The onset of export during leaf development was correlated with changes in metabolism and ultrastructure and with patterns of solute distribution in the developing seventh leaf of sugar beet (Beta vulgaris L.) in order to study the cause of initiation of translocation. Infrared gas analysis of carbon dioxide uptake showed a broad peak for net photosynthesis dm(2 at 35 to 40% final laminar length. Pulse labeling with 14CO2 demonstrated that maximum import of translocate occurred at 25% final laminar length; export was first observed at 35% final laminar length. Between 40 and 50% final laminar length a rapid increase in amount of export occurred, primarily as a result of the increase in the area of leaf which was exporting. Whole leaf autoradiography revealed that onset of phloem loading spread basipetally from the leaf tip; loading was initiated at about 22% final laminar length and was essentially complete by 50% final laminar length. Those areas which clearly exhibited loading no longer imported from other parts of the plant while the area in transition still appeared to import label from source regions.There was little difference between source and sink leaf tissue in the kinetic parameters Kj and Jmas (30) for uptake of exogenous sucrose supplied via free space. The concentration of solutes in sieve elements and companion cells of the sink leaf was highest in the mature tip area and gradually decreased in the direction of the immature base. There appeared to be no dramatic structural transformation within the phloem of the minor veins that was closely correlated with the time when phloem loading or export began. Rather, there appeared to be a gradual differentiation of phloem which resulted in a sizable proportion of the population of minor vein sieve elements and companion cells attaining maturity in the older sink regions prior to initiation of phloem loading. The area of the leaf undergoing development appeared to exhibit the beginnings of phloem loading 30 to 45 hours prior to onset of export. Import continued into the area in transition until the full level of vein loading was attained. Structural maturation of the phloem and onset of phloem loading are felt to be more preparatory in nature rather than immediately causal events which triggered export.The initiation of export out of a developing leaf, we believe, is the result of the increasing solute content within the sieve element and companion cells of the minor veins, in particular.The higher osmotic pressure in the sieve tubes causes a reversal of the previously inward directed gradient and produces a mass

Soils amended with [14C]hexahydro‐1,3,5‐trinitro‐1,3,5‐triazine (RDX) were sampled over 60 d and subjected to exhaustive Soxhlet extraction followed by HPLC analysis. RDX was the only radiolabeled compound observed in soil extracts. Emission of volatile organics and 14CO2 from soil accounted for only 0.31% of the amended radiolabel. Mass balance for RDX‐amended soil was better than 84% throughout the two‐month study. The analytical method developed for plants involved acid hydrolysis, solvent extraction, fractionation on Florisil® adsorbent and separation by HPLC. The described methodology allowed for RDX recovery of 86 ± 3% from fortified bush bean leaf tissue. Further experiments were conducted with bush bean plants maintained on RDX‐containing hydroponic solutions. Hydroponic plants did not emit detectable amounts of 14CO2 or radiolabeled volatile organics. Analysis of the plant tissue indicated bioaccumulation of RDX in the aerial tissues of hydroponic plants exposed for either 1 or 7 d. Metabolism of RDX to polar metabolites was observed in plants exposed for 7 d.

Phloem loading in source leaves of sugar beet (Beta vulgaris, L.) was studied to determine the extent of dependence on energy metabolism and the involvement of a carrier system. Dinitrophenol at a concentration of 4 mM uncoupled respiration, lowered source leaf ATP to approximately 40% of the level in the control leaf and inhibited translocation of exogenously supplied '4C-sucrose to approximately 20% of the control. Dinitrophenol at a concentration of 8 mM inhibited rather than promoted C02 production, indicating a mechanism of inhibition other than uncoupling of respiration. The 8 mM dinitrophenol also reduced ATP to approximately 40% of the level in the control source leaf and reduced translocation of exogenous sucrose to approximately 10% of the control. Application of 4 mM ATP to an untreated source leaf promoted the translocation rate by approximately 80% over the control, while in leaves treated with 4 mM dinitrophenol, 4 mM ATP restored translocation to the control level. No recovery of translocation was observed when ATP was applied to leaves treated with 8 mM dinitrophenol. The results indicate an energyrequiring process for both phloem loading and translocation in the source leaf.Application of '4C-sucrose solutions in a series of concentrations through the upper surface of a source leaf produced a biphasic isotherm for translocation out of the fed region. A similar dual isotherm was obtained for phloem loading with leaf discs floated on 14C-sucrose solutions. The first and possibly the second phases were attributed to active, carriermediated accumulation in the minor vein phloem. Autoradi The source leaf, a sugar-exporting region of a plant, seems to play a key role in providing the driving force for long distance transport of organic solutes throughout the plant. Few translocation studies have been focused on events in the source leaf, and some crucial questions remain unanswered. Is there, for example, an energy-requiring step in the transport of sugar from chloroplasts to sieve elements that is directly related to phloem loading and long distance transport. If an active step is demonstrated, do the kinetics of uptake indicate that this step involves a membrane carrier?In 1939, Curtis and Asai (3) suggested that the osmotic gradient from the mesophyll to the sink tissue was in the opposite direction required for a pressure flow mechanism. Roeckl (26) observed that the mesophyll cells, which produce mobile sugars, have a substantially lower osmotic potential than the phloem exudate. A gradient of increasing concentration toward the sieve tubes would require energy to drive transport in that direction, suggesting the need for an active step between sugar production and translocation out of source leaves. Barrier and Loomis (1) termed the active step "loading," implying that the rate-limiting step is transport into the veins, rather than a simple chemical transformation or synthesis of the translocate. There now seems to be general agreement that an active step does occur in source tissue (10,...

The findings of this report are not to be construed as official Department of the Army positions unless so designated by other authorized documents DISClAIMER This report was prepared as an account of work sponsored by an agency of the United States Government. Neither the United States Government nor any agency thereof, nor Battelle Memorial Institute, nor any of their employees, makes .tny wMr.Jnty, expressed or implied, or assumes .tny legal liability or responsibility for the accur.tcy, completeness, or usefulness of .1ny information, apparatus, product, or process disclosed, or represents th.tt its use would not infringe printely owned rights. Reference herein to any specific commercial product, process, or service by trade name, trademark, manufacturer, or otherwise, does not necessarily constitute or imply its endorsement, recommendation, or favoring by the United States Government of any agency thereof, or Battelle Memorial Institute. The views and opinions of authors expressed herein do not necessarily state or reflect those of the United States Government or any agency thereof.

No abstract