JXB Advance Access originally published online on September 10, 2004
Journal of Experimental Botany 2004 55(408):2571-2579; doi:10.1093/jxb/erh255
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RESEARCH PAPER |
Differential accumulation of Cd in durum wheat cultivars: uptake and retranslocation as sources of variation
Department of Land Resource Science, University of Guelph, Guelph, Ontario, Canada N1G 2W1
* To whom correspondence should be addressed. Fax: +1 519 824 5730. E-mail: bhale{at}uoguelph.ca
Received 4 March 2004; Accepted 15 July 2004
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Abstract |
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Durum wheat (Triticum turgidum L. var. durum) accumulates Cd from the soil depending on various factors. When grown in hydroponic solution containing Cd (20 µg l–1), roots had higher tissue Cd concentrations than shoots or heads. Kyle (the higher grain-Cd accumulating cultivar) had lower root-Cd, and greater shoot-Cd and head-Cd concentrations than Arcola (the lower grain-Cd accumulating cultivar). These cultivar differences were greater at flowering and ripening than at tillering. Much of the root-Cd was lost between the flowering and ripening stages of development. Distribution of 106Cd among plant parts, after a single 24 h feeding, demonstrated that root-to-shoot transfer of Cd in Arcola was similar to that of Kyle at tillering, but it had ceased at flowering in Arcola but not Kyle. None of the Cd in wheat heads at ripening originated from 106Cd exposure in the previous 24 h, suggesting that grain-Cd is a function of total shoot accumulation. Both cultivars demonstrated basipetal translocation of Cd; Arcola at tillering translocated more Cd from shoots to roots than Kyle. The proportion of Cd2+/Cdtotal in the nutrient solution decreased with time, suggesting that plant activity altered the solution chemistry. The alteration probably resulted from either preferential depletion of solution Cd2+ and/or addition of root exudates. Lower grain-Cd accumulation in Arcola possibly resulted from a combination of reduced root-to-shoot transfer of Cd at flowering, as well as enhanced shoot-to-root retranslocation of Cd, at least in younger plants. Plant-mediated changes in solution-Cd speciation did not play a role.
Key words: Accumulation, cadmium, durum wheat, translocation
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Introduction |
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Canadian durum wheat is routinely grown on prairie soils that contain up to 3.8 mg Cd kg–1 with a mean of 0.28 mg Cd kg–1 (Garrett et al., 1998
). Although these concentrations are not linked to phytotoxic effects in wheat, cadmium is accumulated by the plants that results in Canadian-grown durum wheat having grain-cadmium concentrations in the range of 0.10–0.50 mg kg–1, depending on the cultivar (Garrett et al., 1998). Plant accumulation of metals from soils depends both on the bioavailability of the metal and on physiological processes. Soil factors such as cation exchange capacity, organic matter content, concentrations of other cations, and pH influence the proportion of a soil metal which is bioavailable. The ionic form of the metal is usually considered to be the bioavailable form, although ligand-enhanced uptake of Cd has been demonstrated (Berkelaar and Hale, 2003a, b). The concentration of accumulated Cd is usually highest in the roots, followed by shoots and the reproductive structures (Van Bruwaene et al., 1984). Recent studies of Thlaspi caerulescens have demonstrated the presence of a highly selective, high affinity Cd transporter in roots of the hyperaccumulating ecotype (Zhao et al., 2002). Root-to-shoot translocation has been associated with phytochelatin induction (Guo and Marschner, 1996; Larsson et al., 2002) and linked (Salt and Rauser, 1995) or not linked (Florijn and van Beusichem, 1993) to transpiration. Basipetal translocation of rare earth metals has been demonstrated following foliar application (Wang et al., 2001); acropetal translocation of Cd introduced to the phloem has been noted for detached wheat shoots (Herren and Feller, 1997), and whole plants (Harris and Taylor, 2001; Cakmak et al., 2000a, b). Most of what is known about the uptake and translocation of Cd in durum wheat has been determined for young plants; relatively less is known about how cultivar-specific uptake and translocation processes change with the maturity of the plants. Two cultivars of Triticum turgidum L. var. durum with known high (Kyle) and low (Arcola) Cd accumulation in the grain were used (i) to determine whether differences between Kyle and Arcola accumulation patterns for Cd remained consistent throughout the life cycle; (ii) to identify whether Kyle and Arcola varied in their translocation of Cd from root to shoot and vice-versa; and, (iii) to explore whether differential root accumulation of Cd was related to speciation of Cd in the hydroponic culture, specifically whether plant growth influenced speciation differentially between cultivars.
