Xylem to phloem transfer of solutes in fruiting shoots of legumes, studied by a phloem bleeding technique

Publisher Website
Google Scholar
Abstract
Comparisons were made of the levels of various solutes in xylem (tracheal) sap and fruit tip phloem sap of Lupinus albus (L.) and Spartium junceum (L.). Sucrose was present at high concentration (up to 220 mg ml-1) in phloem but was absent from xylem whereas nitrate was detected in xylem (up to 0.14 mg ml-1) but not in phloem. Total amino acids reached 0.5–2.5 mg ml-1 (in xylem) versus 16–40 mg ml-1 in phloem. Phloem: xylem concentration ratios for mineral nutrients (K, Na, Mg, Ca, Fe, Zn, Mn, Cu) spanned the range 0.7 to 20, the ratios generally reflecting an element's phloem mobility and its availability to the xylem from the roots. The accessibility of nitrate to xylem and phloem was studied in Lupinus. Increasing the nitrate supply to roots from 100 to 1000 mg NO3−Nl-1 increased nitrate spill over into xylem, but nitrate always failed to appear in phloem. However, phloem loading of small amounts of nitrate was induced by feeding 750 or 1000 mg NO3−Nl-1 directly to cut shoots via the transpiration stream. Transfer of reduced nitrogen to phloem was demonstrated by feeding 15NO3 to shoots and recovering 15N-enriched amides and amino acids in phloem sap. Increased nitrate supply to roots led to increased amino acid levels in xylem and phloem but did not alter markedly the balance between individual amino acids. The fate of xylem-fed 14C-labelled asparagine, glutamine and aspartic acid and of photosynthetically fed 14CO2 was studied in Spartium, with reference to phloem transport to seeds. Substantial fractions of the 14C of all sources appeared in non-amino compounds. [14C]asparagine passed largely in unchanged form to the phloem whereas the 14C from aspartic acid or glutamine appeared in phloem attached to other amino acids (e.g. asparagine and glutamic acid). Serine, asparagine and glutamine were the main amino compounds labelled in phloem sap after feeding 14CO2. The wide distribution of 14C amongst free and bound amino acids of seeds suggested that extensive metabolism of phloem-borne solutes occurred in the fruits.