The metal micronutrients (MN) copper, iron, manganese, and zinc are transported via the phloem in the course of remobilization and circulation. The extent of these processes and transport species are still largely unknown. The Ricinus seedling was used to study the transport of these metal micronutrients as well as their interactions with the plant‐endogenous chelator nicotianamine (NA) by daily measurements of the concentrations in the seedling parts and in the sieve tube sap obtained from a cut at the hypocotyl hook. The concentrations of these micronutrients in the phloem exudate decreased slightly from day 4 to day 8 of seedling development. Maximum values at day 4 were 65 μM for Zn, 63 μM for Fe, 27 μM for Cu, and 12 μM for Mn. The phloem transport rates reached maxima of 0.12 nmol cm−2h−1 for Zn and Fe at days 6 and 7, corresponding to the maximum exudation rates. The magnitude of these transport rates were in agreement with the net translocation rates estimated by analyses of the concentrations in the individual seedling parts. The NA content of the seedlings increased from day 0 (seed before sowing) until day 8, from 16 nmol to 474 nmol, which corresponds to an average net synthesis rate of about 100 nmol day−1 between the days 4 and 8. The NA:MN ratio was constant at 0.5 in the seedlings within this period. The NA concentrations and the sum of the concentrations of all four micronutrients in the sieve tube sap showed a constant ratio of 1.25 over the entire experimental period. Thus, both complex partners were subject to a cotransport in the phloem. Removal of the supplying endosperm led to a decrease in MN and NA concentrations in the sieve tube sap to about 80% while an average excess of NA of 1.1 was maintained. Since the concentrations of other amino acids, also possible chelators of metal micronutrients, fall to about 10% after removal of the endosperm, their role seems to be negligible as vehicles of MN transport in the phloem. Thus it is suggested that the divalent micronutrients considered in this study are loaded and maybe transported as NA complexes.

The non-proteinogenic amino acid nicotianamine (NA) is ubiquitous among plants. In meristematic tissues it reaches concentrations of about 400/lmol (g fresh weight)-i. NA forms complexes, among others, with the metal micronutrients (MN) copper, zinc, iron and manganese (log KMeNA 18.6-8.8). Calculations of the dissociation curves of the metal-NA complexes based on the complex formation constants and on the acid dissociation constants of NA revealed their stability at the neutral or weak alkaline pH of cytoplasm and sieve tube sap. For the Mn-NA complex, dissociation begins at about pH 6.5, for all others dissociation occurs at more acid pHs. Thus, metal-NA complexes could theoretically persist also in the apoplasm and in xylem sap. The octanol water partition coefficient of NA is about 1 and those of its metal complexes arc in the range of 0.3-0.4. The reason for this shift is perhaps the negative charge of the complexes. The higher lipophilicity of the free NA indicates that the NA supply to sites of requirement is faster than the removal of the complexes as long as membranes are an integral part of the transport paths. Changing phloem transport rates of MN-NA complexes by manipulation of the cotyledon apoplasm of Ricinus communis L. suggest a competition of MN for NA at the site(s) of phloem loading. Thus, NA could control MN transport via phloem including recirculation.

During the first 8 days of germination the Ricinus seedling is supplied with all nutrients by the endosperm via phloem transport. In 4-to 8-days-old seedlings the concentrations and contents of Fe, Cu, Mn and Zn, and nicotianamine (NA) in the endosperm, cotyledons, hypocotyl and roots were estimated. From the data obtained translocation rates and flow profiles for the metals were established. The main sink for Fe, Mn and Zn were the cotyledons whereas Cu was mainly imported into the hypocotyl. Maximum flow rates occurred between days 5 and 7, for Zn between days 6 and 8.The time kinetics of NA and divalent metal ion concentrations and contents are interpreted as co-transport. The role of NA as transport vehicle of micronutrients in the sieve tubes is discussed.Abbreviations: NA, nicotianamine

The concentration of iron within cells has to be precisely regulated because shortage as well as surplus may be precarious for the survival of the cell. The maintenance of iron homeostasis in shoot organs requires an efficient signalling of the leaf cells’ iron status to the uptake sites of the roots. This ‘iron signal’ may be transferred by the phloem. The handling of iron in the symplast and during phloem transport calls for mechanisms taking into account the specific physicochemical properties of this element.Seedlings of Ricinus communis were used as model plants to investigate characteristic features of phloem loading, and of speciation and valence of iron during transport in the sieve tubes. When the storage endosperm is removed from the cotyledons, phloem is loaded from the reserve pool of the mesophyll cells. In this situation, iron and the other micronutrients copper, manganese and zinc are loaded in a constant stoichiometric ratio of 1:1 to the endogenous complexor nicotianamine (NA). Application of the chelators 1,4‐di(4‐phenylsulphonate)‐1,10‐phenanthroline (BPDS) and ethylenediaminetetraacetic acid to the cotyledon apoplast did not decrease the loading rate of iron, indicating symplastic loading. Supply of ferrous ions in various concentrations to the apoplast revealed the existence of two loading systems. One of them is linearly dependent upon the concentration, and remained unsaturated up to an apoplastic concentration of 200 μM. The other one, whose activity steeply inclines already with a slight increase of the apoplastic concentration, is saturable at a supply of 100 μM. The loading of iron is slowed down with time depending on whether iron is supplied to the apoplast as a complex with NA or citrate instead of as free ferrous ions. This effect may be caused by competition of these chelators with an iron uptake receptor in the plasmalemma of the mesophyll cells. In spite of the close relationship between iron and NA during phloem loading, the Fe‐NA complex seems not to be the predominant transport species in the sieve tubes. A molecule of much larger mass than NA probably serves as a transport vehicle, as concluded from microdialysis experiments. Only 4% of the total iron in the sieve tube exudate was found to exist as Fe(II) and about 45% as Fe(III). The residue of more than 50% was tightly bound and not accessible even in the presence of the reductant sodium dithionite and the chelator BPDS. The conclusions regarding the nature of the transport species and the results on the valence of iron in the sieve tubes were confirmed by calculations with the software programs GEOCHEM [Sposito G, Mattigod SV (1979) A computer program for calculating chemical equilibria in soil solutions and other natural water systems. Kearney Foundation of Soil Sci, Univ. of California, Riverside, CA] and PHREEQC [Parkhurst (1995) PHREEQC‐A computer program for speciation, reaction‐path, adjective‐transport, and inverse geochemical calculations. US Geological Survey]. A model is outlined on the basis of the experimental ...

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