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Materials and methods |
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Plant culture and Cd exposure
A semi-automated continuous flow, recirculating hydroponic system was configured so that each cultivar had a separate and equal supply of macro and micro nutrients; pH of the nutrient solution was maintained at 6.0±0.2 using dilute HNO3 or KOH added automatically by feedback control. Each experimental unit (four wheat plants) was grown in a 3.0 l opaque pot with inflow and outflow ports for the circulation of the nutrient solution, maintained steadily at 4.0 l h–1. The solution for each cultivar and Cd exposure concentration was drained and aerated in the corresponding storage tank. Half-strength modified Hoagland's solution was used for the first four weeks of growth; thereafter, 75% strength Hoagland's was used (Hoagland and Arnon, 1950
). Each storage reservoir was renewed every other day with 20 l of fresh solution including the appropriate Cd concentration throughout the life cycle of the plants, in order to replace nutrients removed by the plants; every 2 weeks, the nutrient solution was completely replaced with fresh solution to remove waste products and to re-establish the initial nutrient and cadmium concentrations. The grain caryopses were germinated in glass Petri dishes lined with moist filter paper, in the dark at 23 °C. After the radicle had emerged from the grain (3–4 d), the germinated seeds were placed in 3.5x3.5x4.0 cm rockwool plugs (Grodan, Denmark) moistened with deionized water. Once the first leaf emerged and the roots grew through the bottom of the rockwool, they were transferred to the hydroponic system. Four rockwool plugs, each covered with reflective plastic to discourage algal growth, were transferred to a styrofoam support tray, for floatation on the aerated nutrient solution in the 3.0 l pot. There were seven pots per row, with two rows per treatment for each cultivar. Two horizontal wire grids, held up by four supporting beams attached to the greenhouse bench allowed the wheat to grow to mature height without lodging. During the summer months, the relative humidity ranged from 70–80% while in the autumn, the range was 50–70%. The day length was minimally 16 h, using high pressure sodium lights (430 W) in the greenhouse as supplementary light. At early stages of growth, the lights provided 50–70 µmol m–2 s–1 of photosynthetically active radiation (PAR) at the canopy. At the later stages of development, the canopy received 100 µmol m–2 s–1 of PAR. For the experiment on distribution of Cd among plant parts (Experiment 1), plants were continuously supplied with either 5 µg l–1 or 20 µg l–1 elemental Cd. In the separate stable isotope labelling study (Experiment 2), the plants were only grown in 20 µg l–1 elemental Cd, which was replaced at each life stage (separate plants for each life stage), for 24 h, by either 0 or 20 µg l–1 106Cd in solution, which also contained the same concentrations of KNO3, MgSO4 and Ca(NO3)2.4H2O as in the solution with elemental Cd. The plants were exposed to 106Cd in pots detached from the recirculating system. For the foliar spray experiment (Experiment 3), pots of each cultivar which had not previously been exposed to Cd were removed from the hydroponic culture system and the foliage sprayed with 7.5 ml of Cd solution (500 or 5000 µg Cd l–1, as Cd(NO3)2.4H2O in NANOpure water with Tween 80, pH 5.5) daily in the evening, for three consecutive days; after each spray, the plants were returned to the recirculating system. Plants were harvested on the fifth day after the first spray. At the tillering stage, all the foliage was sprayed; at the in-boot and flowering stages, the bottom half of the foliage was sprayed; at all stages, roots were covered, and at the later two developmental stages, upper foliage was covered to prevent contamination. For the study of solution Cd2+/Cdtotal (Experiment 4), plants were grown in 5.0 or 50 µg Cd l–1; every 4 d, the reservoirs were sampled prior to the replacement of 20 l of Hoagland's solution with fresh solution to replace the nutrients and Cd taken up by the plants.
