Metabolism of Phloem-borne Amino Acids in Maternal Tissues2Pisum sativumL.)

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Abstract
The amino acid composition of the EDTA-induced phloem exudate reaching the fruit and the seed, and of the solutes released by the seed coat during fruit development were determined in glasshouse-grown pea (Pisum sativum L. cv. Finale) supplied either with nitrate-free nutrients (nodulated plants) or with complete medium (non-nodulated plants). The EDTA-promoted exudation technique was used supposedly to collect phloem sap and the empty seed technique supposedly to collect the solutes secreted by the seed coat to the embryo sac cavity. In young seeds embryo sac liquid was sampled directly from the embryo sac. The main carbohydrate transported and secreted was sucrose. The main amino acids reaching the fruit were asparagine, glutamine, and homoserine. Their proportions were steady during a day-night cycle and throughout fruit development. Amino acid composition changes occurred first in the pathway from fruit stalk to seed funicle, due to the formation of threonine (probably from homoserine) and in the seed coat due to production of glutamine, alanine and valine which, together with threonine were the main secreted amino acids. The temporary nitrogen reserves of the pod wall and seed coat were remobilized as asparagine during senescence. Phloem exudate of nodulated plants showed a higher (about twice) proportion of asparagine but lower proportions of homoserine and glutamine than in EDTA-induced phloem exudate of nitrate-fed plants. The two types of nitrogen nutrition also produced some changes in relative proportions of threonine and homoserine secreted by the seed coat.

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