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Articles

Long Distance Translocation of Sucrose, Serine, Leucine, Lysine, and Carbon Dioxide Assimilates

II. Oats ¹

^a Agricultural Research Service, United States Department of Agriculture and Department of Agronomy, University of Wisconsin, Madison, Wisconsin 53706

To establish whether several amino acids were equally able to enter the phloem of oat (Avena sativa L.) plants and be transported, several ¹⁴C-labeled amino acids were applied individually to an abraded spot on a fully expanded source leaf. The base of an immature sink leaf was monitored with a GM tube for time and rate of arrival of radioactivity. Transport of ¹⁴C-sucrose and ¹⁴CO₂ assimilates was measured for a comparison. The applied L-serine, L-lysine, and L-leucine, as well as sucrose, entered the phloem and were transported to the sink leaf at rates between 1.16 and 1.83 cm/min. Transport velocity for CO₂ assimilates was 1.57 cm/min. A heat girdle near the top of the source leaf sheath blocked most transport, which indicated that transport was primarily through the phloem. Mass transfer rates for amino acids were only 3% as great as that for sucrose, suggesting different mechanisms of entry for sucrose than for amino acids into the phloem. The higher percentage of CO₂ assimilates mobilized to the sink leaf was attributed to the greater surface area of minor veins accessible to loading, as compared to those compounds supplied via an abraded spot. Serine was extensively metabolized in the source leaf, and radioactive products in the sink leaf mirrored those in the source leaf. Most radioactivity of lysine and leucine remained within these compounds in the source, path, and sink tissues. We concluded that there was no barrier to entry of amino acids into the phloem and transport therein. Data do not suggest a specific mechanism for entry of amino acids into the phloem.

¹ Research supported by the Agricultural Research Service, United States Department of Agriculture and the College of Agricultural and Life Sciences, University of Wisconsin-Madison.

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