The experiment to characterize elemental Cd distribution among plant parts relative to life stage and cultivar was repeated twice and there were two replicate experimental units (eight plants) for each life stage and cultivar within each repeat. The solution Cd2+/Cdtotal study was conducted once, with two replicate experimental units per external Cd concentration. The isotope labelling and foliar spray studies were each conducted once, with two replicate experimental units (eight plants) for each life stage and cultivar. Each of these four experimental runs required approximately 125 d to complete the wheat life cycle.
Plant sampling and tissue analysis
Roots and shoots were collected at tillering, in-boot, flowering, and ripening stages of development; heads were collected at the latter two growth stages only, and grain at ripening only. The experimental units were randomly chosen from within the cultivar populations; the time of sampling was based on visual inspection of the plant developmental stage, rather than calendar days as the cultivars matured at different rates. The roots were rinsed and soaked in deionized water for 5 min to remove surface solution. The plant tissue samples were placed in paper bags and dried at 80 °C for 72 h. The desiccated plant tissue was finely ground and digested using nitric acid in preparation for Cd analysis (Topper and Kotuby-Amacher, 1990).
The roots were not desorbed of surface-bound Cd, as it has been demonstrated that Ca and Pb washes, ranging in duration from 20–60 min, did not reduce the Cd concentration of roots of Agrostis or maize (Rauser, 1987). This study concluded that only 10–15% and 4–7% of the Cd in the roots was associated with the cell wall in Agrostis and maize, respectively, and suggests that a similarly small fraction of the root Cd is associated with the cytoplasm, with the majority of the Cd in the roots actually occurring in vacuoles. Hart et al. (1998) exposed roots to 70 nM and 125 nM 109Cd for 20 min, and then desorbed the roots using unlabelled Cd at 2 °C, demonstrating that the roots lost very little of their Cd to desorption. The concentrations of Cd used in our studies (5 or 20 µg l–1, which are equivalent to 45 nM and 178 nM, respectively) are in the range of the concentrations used by Hart et al. (1998) so it was reasonable to think that little of this root Cd would be associated with the cell wall.
Elemental Cd concentrations in plant tissues were determined by AAS-GF (model SpectrAA-200 atomic absorption spectrometer with a GTA-96 graphite tube atomizer attachment, Varian, Australia) or by ICP-MS (Perkin-Elmer SCIEX 5000 inductively coupled plasma–mass spectrometer, Geoscience Laboratories, Sudbury, ON). For the ICP-MS analyses conducted at Geoscience Laboratories, calibration solutions, with the analyte concentration for the elements of interest being 100 ppb, were matrix matched for eluent, nutrient solution, and plant digest solutions. With the exception of monoisotopic elements, multiple isotopes of the analytes of interest were measured to ensure that there were no direct analyte or oxide overlaps. Solutions were analysed in blocks of 20, together with long-term instrumental drift monitors, with reagent blanks and calibration standards being measured after each block. 106Cd concentrations were determined using a Perkin Elmer SCIEX 6000 ICP-MS. International certified reference materials were analysed at the beginning and end of each batch of samples. Internal control standards were analysed every 10 samples and a duplicate was run for every 10 samples (Activation Laboratories, Ancaster, ON).
