Heavy Metals in Crop Plants: Transport and Redistribution Processes on the Whole Plant Level

Copper, zinc, manganese, iron, nickel and molybdenum are essential micronutrients for plants. However, when present in excess they may damage the plant or decrease the quality of harvested plant products. Some other heavy metals such as cadmium, lead or mercury are not needed by plants and represent pollutants. The uptake into the roots, the loading into the xylem, the acropetal transport to the shoot with the transpiration stream and the further redistribution in the phloem are crucial for the distribution in aerial plant parts. This review is focused on long-distance transport of heavy metals via xylem and phloem and on interactions between the two transport systems. Phloem transport is the basis for the redistribution within the shoot and for the accumulation in fruits and seeds. Solutes may be transferred from the xylem to the phloem (e.g., in the small bundles in stems of cereals, in minor leaf veins). Nickel is highly phloem-mobile and directed to expanding plant parts. Zinc and to a lesser degree also cadmium are also mobile in the phloem and accumulate in meristems (root tips, shoot apex, axillary buds). Iron and manganese are characterized by poor phloem mobility and are retained in older leaves.

Selective Transport of Zinc, Manganese, Nickel, Cobalt and Cadmium in the Root System and Transfer to the Leaves in Young Wheat Plants

• Background and Aims The uptake, translocation and redistribution of the heavy metals zinc, manganese, nickel, cobalt and cadmium are relevant for plant nutrition as well as for the quality of harvested plant products. The long-distance transport of these heavy metals within the root system and the release to the shoot in young wheat (Triticum aestivum ‘Arina’) plants were investigated. • Methods After the application of 65Zn, 54Mn, 63Ni, 57Co and 109Cd for 24 h to one seminal root (the other seminal roots being excised) of 54-h-old wheat seedlings, the labelled plants were incubated for several days in hydroponic culture on a medium without radionuclides. • Key Results The content of 65Zn decreased quickly in the labelled part of the root. After the transfer of 65Zn from the roots to the shoot, a further redistribution in the phloem from older to younger leaves was observed. In contrast to 65Zn, 109Cd was released more slowly from the roots to the leaves and was subsequently redistributed in the phloem to the youngest leaves only at trace levels. The content of 63Ni decreased quickly in the labelled part of the root, moving to the newly formed parts of the root system and also accumulating transiently in the expanding leaves. The 54Mn content decreased quickly in the labelled part of the root and increased simultaneously in leaf 1. A strong retention in the labelled part of the root was observed after supplying 57Co. • Conclusions The dynamics of redistribution of 65Zn, 54Mn, 63Ni, 57Co and 109Cd differed considerably. The rapid redistribution of 63Ni from older to younger leaves throughout the experiment indicated a high mobility in the phloem, while 54Mn was mobile only in the xylem and 57Co was retained in the labelled root without being loaded into the xylem.

Redistribution of Nickel, Cobalt, Manganese, Zinc, and Cadmium via the Phloem in Young and Maturing Wheat

The phloem mobility of heavy metals is relevant to the redistribution of micronutrients and pollutants and, ultimately, to the quality of harvested plant parts. The relative mobility in wheat may vary considerably between different cations. In the study reported here, radio-labeled nickel (Ni), cobalt (Co), manganese (Mn), zinc (Zn) and cadmium (Cd) were introduced into either intact young winter wheat (Triticum aestivum L. cv. Arina) via a leaf flap, or detached maturing shoots via the cut stem. Elements fed into the lamina of the second leaf of 21-day-old plants were translocated to the younger (expanding) leaves and to the roots but not or only in trace amounts to the first (already fully expanded) leaf. The 63Ni and 65Zn were exported more rapidly compared with the other heavy metals. Most of 54Mn was retained in the originally labeled leaf. The peduncle of some maturing shoots was steam-girdled below the ear to distinguish between xylem and phloem transport. This phloem interruption reduced the content of 63Ni in the ear to about 25%. Intermediate effects were observed for 65Zn, 57Co, and 109Cd. Total 54Mn accumulation in the ear was hardly affected by steam-girdling, indicating a transport of this element within the xylem to the ear. These results suggest that the relative phloem mobility of Ni and Zn in young wheat plants and in maturing wheat shoots is higher than the mobility of Co and Cd, whereas the mobility of Mn is very low.

Effect of locally increased zinc contents on zinc transport from the flag leaf lamina to the maturing grains of wheat

From previous experiments, it was evident that the accumulation of zinc in maturing wheat grains is highly regulated, but the regulatory mechanisms involved are not yet identified. In this study, we determined the transfer of radiolabelled zinc (fed directly into a leaf flap) from the flag leaf lamina to the grains. We also determined how this zinc transfer was affected by feeding additional unlabeled zinc (1 μmol per plant) either into the flag leaf sheath or the peduncle. Most of the 65Zn was retained in the feeding flap. A high percentage of the zinc exported from the flap accumulated in the grains with little accumulation of radiolabel in the other parts of the shoot. Unlabeled zinc remained mainly in the feeding flap and in the parts reached by the transpiration stream from the feeding position. The transfer of radiolabelled zinc was essentially not influenced by unlabeled zinc fed into another plant part. Our results suggest that the loading of zinc into the phloem and the mass flow in the sieve tubes might regulate zinc redistribution within the wheat shoot.