In Vivo Nitrate Reduction in Roots and Shoots of Barley ( Hordeum vulgare L.) Seedlings in Light and Darkness

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 ...

Section: Resultsmentioning

confidence: 99%

“…Results from experiments with root tissues generally indicate that NO3-reduction in roots does not decrease when plants are in darkness (2,25), although a substantial decline in root nitrate reductase activity was noted in one instance (7).…”

mentioning

confidence: 99%

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

Rufty¹,

Israel²,

Volk³

1984

“…Calcium and phosphate absorption also was relatively unaffected by excision (7,13). In contrast, excised barley roots absorbed less NO3 (1) and excised barley and corn roots developed smaller pH gradients (9,12) than intact roots. These results suggest that the effects of excision may be ion specific.…”

mentioning

confidence: 92%

Root Excision Decreases Nutrient Absorption and Gas Fluxes

Bloom

Caldwell²

1988

The roots of barley plants (Hordeum vulgare L. cv Steptoe) were monitored before and after excision for net uptake of carbon dioxide, oxygen, ammonium, potassium, nitrate, and chloride and for their content of sucrose, glucose, fructose, and malic acid. All fluxes began to attenuate within 2 hours after excision. Net potassium uptake returned to control levels 6 hours after excision, but carbon dioxide, oxygen, ammonium, and nitrate fluxes continued to diminish for the remainder of the observation period. The addition of 0.1 molar glucose or 0.1 molar sucrose to excision medium had no significant effect on these changes in ion and gas fluxes. Net chloride uptake was negligible for all treatments. Sugar and malic acid content of the root declined after excision. Sucrose and glucose levels remained depressed for the entire observation period, whereas fructose and malic acid returned to control levels after 9 hours. These results indicate that excision has profound, adverse effects on root respiration and the absorption of mineral nitrogen.Although excision ofplant tissue often produces severe changes in metabolic processes (23), many studies of nutrient absorption have been based on data obtained from excised roots. Use of excised roots wasjustified in some cases. For example, differences in 86Rb+ influx between excised and intact barley or corn roots became insignificant after 2 to 4 h of washing (11,12,15). Calcium and phosphate absorption also was relatively unaffected by excision (7, 13). In contrast, excised barley roots absorbed less NO3 (1) and excised barley and corn roots developed smaller pH gradients (9, 12) than intact roots. These results suggest that the effects of excision may be ion specific.Our research has compared absorption of NH4' and N03 (2, 3, 5) and determined the influence of different nutrient solutions upon respiration (4) Ion and Gas Fluxes. About 2 weeks after germination, when the third leaf had just emerged and the roots were over 15 cm in length, a plant was transferred to a measurement system. Teflon tape was wrapped around the root about 1 cm below the endosperm. A slotted rubber stopper was placed around the wrapped section of root, and the remaining 12-cm portion of root was placed into a Plexiglas/stainless steel cuvette (3). On top of the cuvette, a bath 3 cm deep was fabricated from putty and filled with nutrient solution or nutrient solution plus 0.1 M sucrose or 0.1 M glucose; the upper 1 cm of root sat in this bath, and the shoot was held erect above it. To permit recovery from any transplant shock, this plant was kept for at least 8 h in the dark and 3 h in the light at 500 ,umol m-2s-' PAR before experimental data were taken. The temperature of the root cuvette was 20Cand that of the shoot was 25°C.Ion-selective electrodes simultaneously and continuously monitored depletion or addition of CO2, NH4', K+, and NO3-, and a miniature polarographic electrode monitored depletion of 02 from a nutrient solution flowing through the root cuvette (3,4). This solution initia...