Free cadmium analysis
An ion-exchange technique was used to estimate the amount of Cd2+ in the nutrient solutions (Cantwell et al., 1982); MINEQL+ cannot be used to do so, as it requires precise knowledge of the solution components, something that is not known after plants have removed ions and added organic exudates. Analytical grade resin (25 g of 50W-X8, 50–100 mesh, Bio-Rad) was measured into a 2.5 cm diameter column, washed ten times with NANOpure water, flushed with 200 ml of 4.0 M HCl and rinsed with approximately 2.5 l of NANOpure water to increase the pH to the original pure water level (±0.3) (PerpHect Log R meter, model 320, ATI Orion). The resin was then converted to the Na+ form by adding 500 ml of 3.0 M NaOH followed by a rinsing with NANOpure water until the pH returned to its original level. Lastly, 250 ml of methanol were passed through the resin followed by an equal amount of NANOpure water. The resin was transferred to a high density polyethylene bottle and placed in a clean convection oven for 48 h at 45 °C, followed by storage in a dessicator.
A poly-prep column (0.8x4.0 cm, Bio-Rad) to which a 250 ml reservoir and 2-way stopcock were attached (Bio-Rad) was packed with prepared resin (0.1 g) which was then rinsed with an electrolyte solution (0.2 M NaNO3) until the outflow solution had a pH of 6.0 and thus was in equilibrium with the nutrient solution samples. The flow rate was adjusted to 6 ml min–1, the nutrient solution sample was loaded; once the sample had passed through the resin, 10 ml of NANOpure water was added to the column and then compressed gas was used to eliminate any remaining interstitial solution. The true flow rate was determined from start and stop times; the resin was eluted with 50 ml of 1.5 M HNO3, followed by compressed gas to recover all the Cd released from the resin. The cadmium content of the eluent was determined through ICP-MS as previously described for plant samples. The free cadmium was calculated using the following equation,
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is the distribution coefficient (l g–1), and mass of resin equals 0.1 g.
The distribution coefficient () was determined by passing a solution of known cadmium speciation through the resin; the equation described above was used to determine the value of , with the [Cd]free estimated using MINEQL+.
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Results |
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Experiment 1
Total plant dry mass of Kyle was reduced by exposure to 20 µg l–1 Cd, relative to plant mass at 5 µg l–1 at the tillering and flowering stages of development; plant mass of Arcola at all developmental stages was the same for both exposure concentrations (Table 1). Relative to masses at 5 µg l–1 (data not shown), all parts of the plant (masses of root, shoot, head, and grain) in Kyle were smaller at 20 µg l–1 Cd. Between flowering and ripening, there was indication of a shift in dry mass distribution away from roots and towards heads at both 20 µg l–1 Cd (Table 2) and 5 µg l–1 (data not shown); low precision prevented the declaration of statistically significant changes. The roots of Kyle grown in solution containing 20 µg l–1 Cd had a lower concentration of tissue Cd than Arcola at all life stages; the difference was smallest at tillering, and thereafter, root-Cd concentration was 2-fold greater in Arcola than Kyle (Table 3). For both cultivars, root tissue Cd concentration peaked at the flowering stage of development, and then decreased to concentrations lower than observed at tillering (Table 3). Shoot tissue Cd concentrations were approximately one order of magnitude lower than those in the roots of the same plants; at all life stages, shoot Cd concentration was higher for Kyle than Arcola (Table 3). At both flowering and ripening life stages, head tissue of Kyle had a higher Cd concentration than Arcola, and the difference between these cultivars was observed for grain Cd concentration at harvest (Table 3). Grain Cd concentrations were lower than those for the head; the chaff was removed from the grain before analysis for Cd. The same cultivar and life stage specific differences were noted in plants grown at 5 µg l–1 (data not shown), suggesting that the differences between the cultivars in tissue concentrations of Cd, noted in Table 3, are not due to the phytotoxic effects of Cd on Kyle.
