TRANSPORT OF METAL MICRONUTRIENTS IN THE PHLOEM OF CASTOR BEAN (RICINUS-COMMUNIS) SEEDLINGS

The metal micronutrients (MN) copper, iron, manganese, and zinc are tr ansported via the phloem in the course of remobilization and circulati on. The extent of these processes and transport species are still larg ely unknown. The Ricinus seedling was used to study the transport of t hese metal micronutrients as well as their interactions with the plant -endogenous chelator nicotianamine (NA) by daily measurements of the c oncentrations in the seedling parts and in the sieve tube sap obtained from a cut at the hypocotyl hook. The concentrations of these micronu trients in the phloem exudate decreased slightly from day 4 to day 8 o f seedling development. Maximum values at day 4 were 65 mu M for Zn, 6 3 mu M for Fe, 27 mu M for Cu, and 12 mu M for Mn. The phloem transpor t rates reached maxima of 0.12 nmol cm(-2) h(-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 se edling parts. The NA content of the seedlings increased from day 0 (se ed before sowing) until day 8, from 16 nmol to 474 nmol, which corresp onds to an average net synthesis rate of about 100 nmol day(-1) betwee n the days 4 and 8. The NA:MN ratio was constant at 0.5 in the seedlin gs within this period. The NA concentrations and the sum of the concen trations of all four micronutrients in the sieve tube sap showed a con stant ratio of 1.25 over the entire experimental period. Thus, both co mplex 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 r emoval 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 transpor ted as NA complexes.