Phloem bleeding from legume fruits?A technique for study of fruit nutrition

Abstract

Bleeding from phloem of cut distal tips of attached fruits was demonstrated in the genera Spartium, Genista, Lupinus and Jacksonia. Bleeding occurred over a 2-25 min period enabling 0.5-10 μl of sap to be collected from a fruit. A detailed study of Lupinus albus L. showed that exudation rate declined exponentially after cutting, but without any change with time in solute levels in exudate. Bleeding resumed at its initial rate and solute concentration on recutting the fruit tip.Phloem exudates had a high pH (7.8-8.0), a sucrose content of 100-210 mg ml(-1) but only traces of monosaccharides. Surrounding pod tissues contained only 15-35 mg ml(-1) of sugars (tissue water basis) more than two thirds of this monosaccharide. Amino compounds were present in phloem exudates at 8-28 mg ml(-1), asparagine and glutamine predominating but a wide spectrum of other amino acids being also present. No significant differences in levels of organic solutes were observed in phloem exudates collected from tips of attached versus detached fruits, from phloem exudates collected from fruit tips versus pedicels, or from basal versus distal ends of a detached fruit.Potassium was the major cation (1.5-2.2 mg ml(-1)) of the phloem exudate, Ca(2+) was at a much lower level than either Mg(2+) or Na(+). Trace element levels in phloem exudates appeared to be influenced by availability to the plant from the rooting medium. Nitrate was absent though detectable in non-vascular tissues of the shoot. (14)C- labelled assimilates were detected in exudates of L. albus one hour after feeding a source leaf (14)CO2; sucrose, organic acids and certain amino compounds achieved high specific labelling. (14)CO2 feeding studies coupled with the phloem bleeding technique revealed highly specific source-sink relationships between foliar organs and fruits of the primary inflorescence.