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Masses of Cd in the roots, shoots, heads, and grain of the two cultivars at the three life stages, and both exposure concentrations of Cd, were estimated by multiplying the tissue Cd concentrations by the tissue masses. In plants grown at 20 µg l–1, the amount of Cd sequestered in the roots decreased between the flowering and ripening stages, in both cultivars (Table 4); its loss cannot be accounted for by gains in other plant parts. Many of the same differences were seen in plants grown at 5 µg l–1 (data not shown). Similarly to plants grown at 20 µg l–1, shoot Cd masses at 5 µg l–1 did not change much between flowering and ripening, and the cultivars were not different (data not shown). At neither exposure concentration did the cultivars differ in total plant mass of Cd, but at both exposure concentrations there was a net loss in Cd mass by the plant between flowering and ripening (Table 4; data not shown).
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Experiment 2
After 24 h, 106Cd supplied to the roots of plants at tillering had accumulated in the roots of both cultivars, and to similar concentrations (Fig. 1), although Arcola roots were previously shown to have a higher Cd concentration at tillering than Kyle (Table 3). This suggests that the difference noted in Table 3 arose from the distribution of Cd after uptake, rather than from uptake (averaged over 24 h) itself. At the flowering stage of development, both cultivars accumulated 106Cd in roots (Fig. 1). The increase in root tissue Cd concentration 24 h after the application of 106Cd was smaller at flowering than tillering, which is not consistent with the large increase in root tissue Cd concentration at flowering noted in Table 3 for both cultivars. The roots of both cultivars accumulate 106Cd at ripening (Fig. 1), which is completely contrary to the decrease in concentration between flowering and ripening noted in Table 3.
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0.05).At tillering, the cultivars had similar 106Cd concentrations in the shoot, 24 h after it was supplied to the root (Fig. 2); this observation is inconsistent with the data in Table 3 (lower shoot Cd concentrations in Arcola than Kyle), although the precision of tillering data in Fig. 2 was low. This low precision prevented concluding definitively that 106Cd applied to the roots had moved to the shoots during the 24 h, although this was probably the case. Kyle shoot tissue at flowering accumulated 106Cd from the roots, and Arcola did not (Fig. 2), suggesting that root-to-shoot transfer of Cd is a more prolonged process in Arcola than in Kyle; this observation is consistent with Table 3. At the ripening stage of development, neither cultivar accumulated 106Cd in the shoot within 24 h of its application to the root, suggesting that root-to-shoot transfer of Cd had ceased in both cultivars by this time (Fig. 2), again in agreement with Table 3. Consistent with the shoot accumulation of 106Cd over 24 h at the flowering stage of development in Kyle, but not Arcola, the heads also accumulated 106Cd in Kyle but not Arcola at the flowering stage of development (Fig. 3). By the ripening stage of development, consistent with the shoot 106Cd accumulation (Fig. 2), neither Kyle nor Arcola transported 106Cd supplied to the roots within the previous 24 h into the grain heads (Fig. 3). The data in Fig. 3 are consistent with those in Table 3. Neither cultivar translocated Cd from the root to the grain at ripening (data not shown).
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Experiment 3
The root concentration of Cd was higher in plants which had not been previously exposed to Cd but whose foliage had been sprayed with 5000 µg l–1 Cd, compared with plants which had been treated similarly, but sprayed with 500 µg l–1 Cd, suggesting that basipetal translocation of foliar Cd occurred in both cultivars (Table 5). Basipetal transport of Cd occurred at all life stages (as indicated by the difference in tissue Cd concentrations between the 500 and 5000 µg l–1 spray treatments), but was greater in Arcola than in Kyle at the tillering (both spray concentrations) and in-boot (500 µg l–1 only) stages of development. By contrast, acropetal translocation of foliar applied Cd at the in-boot stage of development occurred to a greater extent in Kyle than Arcola, as shown by the upper shoot Cd concentration; however, the variances of the Kyle observations were so much larger than those for Arcola, that although most of the differences between the two cultivars were statistically significant, comparing them was not consistent with assumptions of homogeneity of variance (Table 5). The cultivars transported foliarly applied Cd to heads in a similar way (Table 5).
