Stimulation of Cadmium Uptake in Relation to the Cadium

The time course ofcadmim uptake by the roots of intact tomato plats (Lycoperskba escuaetum MDL) was measured In a nutrient solutin with a micromolar cadmium concentration untfl all cadmium In the medim was exhausted. Exhaustion taking a few hours, cadmium was repeatedl added to the nutrient solution. Tle initial rate of cadmium uptake was computed for each cadmium addition. This rate sharply increased and ultimately leveled off, the maximum vaue being about three times higber than the value measured after the first cadmium addition. The stimulating effect of cadmium was associated with an inhibitory effect at higher levels of cadmium concentrations. An increase in the net cadmium influx with time could not be explained by the inding of heavy metal to a fixed number of organic comnds. Conceivably, the production of binding sites could be increased and cadmium might play a part in controlling the rate of sites production. Nonessential elements are generally present in plant tissues at very low concentrations. Up to now, no investigation seems to have been done concerning the influence ofincreasing the quantity of nonessential elements in plant tissues on the influx of these elements into cells. This may become of practical interest in view of the increasing level of some. heavy metals, such as Cd, in the environment (3). Cd has been observed to be largely bound nonmetabolically to organic compounds of cell walls (1). This process, including both exchange adsorption and irreversible sequestering in combination with diffusion, could largely account for the uptake of cadmium by excised root tissues (1). Cd is also known to be either an inhibitor or an activator of enzymes such as nitrate reductase or the ion-stimulated ATPase (6, 7, 17), both of which are assumed to be closely associated with ion uptake (4, 8). The purpose of this work was to investigate the effect of Cd absorption by roots of intact plants on its subsequent rate of uptake with reference to this inhibitor-activator role. 1 Publication No. 1489 of the Biology-Radiation Protection Programme D. G. XII of the Commission of the European Communities. 2"ASpirant de recherche du Fonds National Belge de la Recherche Scientifique" at the time of the research. 3 Mailing address: Plant Physiology Department, Faculty of Sciences, Catholic University of Louvain, 1348 Louvain-la-Neuve, Belgium. MATERIALS AND METHODS Tomato seeds, Lycopersicon esculentum Mill., cv. Moneymaker, were germinated for 5 days in darkness at 28 C on wet filter paper. The seedlings were then grown in a climate-controlled growth chamber on a nutrient solution which was renewed every week. The composition of the nutrient solution, prepared with deionized water and chemically pure reagents, was in mm: 6 KNO3, 5 Ca(NO3)2 * 4H20, 2 MgSO4 7H20, 1 KH2PO4, 9.1 x 10-3 MnSO4 H20, 7.6 x 10ZnSO4.7H20, 3.1 x l0-4 CuSO4 -5H20, 7.0 x l0-4 MoO3*H20, 4.6 x 10-2 H3BO3, 8.2 x 10-2 NaFe (EDTA). At the time of the experiment, the plants were 7 weeks old, had flower buds, and the initial growth rate had decreased considerably. Cd uptake was measured by its depletion in the solution through the use ofan apparatus especially built for kinetic studies on intact plants (18). Three plants were fixed on a perforated black Lucite disc separating the two compartments of the apparatus. The first compartment contained the aerial part of the plants and temperature, humidity, and light intensity were kept at 24 C ± 1, 50%o ± 10; and 13,000 lux ± 200, respectively. The roots were in the bottom compartment which was equipped with automatic regulation and continuous monitoring apparatus. The nutrient solution, devoid of micronutrients, was kept at a constant temperature (23 C ± 0.5), pH (maintained at 5 ± 0.5 by the addition ofHN03), and volume (maintained at 4.25 liters by water addition). Water was injected into the root compartment from a graduated container from which the consumption was read. The top layer of the nutrient solution activated an electrical contact starting a peristaltic water pump. The solution was homogenized by an electromagnetic 50-Hz vibrator. Aeration of the medium was controlled, and the whole set-up made quick renewal of the solution possible. The [ll5mCd] (Radiochemical Centre of Amersham, U.K., code CAS I) was used as Cd tracer and was measured by continuous P8 counting. Whenever supplementary stable Cd had to be added it was in the form of Cd(NO3)2. To reduce Cd adsorption to inert material all surfaces of the experimental set-up (except the pH electrode and the mylar membrane in front of the detector) were made of PTFE.4 No long term adsorption was detected at pH 5 with 0.1 LM Cd. The plants used in the experiments were acclimatized during 24 hr to the conditions ruling within the apparatus; only then were Cd injections in the root compartment started. The sequence of Cd treatments is specified below. After each absorption period, 'Abbreviation: PTFE: polytetrafluorethylene (Teflon). 554 www.plantphysiol.org on June 29, 2017 Published by Downlo ded from Copyright © 1978 American Society of Plant Biologists. All rights reserved. STIMULATION OF CADMIUM UPTAKE the old solution was completely removed and the roots were washed for 5 min with a Cd-free solution before the new solution was applied. RESULTS AND DISCUSSION In general, after adsorption in the apparent free space, the depletion curve is a linear function for a short period of time (2). The correlation coefficient computed from a least square linear regression during the first 3 hr was higher than 0.98 with P < 0.001. The rate of absorption at the initial concentration could be satisfactorily estimated from the slope of the straight line. Cutler and Rains (1), who used excised root tissue of barley, found an intermediary nonlinear phase for Cd uptake which lasted about 30 min, where nonmetabolic sequestering predominated. Such a nonlinear phase was not observed in the present measurements. Plants absorbed all of the Cd in the nutrient solution at initial Cd concentrations ranging from 0.1 to 2.8 ,lM. These same plants required much less time to exhaust Cd in the medium during subsequent runs of Cd absorption. In Figure 1, at an initial Cd concentration of 2.8 ,UM, the first reduction of 97% of the available Cd took about 24 hr. During a second run, 42 hr later, the same plants absorbed a similar percentage in only 8 hr. At a lower initial concentration of 0.1 lMm Cd, Cd exhaustion took much less time, which permitted many more runs during a similar period of time. The stimulation of the absorption is obvious from Figure 2 where the Cd influx of two replicates is expressed by the initial rate of uptake. From run to run this increase followed a pattern similar to an S-curve as the increase in Cd influx leveled off after the sixth run. The existence of maximum Cd influx was clearly shown in experiments with a 1 uM initial Cd concentration, where the maximum was reached after only two complete runs (Fig. 3a). Such an increase of uptake occurring within a few hr cannot be explained by concurrent growth and enlargement of the root surface. This is indirectly confirmed by the difference between the