The Role of the Stem in Phloem Loading of Minerals in Lupinus albus L. cv. Ultra

Collections of phloem sap made over a 40-day period from a variety of locations on nodulated white lupin plants (Lupinus albus L. cv. ultra) showed considerable enrichment with K+ and Mg2+ in the phloem streams destined for the shoot apices or fruits relative to the streams arising from the leaflets (up to 5.5 times). Sodium showed enrichment in the streams destined for the roots (up to 2.5 times) but only when present in the water supply at a high level (3 mM). The stem, in view of its central location in the transport pathway, is seen as an organ capable of redistributing minerals in the phloem independently of photosynthate.