“…The shoot and root were maintained at 25 and 20°C, respectively. These conditions differed from the growth conditions; the light level was higher and root temperature lower during the experiment to ensure that NO3-absorption and assimilation would not be carbohydrate-limited (1,4 To examine NO3-induction of NO3-absorption, there were seven treatments: (a) control plants received NO3-during growth and during the experiment; (b) nitrogen-deprived plants had no exposure to nitrogen until they received NO3-at the start of the experiment; (c) nitrogen-deprived and disturbed plants also had no previous exposure to nitrogen and, at the start of the experiment, were manipulated as described below and began to receive NO3-; (d) NH4+-tested plants received NH4' during growth and both NH4' and N03-during the experiment; (e) NH4+-grown plants received NH4' during growth and only NO3-during the experiment; () NH4+-grown and disturbed plants received NH4' during growth and, at the start of the experiment, were manipulated as described below and began to receive only NO3-; and (g) plants designated as 'NH4+-l0 h' were grown without nitrogen in the medium, were exposed to NH4' for 10 h before the experiment, and received only NO3-during the experiment.The two 'disturbed' treatments simulated the transplant shock that occurs when plants are physically transferred from one solution to another. At the start of the experiment, these plants were gently removed from the root cuvette, shaken lightly one time to remove some solution from the roots, and then immediately reinserted into the cuvette.…”

mentioning

confidence: 99%

Effects of Exposure to Ammonium and Transplant Shock upon the Induction of Nitrate Absorption

Bloom

Sukrapanna²

1990

In barley (Hordeum vulgare L. cv Steptoe) seedlings, the time course for induction of root nitrate absorption varied significantly with pretreatment. Net nitrate uptake of nitrogen-deprived plants more than doubled during the 12 hours after first exposure to nitrate. For these plants, gentle physical disturbance of the roots inhibited net nitrate absorption for more than 6 hours and potassium absorption for 2 hours. Pretreatment with ammonium appeared sufficient to induce nitrate absorption; plants either grown for 2 weeks on or exposed for only 10 hours to a medium containing ammonium as a sole nitrogen source showed high rates of net nitrate uptake when first shifted to a medium containing nitrate. Gentle physical manipulation of these plants inhibited nitrate absorption for 2 hours and potassium absorption for more than 12 hours. These results indicate (a) that experimental protocols should avoid physical manipulation of the roots whenever possible and (b) that ammonium or a product of ammonium assimilation can induce nitrate absorption.The presence of NO3-in the rhizosphere induces root absorption of NO3-(10, 11, 14, for reviews). For example, the barley cultivar Steptoe (Hordeum vulgare L.) showed a sixfold increase in the rate of net NO3-uptake during the 12 h after first exposure to ical manipulation diminishes calcium absorption (23), phosphate absorption and energy charge (13), or root pressure and exudation (20). To minimize transplant shock, we placed the roots of an intact plant in a cuvette at least 10 h before measurements began and shifted from one medium to another by changing the solution that flowed through the cuvette (3). Most studies of NO3-induction of NO3-absorption have initiated the N03--induction period by physically transferring plants from one solution to another, a procedure that might produce transplant shock. In this study, we examined whether transplant shock might influence the induction of NO3-absorption.The absorption of counterions such as K+ and NH4' affect the induction of NO3-absorption (18, 24). For example, absorption of K+ moderated the inhibitory effects of NH4' on this process (24). We thus monitored K+ and NO3-absorption concurrently during the following experiments. MATERIALS AND METHODSBarley seeds (Hordeum vulgare L. cv Steptoe) were surfaced sterilized for 1 min in 2.6% NaClO, thoroughly washed with distilled water, and germinated on wet toweling. Six 3-d-old seedlings were suspended above light-tight root boxes holding 2 L of an aerated nutrient solution. The solution consisted of 2.5 mm CaSO4, 10 mm KH2PO4, 5.0 mM MgSO4, micronutrients and iron as described previously (3), and contained either 0.6 mM NH4NO3 for the NH4' and NO3-absorption kinetic experiments, 0.6 mm KNO3 for the control treatment, or 0.6 mM NH4H2PO4 for the treatments designated as NH4+-grown and NH4+-tested. The pH of the solution was adjusted to 6.0 with H2SO4 or KOH. Each day, the root box was replaced with one that had been sterilized with isopropyl alcohol and filled with fresh nu...