amount of substrate reduced. Similar transients were observed in a number of experiments when NaCl or NH₄Cl were added.The addition of NO₂ ^- at concentrations of 10^-4 M and above resulted in marked inhibition of both O₂ evolution and CO₂ fixation. NO₂ ^- appears to inhibit by blocking the reduction of NADP. NO₃ ^- at similar concentrations had no such effect.An increase in the soluble amino nitrogen content of the chloroplasts was observed when NO₃ ^- or NO₂ ^- was reduced. There was, however, no increase in the incorporation of ¹⁴C from ¹⁴CO₂ into amino acids under these conditions. Even with the addition of ammonia the amount of ¹⁴C incorporated into the amino acids was not changed from less than 5% of the total ¹⁴C fixed. We conclude that while intact chloroplasts do have the ability to reduce both NO₃ ^- and NO₂ ^- at low rates, they do not synthesize appreciable amounts of amino acid directly, and this fact must be considered when formulating any pathways for nitrogen metabolism during photosynthesis.Supported in part by the National Research Council of Canada.