The Composition of Phloem Exudate and Xylem Sap from Broccoli (Brassica oleraceavar. italica) Supplied with NH4+, NO⊟3or NH4NO3

Abstract

Shelp, B. J. 1987. The composition of phloem exudate and xylem sap from broccoli (Brassica oleracea var. italica) supplied with NH⁺₄, NO^⊟₃ or NH₄NO₃.—J. exp. Bot. 38: 1619–1636. The detailed composition of xylem sap and exudate from stem incisions of attached inflorescences of broccoli (Brassica oleracea var. italica) was compared in plants supplied with NH⁺₄, NO^⊟₃ or NH₄NO₃. A phloem origin for the exudate was suggested from the high levels of sugars (71–133 mg cm^-3), amino acids (8·1-26·7 mg cm^⊟3) and K. (2·3–3·8 mg cm^⊟3), the low levels of NO^⊟₃ and Ca, the high C: N (w/w) ratios (8·3–33), and the alkaline pH (7·2–7·3). In contrast, the xylem sap was mildly acidic (pH 5·6–6·0), and possessed lower levels of all organic and inorganic solutes but NO^⊟₃ and Ca, and lower ratios of K: Ca, Mg: Ca and C: N (0·6–4·4). Glutamine was the predominant o-phthalaldehyde-reactive amino compound in both transport fluids with the next most abundant amino acids dependent on sap type and N-form. Together with arginine, γ-aminobutyric acid, which was found only in the xylem stream, was enhanced by NH⁺₄compared to NO^⊟₃ -nutrition suggesting that glutamate metabolism was stimulated in the roots. Under limiting N the amino acid concentrations in the transport fluids were greater with NH⁺₄ than with NO₃^⊟. NO₃^⊟ reduction occurred in both the root and shoot with the latter site predominating over the entire N range (0-300 mol m^⊟3). Even though the composition of nitrogenous solutes in the xylem was dependent on cultivar and N source, the composition of the phloem streams supplying the developing inflorescence was relatively unaffected. The data on the element composition of organs and phloem sap are interpreted to suggest that, in spite of the restricted mobility of some elements such as B and Mn, a significant proportion of their total supply to developing sinks is carried in the phloem stream.