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

Abstract

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 '4C-sucrose and "4CO2 assimilates was measured for a comparison. The applied L-serine, L-lysine, and L-leucine, as weli 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 primarnly 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 conduded 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. ity in the amino acid fraction of the flag leaf blade or its sheath (Cataldo, Schrader, and Peterson, unpublished data). The possible existence of some mechanism for limiting the entry of amino acids into the transport path was suggested by this observation.There are few previous reports on amino acid transport in the phloem of gramineous species. Yamaguchi and Islam (22) applied several 14C-labeled amino acids to the first leaf of barley (Hordeum vulgare L.) seedlings. Whole plant autoradiography after 24 hr revealed radioactivity in the expanding second leaf and the roots, which indicated probable phloem mobility. With the exception of applied valine, the translocated products were not identified. Amounts of radioactivity extracted from the bud varied among different applied amino acids, suggesting greater transport of some than of others.The objective of our experiments was to determine if several amino acids were able to enter the phloem with equal facility, and be transported from a source to a sink tissue. We applied radioactive amino acid solutions to an abraded spot on a fully expanded source leaf, and monitored an expanding sink leaf with a GM tube for the arrival of radioactivity. We selected L-serine, an amino acid which was rapidly labeled during photosynthesis in '4C02, and L-leucine and L-lysine, amino acids in which label had not been detected (Cataldo et al., unpublished data). The movement of sucrose, which is known to be...

Relationship between Photosynthesis and Protein Synthesis in Maize

“…In the vascular bundles only a few amino acids are labeled, in accordance with the results obtained in soybean (12) and in oat (23). This could suggest the possible existence of some mechanism limiting the entry of amino acids into the transport path, an assumption supported by the absence of 14C glycine in the intermediary organs.…”

Distribution of nitrogen absorbed during different times of growth in the plant parts of wheat and contribution to the grain amino acids

“…The wide spread of HC label in the phloem amino acids in the former may suggest that these amino acids are derived from protein breakdown. It was reported that lysine and leucine could be transferred in the soybean and oats phloems without being metabolized, whereas serine was metabolized in the transport path (5, 19).…”

Amino Acids Translocated from Turgid and Water-stressed Barley Leaves

“…In unstressed leaves, label began to appear in the monitored area of the sheath about 80 min after the application of either '4CO2, ['4C]glutamate ( Fig. 2A) The results above show only that glutamate and proline can be transported in the phloem if they are applied to the leaf in substrate amounts; it cannot be inferred that these amino acids are endogenous species that normally carry the bulk of the N. In oats (11) and soybean (7), amino acids which are not important phloem constituents enter and move in the phloem as readily as serine (which probably is a major amino acid in the phloem) when they are applied in 0. ['4CJglutamate of high specific radioactivity were supplied to label the endogenous pools of free glutamate, glutamine, and proline in stressed blades.…”