Cadmium uptake and translocation in seedlings of near isogenic lines of durum wheat that differ in grain cadmium accumulation

Abstract

Background: Cadmium (Cd) concentrations in durum wheat (Triticum turgidum L. var durum) grain grown in North American prairie soils often exceed proposed international trade standards. To understand the physiological processes responsible for elevated Cd accumulation in shoots and grain, Cd uptake and translocation were studied in seedlings of a pair of near-isogenic durum wheat lines, high and low for Cd accumulation in grain.

Results: In short-term studies (<3 h) using 109Cd-labelled nutrient solutions, there were no differences between lines in time- or concentration-dependent 109Cd accumulation by roots. In contrast, rates of 109Cd translocation from roots to shoots following longer exposure (48-60 h) were 1.8-fold higher in the high Cd-accumulating line, despite equal whole-plant 109Cd accumulation in the lines. Over the same period, the 109Cd concentration in root-pressure xylem exudates was 1.7 to 1.9-fold higher in the high Cd-accumulating line. There were no differences between the lines in 65Zn accumulation or partitioning that could account for the difference between lines in 109Cd translocation.

Conclusion: These results suggest that restricted root-to-shoot Cd translocation may limit Cd accumulation in durum wheat grain by directly controlling Cd translocation from roots during grain filling, or by controlling the size of shoot Cd pools that can be remobilised to the grain.

Figure 1

Short-term ¹⁰⁹Cd accumulation in roots of durum wheat seedlings. Time-course of ¹⁰⁹Cd accumulation in intact roots (A) of high (TL-H) and low (TL-L) Cd-accumulating isolines of durum wheat. Roots of 6-d old seedlings were exposed for up to 180 min in 15 mL of complete nutrient solution containing 50 pM ¹⁰⁹Cd and 0.5 μM ⁶⁵Zn. Desorbable ¹⁰⁹Cd (B) was removed following treatment by a 30 min (2°C) wash in non-radiolabelled nutrient solution containing 50 μM DTPA. Inset shows desorbable ¹⁰⁹Cd expressed as a percentage of total ¹⁰⁹Cd accumulation in the roots. Means and standard errors of 5 replicates (3 plants per replicate) are plotted.

Figure 2

Spatial variation in ¹⁰⁹Cd accumulation in roots of durum wheat seedlings. Time-course of ¹⁰⁹Cd accumulation in root tips (1 cm) and basal root sections of high (TL-H) and low (TL-L) Cd-accumulating isolines of durum wheat. Intact roots of 6-d old seedlings were exposed for up to 180 min in 15 mL of complete nutrient solution containing 50 pM ¹⁰⁹Cd. Roots were desorbed for 30 min (2°C) prior to harvest in non-radiolabelled nutrient solution containing 50 μM DTPA. The 3 longest roots per plant were separated into root tips and remaining basal portions. Means and standard errors of 3 replicates (5 plants per replicate) are plotted.

Figure 3

Concentration-dependent Cd accumulation in roots of durum wheat seedlings. Concentration-dependent Cd accumulation in roots (A) of high and low Cd-accumulating isolines of durum wheat. Intact roots of 6-d old seedlings were exposed for 30 min to 15 mL of complete nutrient solution containing 5–1800 nM Cd labelled with ¹⁰⁹Cd. Desorbable Cd (B) was removed following treatment by a 30 min (2°C) wash in non-radiolabelled nutrient solution containing 50 μM DTPA. Inset shows desorbable Cd expressed as a percentage of total Cd accumulation in the roots. Means and standard errors of 5 replicates (3 plants per replicate) are plotted.

Figure 4

Time-course of Cd desorption from roots of durum wheat seedlings. Time-course of Cd desorption from intact roots of a low (TL-L) Cd-accumulating isoline of durum wheat. Roots of 6-d old seedlings were exposed for 30 min to 15 mL of complete nutrient solution containing 50 pM ¹⁰⁹Cd or 0.5 μM Cd (labelled with ¹⁰⁹Cd). Roots were desorbed in non-radiolabelled nutrient solution containing 50 μM DTPA for up to 60 min (2°C) prior to harvest. Inset shows Cd remaining in the roots as a percentage of Cd accumulation prior to desorption (0 min). Means and standard errors of 5 replicates (3 plants per replicate) are plotted.

Figure 5

Long-term ¹⁰⁹Cd accumulation in roots of durum wheat seedlings. Time-course of ¹⁰⁹Cd accumulation in intact roots (A) of high (TL-H) and low (TL-L) Cd-accumulating isolines of durum wheat. Roots of 6-d old seedlings were exposed for up to 60 h (solutions changed every 12 h) in 15 mL of complete nutrient solution containing 50 pM ¹⁰⁹Cd and 0.5 μM ⁶⁵Zn. Desorbable ¹⁰⁹Cd (B) was removed following treatment by a 30 min (2°C) wash in non-radiolabelled nutrient solution containing 50 μM DTPA. Inset shows desorbable ¹⁰⁹Cd expressed as a percentage of total ¹⁰⁹Cd accumulation in the roots. Means and standard errors of 7 replicates (3 plants per replicate) are plotted.

Figure 6

Long-term shoot and whole-plant ¹⁰⁹Cd accumulation in durum wheat seedlings. Time-course of ¹⁰⁹Cd translocation to shoots (A) and Cd accumulation per plant (B) of high (TL-H) and low (TL-L) Cd-accumulating isolines of durum wheat. Roots of 6-d old seedlings were exposed for up to 60 h (solutions changed every 12 h) in complete nutrient solution containing 50 pM ¹⁰⁹Cd and 0.5 μM ⁶⁵Zn, followed by a 30 min (2°C) desorption in non-radiolabelled nutrient solution containing 50 μM DTPA. Means and standard errors of 7 replicates (3 plants per replicate) are plotted.

Figure 7

Long-term ⁶⁵Zn accumulation in durum wheat seedlings. Time-course of ⁶⁵Zn accumulation in intact roots (A), ⁶⁵Zn translocation to shoots (B), and ⁶⁵Zn accumulation per plant (C) of high (TL-H) and low (TL-L) Cd-accumulating isolines of durum wheat. Roots of 6-d old seedlings were exposed for up to 60 h (solutions changed every 12 h) in 15 mL of complete nutrient solution containing 50 pM ¹⁰⁹Cd and 0.5 μM ⁶⁵Zn. Desorbable ⁶⁵Zn was removed following treatment by a 30 min (2°C) wash in non-radiolabelled nutrient solution containing 50 μM DTPA. Inset shows desorbable ⁶⁵Zn expressed as a percentage of total ⁶⁵Zn accumulation in the roots. Means and standard errors of 7 replicates (3 plants per replicate) are plotted.

Figure 8

¹⁰⁹Cd transport in xylem sap of durum wheat seedlings. Time-course of ¹⁰⁹Cd concentration in xylem sap (A) and the rate of xylem sap exudation (B) of high (TL-H) and low (TL-L) Cd-accumulating isolines of durum wheat. Roots of 6-d old seedlings were exposed for between 12 and 60 h in 15 mL of complete nutrient solution containing 25 pM ¹⁰⁹Cd (solutions changed every 12 h). Shoots were then excised 5–7 mm above the roots and xylem sap collected for 6 h. Means and standard errors of 10 replicates are plotted.