Influence of varied zinc supply on re-translocation of cadmium (Cd-109) and rubidium (Rb-86) applied on mature leaf of durum wheat seedlings

Effect of varied zinc (Zn) supply (0, 0.1, 1, 5 mu M) on re-translocation of radio-labeled cadmium (Cd-109) and rubidium (Rb-86) from mature leaf to root and other parts of shoot was studied in 11-day-old durum wheat (Triticum durum cv. C-1252) plants grown in nutrient solution under controlled environmental conditions. Application of Cd-109 and Rb-86 was carried out by immersing the tips (3 cm) of mature leaf in radio-labeled solutions for 10 s at three different times over a 42 h period. Differences in Zn supply for 11 days did not affect plant growth nor did it cause visual leaf symptoms, such as necrosis and chlorosis, at either the lowest or the highest Zn supply. Only at the nil Zn supply (0 mu M), shoot and root dry weights tended to decrease and increase, respectively, causing a lower shoot/root dry weight ratio. Partitioning of more dry matter to roots rather than shoots, a typical phenomena for Zn-deficient plants in nutrient solution experiments, indicated existence of a mild Zn deficiency stress at the nil-Zn treatment. Irrespective of Zn supply, plants could, on average, retranslocate 3.8% and 38% of the total absorbed Cd-109 and Rb-86 from the treated leaf to roots and other parts of shoots within 42 h, respectively. At nil-Zn treatment, 2.8% of the total absorbed Cd-109 was re-translocated from the treated leaf, particularly into roots. The highest re-translocation of Cd-109 (6.5%) was found in plants supplied with 0.1 mu M Zn. Increases in Zn supply from 0.1 mu M reduced Cd-109 re-translocation from 6.5% to 4.3% at 1 mu M Zn and 1.3% at 5 mu M Zn. With the exception of the nil-Zn treatment, the proportion of re-translocated Cd-109 was greater in the remainder of the shoot than in the roots. Contrary to the Cd-109 results, re-translocation of Rb-86 was not (at 0, 0.1 and 1 mu M Zn), or only slightly (at 5 mu M), affected by changing Zn supply. The results indicate an inhibitory action of increased concentrations of Zn in shoot tissues on phloem-mediated Cd transport. This effect is discussed in relation to competitive inhibition of Cd loading into phloem sap by Zn.