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Experiment 4
The proportion of total Cd as Cd2+ in the nutrient solution declined as the number of days post-solution change increased (Table 6); the indication that the proportion of total Cd as Cd2+ in the nutrient solution was lower for Arcola than Kyle on each measurement day was only just not statistically significant at the 5% level (Table 6). There was a decrease in total Cd in solution relative to number of days post-solution change, in both cultivars; the relative decrease over time was greater for Cd2+ than for total Cd (Table 6). The speciation of Cd in the solution changed with time, which could result from differential uptake of various Cd species by the plants or exudation of organic compounds from roots, following which the original equilibration among species was not re-established.
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Discussion |
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The general objective of this study was to understand the roles of uptake and retranslocation of Cd, and Cd speciation in the rooting solution, as possible modifiers of Cd accumulation in two cultivars of durum wheat. The first specific objective of this study was to determine whether differences between Kyle and Arcola accumulation patterns for Cd remained consistent throughout the life cycle: Arcola accumulated more Cd in roots than Kyle during continuous exposure to Cd, and the reverse was true for shoot-Cd. These data are consistent with those of Archambault et al. (2001)
who surveyed isoline pairs (high and low grain-Cd accumulators) of durum wheat, and demonstrated that the root/shoot Cd ratio in seedlings was consistently higher for the low grain-Cd accumulating isoline in each pair. However, in their study, both root and shoot Cd concentrations were lower in the low grain-Cd accumulating isoline, by contrast with this study where the order of the low and high-grain Cd accumulators was reversed between root and shoot. The same pattern of cultivar differences was noted in this study, in the presence or absence of phytotoxicity. Kyle growth was sensitive to relatively low concentrations of Cd, whereas growth of Arcola was not; this difference may relate to the larger internal dose of Cd to Kyle shoots. The approximately 65% and 80% losses in mass of Cd in the roots at the ripening stage, relative to the flowering stage, of plants grown at 5 µg l–1 and 20 µg l–1, respectively, was an unexpected outcome. It could not be explained completely by the export of Cd to the shoots (<10% of the amount of Cd lost from the root) nor by loss of root mass (30% of the mass at the previous life stage, if the absolute values are compared, thus ignoring the fact that the root masses were not statistically different). This suggests that it must include loss of some cytoplasmic Cd from the root, as Rauser (1987) demonstrated that cell-wall bound Cd accounts for less than 15% of total root Cd. Gregory et al. (1979) demonstrated a 50% loss of root sulphur between anthesis and harvest, and hypothesized that most of this was exudation from the root to the soil. Cd is a 
-block metal (avid S-seeker), and more than half of root Cd has been shown to be complexed with cysteine-rich peptides (Rauser and Meuwly, 1995), thus it is speculated that S losses from roots of durum wheat at the ripening stage may have shown similar losses as Cd. However, this turned out not to be the case; S losses from roots were proportional to dry weight losses between the flowering and ripening stages of development (data not shown). It is speculated here that losses of Cd from roots which are larger than dry weight losses, probably result from the decomposition of root cell integrity leading to leakage from the cytoplasm and vacuole.
These data describing the accumulation of Cd in different tissues of durum wheat are consistent with other studies measuring the attenuation of metal concentration between roots and shoots. Generally, root concentrations are higher than those for shoots; most of the root-Cd in plants is probably in the apoplast, including that which is surface adsorbed. Only a fraction of that passes into xylem vessels, and thus is available for translocation to the shoot. Salt and Rauser (1995) noted that root Cd concentrations in Brassica juncea and Thlaspi caerulescens were several fold higher than shoot Cd concentrations, when the plants were grown in hydroponic solution. Florijn and van Beusichem (1993) demonstrated that the ratio of root[Cd]:shoot[Cd] for inbred maize lines ranged from 1 to 55, with most being greater than 10. Inter- or intraspecific variation in root-Cd accumulation has been attributed to variation in expression or occurrence of high affinity Cd transporters in plasma membranes of root cells (Lombi et al., 2001; Zhao et al., 2002). Berkelaar and Hale (2000) demonstrated that some of the enhanced Cd accumulation by Arcola seedling roots could be explained by a greater frequency of root tips in Arcola than Kyle; the tip is the most active section of the root for Cd flux (Piñeros et al., 1998).
