Nitrate Translocation by Detopped Corn Seedlings

Abstract

Five-day-old seedlings of corn (Zea mays L.) grown without nitrate were decapitated and exposed to 0.5 mM KNOs or 0.5 mM KCI in aerated solutions at 30 C. Uptake of nitrate, chloride, and potassium was determined by replacing solutions hourly and measuring their depletion. Translocation of these ions and of organic nitrogen was determined by hourly analysis of the vascular exudate. Nitrate reduetion was estimated by the difference between nitrate uptake and nitrate recovered in the tissue and exudate. Nitrate uptake exhibited its usual pattern of apparent induction resulting in the development of an accelerated uptake phase. Chloride uptake remained fairly constant throughout the experimental period. Translocation of nitrate increased progressively for at least 7 hours whereas chloride translocation reached a maximum about the 3d hour and then declined to a lower rate than nitrate translocation. Nitrate uptake and translocation were restricted by anaerobiosis, by 20 and 40 C relative to 30 C, and by 0.05 mM 6-methylpurine, an RNA-synthesis inhibitor. Accumulation, reduction and translocation of nitrate had different sensitivities to all these factors. (17,18). Data are presented to support the concept that continuous nitrate uptake is required to maintain significant translocation of nitrate. In addition, the translocation of organic nitrogen was strongly dependent upon continuous nitrate uptake. MATERIALS AND METHODSThe procedures were, in general, similar to those described by Jackson et al. (17). All experiments were conducted with corn (Zea mays L.) hybrid DeKalb XL45. Four days after germination in darkness with 0.1 mm CaSO, the secondary roots were excised and five seedlings were assembled in a Plexiglas holder especially designed to fit the top of a 75-ml Taylor tube. Seedlings were then placed in a pretreatment solution for 24 to 36 hr in darkness prior to initiating the experiment. The pretreatment solutions contained per liter 0.25 mM (NH)2SO,, 0.6 mmY K2SO4, 0.4 mM KH2PO4, 1.6 mM MgSO4, 0.8 mm CaSO4, and 250 mg CaCO,. Iron was supplied as Fe-EDTA at 1.8 mg per liter; the remaining five micronutrients were present at one-fifth the concentration of Hoagland's solution (13).The standard uptake solution contained 0.5 mM KNO or 0.5 mM KCl plus 0.25 mm CaSO4, pH 6. For the experiments involving temperature, 6-methylpurine, or anaerobiosis variables, the uptake solutions also contained 1 mm Na 2-[Nmorpholino]ethanesulfonate as a buffer and the bactericide chloramphenicol at 20 jtg mP. Except where indicated, solutions were continuously aerated and the temperature was maintained at 30 C. Solutions were changed hourly and nitrate, chloride, and potassium uptake determined from depletion of the solutions. Five plants per 70 ml were employed for each replication. Four of these five-seedling units were used as replicates for each treatment and the data presented are the averages of the four replicates.Immediately after placing the plants in the uptake solutions, the mesocotyls were excised below the firs...

Nitrate Reductase Activity in Maize (Zea mays L.) Leaves

“…(8) found that NRA increased while nitrate was present in the exogenous medium, but fell as soon as the nitrate was removed, even though total cellular nitrate levels were relatively constant. Similar relationships between exogenous nitrate supply and NRA have also been demonstrated in excised maize leaves (21), terminal buds of peas (10), roots (2,6,15,18), and cotton ovules (17).…”

Differential Inhibition by Ferulic Acid of Nitrate and Ammonium Uptake in Zea mays L.

“…The next experiment was conducted with and without 250 and 500 gM FA, the concentrations at which maximal inhibitions were approached for NO3-uptake and NH4+ uptake, respectively. (5,15,19,20). During the first hour, altered appreciably (Fig.…”

Nitrate induction of root hydraulic conductivity in maize is not correlated with aquaporin expression

“…Although variation in transpiration rate and stomatal conductance in response to nitrate availability has been observed in a number of crop species (maize, tomato, cotton, barley), this response was mediated by adjustments in root hydraulic properties rather then via direct regulation of stomatal conductivity (Chapin et al 1988;Radin 1990;Radin and Matthews 1989). The sensitivity of root hydraulic resistance to nitrate concentrations Carvajal et al 1996;Ezeta and Jackson 1975;Gloser et al 2007;Horau et al 1996), provides the basis for an alternative scenario in which water Xow is enhanced only to roots exposed to high nitrate levels (Gorska et al 2008). An important outcome of such a local response is that it will magnify the positive eVect of the altered root resistance on total nitrate uptake by prioritizing water uptake from roots exposed to nutrient rich patches.…”