Phloem unloading in developing seeds ofVicia faba L.

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Abstract
Several characteristics of the process of phloem unloading in the seed coat of developing seeds ofVicia faba L. were investigated. Prior to the procedure of measuring the release of solutes by the seed coat, the embryo was removed from each ovule studied. First, a window was made in the pod wall and, subsequently, a rectangular incision was made in the seed coat of several ovules within the same pod. After the removal of the embryo from a developing seed, the “empty” seed coat was filled with a solution (pH 5.5) with or without inhibitors. Following another procedure excised seed-coat halves were incubated in a “wash-out” solution. The effects ofp-chloromercuribenzenesulfonic acid, low temperature, NaN3, carbonylcyanide-m-chlorophenylhydrazone and 25 mM K+ were measured. The pattern of solute unloading from the seed coat was not clearly influenced by 25 mM K+. All other treatments strongly inhibited the release of sucrose ana amino acids (L-valine, L-asparagine, α-aminoisobutyric acid). In many experiments, the amount of label entering “empty” ovules filled with a solution without inhibitor did not differ much from the amount of label entering an intact ovule within the same pod. Moreover, in some experiments in which the absolute amounts of sucrose and asparagine were measured, the release of these solutes by the seed coat of an “empty” ovule was remarkably constant over a period of several hours after the surgical treatment. From these data it can be concluded that, at least in short-term experiments (6 h or somewhat longer), the presence of the developing embryo is not necessary for intensive phloem unloading. “Empty” ovules of legumes represent a valuable system for detailed studies on the regulation of phloem unloading.