Nitrate Reduction by Roots of Soybean (<i>Glycine max</i> [L.] Merr.) Seedlings

Crafts‐Brandner, Steven J.; Harper, James E.

doi:10.1104/pp.69.6.1298

Cited by 47 publications

(37 citation statements)

References 14 publications

(13 reference statements)

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“…In N03-fed soybean plants, considerable nitrate reductase activity is found both in roots and leaves (6,16,23). Also, in this experimental system concentrations of NO3-and reduced-N in xylem exudate are nearly equal (27).…”

Section: Resultsmentioning

confidence: 99%

Assimilation of ¹⁵NO₃⁻ Taken Up by Plants in the Light and in the Dark

Rufty¹,

Israel²,

Volk³

1984

Plant Physiol.

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An experiment was conducted to determine the extent that NO3-taken up in the dark was assimilated and utilized differently by plants than NO3-taken up in the light. Vegetative, nonnodulated soybean plants (Glycine max L. Merrill, 'Ransom') were exposed to '5NO3-throughout light (9 hours) or dark (15 hours) phases of the photoperiod and then returned to solutions containing '4NO3-, with plants sampled subsequently at each light/dark transition over 3 days. The rates of '5N03-absorption were nearly equal in the light and dark (8A2 and 7.93 micromoles per hour, respectively) however, the whole-plant rate of '5N03-reduction during the dark uptake period (2.58 micromoles per hour) was 46% of that in the light (5.63 micromoles per hour). The lower rate of reduction in the dark was associated with both substantial retention of absorbed '5NO3-in roots and decreased efficiency of reduction of '5NO3-in the shoot. The rate of incorporation of '5N into the insoluble reduced-N fraction of roots in darkness (1.10 micromoles per hour) was somewhat greater than that in the light (0.92 micromoles per hour), despite the lower rate of whole-plant '5NO3-reduction in darkness.A hrge portion of the '5NO3-retained in the root in darkness was translocated and incorporated into insoluble reduced-N in the shoot in the following light period, at a rate which was similar to the rate of whole-plant reduction of 'I5NO3-acquired during the light period. Taking into account reduction of NO3-from all endogenous pools, it was apparent that plant reduction in a given light period (A13.21 micromoles per hour) exceeded considerably the rate of acquisition of exogenous NO3-(8.42 micromoles per hour) during that period. The primary source of substrate for NO3-reduction in the dark was exogenous NO3-being concurrently absorbed. In general, these data support the view that a relatively small portion (<20%) ofthe whole-plant reduction ofNO3-in the light occurred in the root system. Plants can assimilate NO3-during both the light and dark portions of the photoperiod, although rates of assimilation may differ. In experiments using '"NO3-and excised leaves or leaf disks, it has been amply demonstrated that N03 reduction can occur in darkness (8,11,24,32), and the rate of reduction is associated with availability ofcarbohydrate (2, 15). Nevertheless, it was found consistently that reduction rates were lower in darkness. Furthermore, activities of nitrate reductase in leaves ' The extent to which the rate of NO3-assimilation in intact plants is diminished during the dark phase of a 'normal' photoperiod has not been examined. The present experiment was conducted with that intent. Nonnodulated vegetative soybean plants were exposed to '5NO3 throughout light or dark periods to determine whole-plant NO3-assimilation during the period in which it was absorbed. In addition, following exposure to '5N03, sets of plants were returned to nutrient solutions containing '4NO3-. The subsequent partitioning of '5N among plant parts and the reduction ofendogenous ...

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Section: Resultsmentioning

confidence: 99%

Assimilation of ¹⁵NO₃⁻ Taken Up by Plants in the Light and in the Dark

Rufty¹,

Israel²,

Volk³

1984

Plant Physiol.

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“…In plants, green leaves are one the main sites of nitrate assimilation, which depends on the translocation of nitrate from the roots via xylem but roots can also make an important contribution, depending on the species (Smirnoff and Stewart, 1985). In soybean, the roots can account for as much as 30% of the total nitrate assimilation in the plant (Crafts-Brandner and Harper, 1982). However, under anaerobic conditions assimilation of nitrate in the roots may be seriously restricted.…”

Section: Discussionmentioning

confidence: 99%

Nitrate uptake and metabolism by roots of soybean plants under oxygen deficiency.

