Mineral nutrition and transport in xylem and phloem of Banksia prionotes (Proteaceae), a tree with dimorphic root morphology

Xylem sap of proteoid roots, lateral roots, sinker root and age classes of trunk segments, and phloem sap of mid-trunks were collected from trees of Banksia prionotes (Proteaceae) in native habitat on highly impoverished sands in south-western Australia. Proteoid roots were major exporters of phosphate, K+ and amino acids during the wet winter season and showed in vitro nitrate reductase activity during periods of soil nitrification. Other parts of the root served as general sources of Na+, Cl-, Mg2+, Ca2+, and SO42-. Lateral root xylem sap was more concentrated in virtually all solutes than that of sinker roots, even during the dry summer following senescence of proteoid roots, Gradients in xylem sap concentration up the main trunks suggested lateral abstraction and storage of incoming phosphate in basal stem parts during winter and a subsequent release to the xylem in summer. Phloem sap was many times more concentrated in nutrient ions than xylem sap, and, like xylem sap, showed unusually low K+ and H2PO4- relative to Na+, Cl- and SO42-, suggesting a sparing role of the latter three ions in meeting ionic requirements of transport, Amino acid analyses showed higher overall concentrations in phloem than xylem sap, but much lower proportions of total amino N as glutamine and asparagine in the former sap, This suggested utilization of xylem-derived amide N by leaves for growth and synthesis of phloem-mobile amino acids.