Spontaneous phloem exudation accompanying abscission in Lupinus mutabilis (Sweet)

Abstract

Like some of the Mediterranean members of the genus Lupinus, the New World lupin, Lupinus mutabilis (Sweet; cv. Inti), exhibits prolonged (20-40 min) exudation of phloem sap from incisions made in stems, in the raceme and at the tips and sutures of developing fruits. Just prior to or immediately following abscission of flowers of L. mutabilis there was also spontaneous exudation from the proximal face of the abscission zone at the base of the pedicel. This is not a recorded feature of other lupins. Analysis of solutes in this exudate was consistent with its having been derived directly from phloem. The major solutes were sucrose (0.94±0.04 M), amino acids (188±11 mM, 45% as asparagine and 15% as glutamine), K ion (52 mM), and total phosphorus (17 mM). Microscopic examination of the proximal face of the pedicel abscission zone at or following abscission showed little or no breakage of the cells at the zone. The major solutes of spontaneous exudate were similar to those in exudates collected from incisions made in the supporting raceme, upper stem and branches, at the tips and sutures of developing fruits and in the mid- and basal stem regions. However, there were significant compositional differences among minor constituents. The spontaneous exudate had a higher level of Ca ion and, consequently, a narrower Mg/Ca ratio (2.8) than exudate from incisions in the adjacent raceme (9.3) or fruits (15.7). There were also higher concentrations of trace elements (Mn, Zn and Cu) but lower concentrations (3 ng ml -1 ) of cytokinins compared to exudates collected from incisions (20 ng ml -1 ). The relative contents of K and Na ions in exudates from incisions at different sites on the plant showed evidence of selective phloem loading and downward translocation of Na ion and selective loading and upward translocation of K ion.