Brandão

Sodek

2009

Braz. J. Plant Physiol.

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Nitrate is reported to improve tolerance of plants towards oxygen deficiency imposed by waterlogging of the root system, but little is known of the mechanism underlying the phenomenon. We studied the metabolism of nitrate in roots under hypoxia, using soybean plants growing in a hydroponic system after suspending aeration and covering the surface of the nutrient solution with mineral oil. Nitrate depletion from the medium was greater under hypoxia than normoxia, but in the presence of chloramphenicol, consumption under hypoxia was significantly reduced. Nitrite accumulated in the medium under hypoxia and this was partially eliminated by chloramphenicol. Nitrate consumption sensitive to chloramphenicol was attributed to bacterial activity. Endogenous root nitrate was strongly reduced under hypoxia indicating mobilization. Although the transport of nitrate to the shoot via the xylem was also reduced under hypoxia, the severity of this reduction was dependent on the concentration of nitrate in the medium, suggesting that at least some of the nitrate in the xylem came from the medium. Root nitrate reductase was also strongly reduced under hypoxia, but recovered rapidly on return to normoxia. Overall, the data are consistent with two main metabolic fates for chloramphenicolinsensitive nitrate depletion under hypoxia: the reduction of some nitrate to nitrite (despite the reduced nitrate reductase activity) followed by its release to the medium (at least one-third of the nitrate consumed followed this route), and the transport of nitrate to the shoot. Nevertheless, it is highly unlikely that these metabolic routes account for all the nitrate consumed.Key words: Glycine max, hypoxia, nitrate reductase, nitrite, waterlogging. absorção e metabolismo do nitrato em raízes de soja sob deficiência de oxigênio. É conhecido que o nitrato melhora a tolerância de plantas à deficiência de oxigênio provocada pela inundação do sistema radicular. Porém, pouco se sabe sobre as causas deste fenômeno. Realizamos um estudo do metabolismo do nitrato em raízes de soja sob hipóxia, empregando plantas cultivadas em sistema hidropônico, após suspender a aeração e cobrir a superfície da solução nutritiva com óleo mineral. O consumo de nitrato no meio foi maior sob hipóxia do que normóxia. Porém, na presença de cloranfenicol o consumo em hipóxia foi reduzido de forma significativa. Houve acúmulo do nitrito no meio sob hipóxia e este acúmulo foi parcialmente eliminado por cloranfenicol. A redução do consumo de nitrato na presença de cloranfenicol foi atribuída à atividade bacteriana. O nitrato endógeno da raiz foi bastante reduzido sob hipóxia, indicando mobilização do mesmo. Apesar do transporte do nitrato até a parte aérea, via xilema, ter sido também reduzido sob hipóxia, a intensidade dessa redução foi dependente da concentração de nitrato no meio, o que indica que pelo menos uma porção do nitrato encontrado no xilema teve origem no meio. A atividade da redutase do nitrato da raiz também foi fortemente reduzida sob hipóxia, porém rec...

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Section: Introductionmentioning

confidence: 99%

“…The pH of fresh solutions was 5.5. The N03-medium contained 2 mm KNO3, 1 mm Ca(NO3)2, 1 mM MgSO4, 1 mM KH2PO4, 0.1 mM 50 ,uM KCI,30 ,M H3BO3,5 ,uM MnSO4,1 M ZnSO4, 1I M CuSO4 and 0.1 M (NH4)6Mo7024.In the urea medium KNO3 and Ca(NO3)2 were replaced by 2 mm urea, 1 mm K2SO4, and 1 mM CaCl2. Solutions were renewed every 3 or 4 d, in order to minimize pH shift (less than 0.4 pH unit in urea medium, and less than 0.3 pH unit in NO3-medium) and nutrient depletion (maximum decrease of NO3-concentration: 15%).…”

mentioning

confidence: 99%

Charge Balance in NO₃⁻-Fed Soybean

Touraine

Grignon²,

Grignon³

1988

Plant Physiol.