The second objective of this study was to identify whether the differences in net accumulation of Cd by Kyle and Arcola could be explained by variation in translocation or retranslocation of Cd among plant parts. The results of the stable isotope and foliar spray experiments both gave positive answers to this question, and gave insights that were not available from the long-term exposure and accumulation study. The stable isotope experiment demonstrated that Arcola ceased Cd translocation from root to shoot earlier in development than Kyle, and the foliar spray experiment demonstrated that both cultivars translocated Cd basipetally, and young Arcola plants did so to a greater extent than Kyle. Harris and Taylor (2001) introduced 109Cd to the phloem of low and high Cd accumulating isolines of durum wheat via a leaf flap, and observed that the accumulation of 109Cd in the grain was correlated with its loss from the stem. They concluded that remobilization of stem-Cd could be a significant source of Cd to the grain, and that variation in this could be responsible for genetically controlled differences in Cd accumulation in grain. These data support this view of the importance of shoots as a source of Cd to heads, as there was no uptake from the roots into the shoots of Arcola at flowering, so the observed import of Cd into Arcola heads at flowering had to be the result of mobilization of Cd in the shoot. However, these data suggest that differences between high and low grain-Cd cultivars may also lie in whether or not roots are active in exporting Cd, something that was not investigated by Harris and Taylor (2001).
The foliar spray experiment in this study also hinted that remobilization of shoot-Cd as a source of Cd to heads is an important distinction between cultivars, as Cd in Kyle upper shoot (translocated from the lower shoot to which the foliar spray was applied) was always higher than that of Arcola. Cakmak et al. (2000a) noted that the fate of 109Cd applied to foliage was subject to genetic variability, as was noted in the present study, and that about twice as much was moved to the roots (2.5% of applied), as was moved to the remainder of the shoots (<1% of applied). By contrast, this study's data suggest the opposite, that approximately 3% of the Cd applied to the lower foliage was found in the upper foliage, whereas <1% was found in the root. The foliar spray data showing that Arcola at tillering translocates more Cd to roots than Kyle offers reconciliation to the conflict between the data in Table 3 and Fig. 1, as the foliar spray experiment suggests that greater chronic accumulation of Cd in Arcola roots results from greater phloem import, rather than greater uptake. This enhanced phloem mobility, noted in Arcola, may be an important mechanism by which Cd is diverted from heads to low grain-Cd cultivars.
The physiological bases of these different translocation patterns were not explored in this study, but have been the subject of considerable speculation. Salt and Rauser (1995) suggested that root to shoot translocation of Cd in Thlaspi caerulescens and Brassica juncea was due to transpiration, as abscisic acid (ABA) prevented shoot-Cd accumulation. However, Florijn and van Beusichem (1993) found no correlation between the root/shoot ratio of Cd and transpiration for inbred maize lines, suggesting that the observations of cultivar-specific root/shoot Cd ratios were probably not due to differences in transpiration between Kyle and Arcola. Herren and Feller (1997) introduced Cd, Sr, and Rb into the xylem and phloem of mature wheat shoots, and measured the acropetal translocation; they identified that Cd was moderately phloem mobile, supporting these observations, as basipetal translocation could only occur via phloem. Guo and Marschner (1996) and Larsson et al. (2002) observed that Cd pre-exposure (the plants in the stable isotope study here were similarly exposed to elemental Cd up until the introduction of 106Cd) enhanced root-to-shoot transfer of Cd, probably through the production of phytochelatins. Phytochelatin induction was not measured in the plants in this study, but its variation between the cultivars could be a cause for differential Cd translocation patterns. Increased Zn supply has been demonstrated to reduce the retranslocation of Cd from the site of application on the leaf, from 6.5% to 1.3% of the total applied (Cakmak et al., 2000b). In a split root experiment, Welch et al. (1999) showed that increasing Zn supply to one half of a root inhibited translocation of Cd from that half to the other half of the root. They suggest that under conditions of Zn deficiency, phloem mobility of Cd is enhanced, and that phytochelatin induction may have a role.
