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

Abstract

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...

A stay‐green mutation of Lolium perenne affects NO3− uptake and translocation of N during prolonged N starvation

“…None of the NO.,' depletion timecourses in either normal or stay-green plants exhibited lag-phases associated with induction of the transport system (data not shown), nor was there preliminary depression in net uptake of NO3" characteristic of 'transplant shock' in response to physical disturbance (Bloom & Sukrapanna, 1990). The absence of 'transplant shock' was expected, given that the plants were grown in flowing nutrient solutions and that their root systems were therefore acclimated to bulk disturbance.…”

“…The estimation of kinetic parameters for nutrient uptake from continuous nutrient depletion experiments is open to criticisms on various grounds, including the validity of interpretations based on 'net' uptake data, the effects of physical disturbance on plants, the accuracy of measurements of very low concentrations, errors in model fitting and the interpretation of isotherms in terms of multiphases (Borstlap, 1981;Clarkson, 1986;Bloom & Sukrapanna, 1990), In the present study no attempt was made to resolve the data into more than a single phase covering the concentration range of < 1-250//.M NO3", Further, the estimated kinetic parameters were interpreted with caution, particularly since the nature of the depletion technique meant that the concentration of NO,," in the For senescent leaves there was a significant effect of genotype (P<0-01) and day (P< 0-001), and a significant interaction between genotype and day {P < 0-01).…”

Energy requirements for maintenance of ion concentrations in roots

“…Possible reasons for this high value are: (a) all anions are included in efflux, (b) nutrient supply was high during growth, (c) some organic ions may have been present, or (d) perturbation effects lasted for more than 30 min, but do not occur in labelling studies. Treatments can influence membrane potential and thus ion transport (Rincon and Hanson 1986), ion uptake (Bloom and , Sukrapanna 1990) and root pressure (Miller 1987). The labelling studies in the literature show a large variation in ion efflux from roots (Tab.…”

Measuring Nitrogen and Phosphorus Uptake by Intact Roots of Mature Acer saccharum Marsh., Pinus resinosa Ait., and Picea abies (L.) Karst

“…This generally involves gently excavating and washing roots (BassiriRad et al, 1999;Gessler et al, 1998Gessler et al, , 2002Rennenberg et al, 1996). The excavation processes is of particular concern as roots are highly sensitive to disturbance (Aslam et al, 1996;Bloom and Sukrapanna, 1990;Rincon and Hanson, 1986) and excavation visibly disturbs roots and severs mycorrhizal hyphae, likely altering uptake rates.…”