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Soybeans (Glycine max L. Merr., cv Kingsoy) were grown on media containing N03-or urea. The enrichments of shoots in K@, N03-, and total reduced N (Nr), relative to that in Ca2", were compared to the ratios K+/Ca2+,NO3j/Ca2+, and Nr/Ca2+ in the xylem saps, to estimate the cycling of K+, and Nr. The net production of carboxylates (R-) was estimated from the difference between the sums of the main cations and inorganic anions. The estimate for shoots was compared to the theoretical production of R-associated with N03-assimilation in these organs, and the difference was attributed to export of R-to roots. The net exchange rates of H+ and OH-between the medium and roots were monitored. The shoots were the site of more than 90% of total N03-reduction, and Nr was cycling through the plants at a high rate. Alkalinization of the medium by N03-fed plants was interrupted by stem girdling, and not restored by glucose addition to the medium. It was concluded that the majority of the base excreted in N03-medium originated from Rproduced in the shoots, and transported to the roots together with K+.As expected, cycling of K+ and reduced N was favoured by N03-nutrition as compared to urea nutrition. Plant roots absorb ions at different rates, so that cation and anion uptakes may not be in equilibrium. (12).Nitrogen contents of plant tissues are greater than those of other mineral elements. This implies that the ionic imbalance of absorption depends on whether N is present in the medium as a cation or as an anion (23). OH-generated by reduction of N03-in the roots may be released in the medium. This kind of cytoplasmic detoxification is not possible in shoots, due to the small volume ofleafapoplast, and to the limited ability ofphloem to transport OH-and HCO3-(1, 22). OH-generated in aerial organs is converted in ionized carboxylic groups, but the calculated accumulation of organic anions is often smaller than the NO3 reduction in shoots. A model was proposed by Ben Zioni and co-workers (4) to explain this discrepancy. According to this model, malate synthesized in the leaves is loaded in the phloem and transported together with K+ into the roots where it is decarboxylated. The HCO3-produced is exchanged for NO3-, which is transported into the shoots together with K+. Thus, the transport of K+ carboxylates by phloem, and K+ cycling are means by which NO3-reduction in shoots controls NO3-uptake by roots.The validity of the Ben Zioni model has been questioned for barley (6), corn (16), tomato (17), and inoculated soybean (cv Ransom) (15), which were reported to accumulate carboxylates in amounts approximately equivalent to N03 reduction in shoots. Castor oil plants excrete more than 50% of the OH-they produced by assimilating N and S (1, 18, 29). In these plants, shoots were responsible for approximately 40 to 60% NO3-assimilation and they returned to the medium approximately 15 to 20% (29) to 60% (1) of the OH-they produced.In these experiments, the majority of excreted OH-originated from roots. Evidence for the cy...

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Nitrate Reduction by Roots of Soybean (Glycine max [L.] Merr.) Seedlings

Cited by 47 publications

References 14 publications

Assimilation of ¹⁵NO₃⁻ Taken Up by Plants in the Light and in the Dark

Assimilation of ¹⁵NO₃⁻ Taken Up by Plants in the Light and in the Dark

Nitrate uptake and metabolism by roots of soybean plants under oxygen deficiency.

Charge Balance in NO₃⁻-Fed Soybean

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Nitrate Reduction by Roots of Soybean (Glycine max [L.] Merr.) Seedlings

Cited by 47 publications

References 14 publications

Assimilation of 15NO3− Taken Up by Plants in the Light and in the Dark

Assimilation of 15NO3− Taken Up by Plants in the Light and in the Dark

Nitrate uptake and metabolism by roots of soybean plants under oxygen deficiency.

Charge Balance in NO3−-Fed Soybean

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Assimilation of ¹⁵NO₃⁻ Taken Up by Plants in the Light and in the Dark

Assimilation of ¹⁵NO₃⁻ Taken Up by Plants in the Light and in the Dark

Charge Balance in NO₃⁻-Fed Soybean