The third objective of this study was to explore whether differential root accumulation of Cd was related to speciation of Cd in the hydroponic culture, specifically whether plant growth influenced speciation differentially between cultivars: the cultivars similarly reduced the initial percentage of total solution Cd present as Cd2+. The free ion is probably most readily taken up by plants (Free Ion Activity Model, FIM) (Morel and Hering, 1993), thus the Cd2+/Cdtotal data from this study are consistent with the depletion of Cd2+ by the greater accumulation of Cd by Arcola. The amount of free Cd remaining in the hydroponic system was very large relative to that accumulated in plant tissue, so depletion of Cd2+, while measurable, was not likely to be limiting to tissue Cd accumulation, as suggested by Archambault et al. (2001). The change in speciation noted in the present study could also be a result of the addition of complexing ligands to the solution by the plants, differentially between cultivars. Krishnamurti et al. (1996) determined the ‘cadmium availability index’ (CAI) in rhizosphere soil of Arcola and Kyle at 2 and 7 weeks of age. The young plants (similar in age to those for the Cd2+/Cdtotal measurements used here) had appreciable amounts of low molecular weight organic acids (LMWOAs) in the root exudates, whereas the 7-week-old plants did not. Rhizosphere soil of 2-week-old Arcola had a higher CAI than Kyle when grown on a soil with high Cd availability and phosphate fertilizer, although the rhizosphere soil pH was the same for both cultivars. The Cd2+/Cdtotal data from this study are consistent with Krishnamurti et al. (1996), as the increase in CAI by Arcola was attributed to complexation of particle-bound Cd by LMWOAs. There was no particle-bound Cd in the nutrient solution in this study; however, the high affinity of the LMWOAs for Cd would probably lead to the complexation of Cd2+ in the nutrient solution. While this would seem counter to the greater accumulation of Cd by Arcola roots, exceptions to the FIM have been noted for Cd, indicating that uptake of Cd by plants is somewhat independent of the free ion concentration, in the presence of Cdcitrate, CdSO2, or CdEDTA (Berkelaar and Hale, 2003a, b).
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Conclusions |
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This study suggests that the enhanced accumulation of Cd in the roots of Arcola relative to Kyle is probably not caused by plant-mediated changes in solution-Cd speciation, although the speciation of the nutrient solution surrounding both Arcola and Kyle roots was altered. The reduced accumulation of Cd in shoots of Arcola results from the reduced transfer from the root to the shoot even when root-Cd accumulation is increasing, as well as greater shoot-to-root translocation of Cd than in Kyle, at least in young plants. The smaller shoot-Cd pool in Arcola is probably the cause of lower grain-Cd accumulation in this cultivar, as neither cultivar transferred root-Cd to the grain during the ripening stage of development.
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References |
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Archambault DJ, Marentes E, Buckley W, Clarke J, Taylor GJ. 2001. A rapid seedling-based bioassay for identifying low cadmium-accumulating individuals of durum wheat (Triticum turgidum L.). Euphytica 117, 175–182.[CrossRef]
Berkelaar EJ, Hale BA. 2000. The relationship between root morphology and cadmium accumulation in seedlings of two durum wheat cultivars. Canadian Journal of Botany 78, 381–387.
Berkelaar EJ, Hale BA. 2003a. Accumulation of cadmium by durum wheat roots: bases for citrate-mediated exceptions to the Free Ion Model? Environmental Toxicology and Chemistry 22, 1155–1161.[CrossRef][Web of Science][Medline]
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