Comparison of Ethylenediaminetetraacetate-Enhanced Exudation from Detached and Translocation from Attached Bean Leaves

A technique for collection of phloem exudate from detached leaves using 20 millimolar EDTA (pH 7.0) has previously been developed (King, Zeevaart 1974 Plant Physiol 53: 96-103). It was the aim of the present study to determine the efficiency of this technique in relation to undisturbed export from attached leaves. Paired primary leaves of bean seedlings (Phaseolus vulgaris L. cv Montcalm) were used to minimize variations in plant material. Attached leaves, exposed to (14)CO(2) for 10 minutes with subsequent excision of one of the leaves and collection of the exudate over a 12-hour period, showed a 25% export of total assimilated (14)C from the attached versus 15% of total assimilated (14)C in the form of exudation from the detached ones. Leaf excision changed the labeling pattern within the leaf, increasing% total leaf (14)C-activity in the ethanolic fraction, while decreasing activity in the starch fraction, as compared to attached leaves. This was presumably caused by a lack of translocation from the detached leaves. Excision did not affect dark respiration. However, measurements of total nonstructural carbohydrates in leaf starch and neutral fractions indicated no significant differences between attached and leaves detached in EDTA. Thus, in terms of actual carbon export, and accompanying distribution of nonexported carbohydrate within the leaf, EDTA-enhanced exudation compares favorably with translocation from attached leaves.