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 o f radio-labeled cadmium (Cd-109) and rubidium (Rb-86) from mature leaf to r oot and other parts of shoot was studied in 11-day-old durum wheat (Triticu m durum cv. C-1252) plants grown in nutrient solution under controlled envi ronmental conditions. Application of Cd-109 and Rb-86 was carried out by im mersing 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 1 1 days did not affect plant growth nor did it cause visual leaf symptoms, s uch as necrosis and chlorosis, at either the lowest or the highest Zn suppl y. 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 typi cal phenomena for Zn-deficient plants in nutrient solution experiments, ind icated existence of a mild Zn deficiency stress at the nil-Zn treatment. Ir respective of Zn supply, plants could, on average, retranslocate 3.8% and 3 8% of the total absorbed Cd-109 and Rb-86 from the treated leaf to roots an d 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, p articularly into roots. The highest re-translocation of Cd-109 (6.5%) was f ound in plants supplied with 0.1 mu M Zn. Increases in Zn supply from 0.1 m u 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 o f 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 no t (at 0, 0.1 and 1 mu M Zn), or only slightly (at 5 mu M), affected by chan ging Zn supply. The results indicate an inhibitory action of increased conc entrations of Zn in shoot tissues on phloem-mediated Cd transport. This eff ect is discussed in relation to competitive inhibition of Cd loading into p hloem sap by Zn.