Endogenous NO 3 − in the Root as a Source of Substrate for Reduction in the Light

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...

Section: Discussionsupporting

confidence: 88%

Charge Balance in NO₃⁻-Fed Soybean

Touraine

Grignon²,

Grignon³

1988

“…Although soluble-'5N content was higher in the Na-treated plants, total soluble-N and nitrate-N content of the K-treated plants were about 126% and 182% of the Na-treated plants, respectively. Rufty et al (14) reported that "N was readily assimilated into insoluble macromolecules following 5NO3-reduction without staying in soluble reduced nitrogen in soybean plants. Accordingly, much more IN03-would be expected to remain in the K-treated plants than in the Na-treated plants.…”

Section: Resultsmentioning

confidence: 99%

Sodium Stimulates Growth of Amaranthus tricolor L. Plants through Enhanced Nitrate Assimilation

Ohta¹,

Yasuoka²,

Matoh³

et al. 1989

Effects of Na application on the capacity of N03-assimilation were studied in Na-deficient Amaranthus tricolor L. cv Tricolor plants. On day 30 after germination, Na-deficient A. tricolor plants received either 0.5 millimolar NaCI or KCI. The level of nitrate reductase activity doubled within 24 hours by the addition of Na and the enhanced level was maintained thereafter. When the plants were exposed to 2 millimolar 15N03-, total 15N taken up by the plants was greater in the Na-treated plants than in the Ktreated plants within 24 hours of the Na treatment. Incorporation of 15N into the 80% ethanol-insoluble nitrogen fraction of the Natreated plants in the light period was about 260% of those of the K-treated plants indicating greater capacity of N03-assimilation in the Na-treated plants. From these results, it was demonstrated that Na application to the Na-deficient A. tricolor plants promoted N03-reduction and its subsequent assimilation into protein, resulting in growth enhancement.As Na is required by C4 plants for their normal growth and not by C3 plants, Na is anticipated to be involved in C4 photosynthetic pathway (2). However, some C4 plants, such as corn and sugarcane have not been shown to require Na (6). Accordingly, it is probable that some metabolic reactions other than those of the C4 photosynthetic pathway occur which require Na. Recently, we reported that N03-uptake and induction ofNR2 activity in the Na-deficient Amaranthus tricolor plants were stimulated by Na (1 1, 12). In this paper, we present evidence that the enhanced level of NR activity by Na had significant effects on the growth of A. tricolor plants. MATERIALS AND METHODS Plant MaterialsSeedlings of Amaranthus tricolor L. cv Tricolor were cultured under Na-deficient conditions as described previously (8, 11). The basal culture solution (pH 6.0) prepared in distilled and deionized water contained 2 mM KNO3, 1 mM CaC2, 0.25 mm (NH4)2HP04 and 0.5 mM MgS04. 7H20. The composition of the micronutrients was that of Arnon's cited by Hewitt (5) except that all the iron was supplied as ferric citrate. The concentration of Na as an impurity in the culture 'Present Address;

“…Previous studies have revealed that a larger proportion of the NO3-taken up by roots is translocated to the shoot in light than in darkness (22,24,29). The observation raises the possibility that xylem transport of NO3-is highly sensitive to a carbohydrate limitation.…”

mentioning

confidence: 89%

“…Utilization of carbohydrate reserves, the shoot being the main source, was most rapid during this time, and root respiration and NO3-uptake maintained at relatively high rates. A third phase was distinguishable over the next 12 h ofdarkness (h [24][25][26][27][28][29][30][31][32][33][34][35][36] Fig. 3A and 3B).…”

Section: Tissue Analysismentioning

confidence: 99%

Effects of Altered Carbohydrate Availability on Whole-Plant Assimilation of ¹⁵NO₃⁻

Rufty¹,

MacKown²,

Volk³

1989

Self Cite

An experiment was conducted to investigate the relative changes in N03-assimilatory processes which occurred in response to decreasing carbohydrate availability. Young tobacco plants (Nicotiana tabacum [L.], cv NC 2326) growing in solution culture were exposed to 1.0 millimolar 15N03-for 6 hour intervals during a normal 12 hour light period and a subsequent period of darkness lasting 42 hours. Uptake of 15NO3-decreased to 71 to 83% of the uptake rate in the light during the initial 18 hours of darkness; uptake then decreased sharply over the next 12 hours of darkness to 11 to 17% of the light rate, coincident with depletion of tissue carbohydrate reserves and a marked decline in root respiration. Changes also occurred in endogenous 15NO3-assimilation processes, which were distinctly different than those in '5NO3-uptake. During the extended dark period, translocation of absorbed '5N out of the root to the shoot varied rhythmically. The adjustments were independent of 15NO3-uptake rate and carbohydrate status, but were reciprocally related to rhythmic adjustments in stomatal resistance and, presumably, water movement through the root system. Whole plant reduction of '5NO3-always was limited more than uptake. The assimilation of '5N into insoluble reduced-N in roots remained a constant proportion of uptake throughout, while assimilation in the shoot declined markedly in the first 18 hours of darkness before stabilizing at a low level. N03-uptake (6, 11, 25), and reduction (1, 27) observed in darkness. Accordingly, our experiment involved exposure of young tobacco plants to '5NO3-for 6 h intervals during a normal 12 h light period and during a following 42 h period of extended darkness. The approach allowed assessment of the relative sensitivities of NO3-assimilation activities in plants with widely varying carbohydrate availabilities.Of particular interest in this experiment was the regulation of NO3-transport from the root into the xylem. Previous studies have revealed that a larger proportion of the NO3-taken up by roots is translocated to the shoot in light than in darkness (22,24,29). The observation raises the possibility that xylem transport of NO3-is highly sensitive to a carbohydrate limitation. An alternative explanation is that the lower rate of xylem transport in darkness is closely associated with decreased water movement through the root and vascular system. Since stomatal opening and closure, and transpiration, continue to oscillate rhythmically in periods of extended darkness (13,16)