Knowledge on xylem, and especially phloem transport of micronutrients is important in developing our capacity to enhance
loading of micronutrients into developing grain (which has consequences for human nutrition), to increase nutrient-use
efficiency through increased remobilisation and re-utilisation of nutrients, to effectively nourish seedlings in the early stages
of development, and to enhance tolerance to a number of biotic and abiotic stresses. Literature on xylem and phloem
transport of zinc and manganese is assessed.
The recent reports on Zn indicate high phloem mobility from leaves to roots, from leaves and stems to developing grain,
and from one root to another. Tissue demand for Zn is a powerful regulator of Zn phloem transport. Loading of Zn into
developing wheat grain occurs mostly in the phloem, with the transfer from xylem to phloem in the rachis and the peduncle.
Mobility of Mn in the phloem is considerably lower than that of Zn. Manganese can be translocated out of leaves, but
transport toward roots is frequently insufficient, and loading of Mn into developing grain is poor in most crop species.
Hence, xylem transport is more important than phloem transport in loading of Mn into developing wheat grain. Insights into
mechanisms of Mn phloem transport can be gained from comparing Lupinus angustifolius, species that is poor at loading
Mn into grain, with L. albus, species that loads up to eight times more Mn into grain under identical conditions.
In conclusion, Zn is classified as highly mobile in the phloem, while relatively poor mobility of Mn in the phloem of most
plant species stresses the importance of xylem transport, even in grain loading.