 During winter , asparagine is metabolised immediately , but other nutrients , especially phosphates and glutamine , are removed from the xylem sap and stored in mature stem , bark and leaf tissues for release back into the xylem just before shoot growth begins in mid summer . This is also the time when the oldest leaves senesce and die , returning nutrients to the plant at the time when they are needed most . When glutamine eventually reaches the leaves , it is broken down and used to synthesise protein and non @-@ amide amino acids such as aspartate , threonine , serine , glutamate , glycine , alanine and cystine . Together with sucrose and other solutes , these are then circulated in the phloem . The phloem sap of B. prionotes is unusual in having an extremely low ratio of potassium to sodium cations , and very low concentrations of phosphate and amino acids compared to chloride and sulfate anions . The low levels of potassium and phosphate reflect the extremely low availability of these minerals in the soil . The unusually high levels of sodium and chloride — at concentrations usually only seen under saline conditions — may be due to the necessity of maintaining turgor pressure ; that is , with so little potassium and phosphate available , and that needed in the building of new tissue , B. prionotes is forced to circulate whatever other ions are available in order to maintain turgor . 
