Sovonick S A, Geiger D R, Fellows R J
Department of Biology, University of Dayton, Dayton, Ohio 45469.
Plant Physiol. 1974 Dec;54(6):886-91. doi: 10.1104/pp.54.6.886.
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 (14)C-sucrose to approximately 20% of the control. Dinitrophenol at a concentration of 8 mm inhibited rather than promoted CO(2) 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 energy-requiring process for both phloem loading and translocation in the source leaf.Application of (14)C-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 (14)C-sucrose solutions. The first and possibly the second phases were attributed to active, carrier-mediated accumulation in the minor vein phloem. Autoradiography of the tissue confirmed that most of the sucrose was localized in the minor veins. Data from uptake through the abraded surface of intact leaves, the most reliable method, were analyzed by the Hofstee method. Kinetic parameters, analogous to Km and V(max) of enzyme studies, were calculated to be: K(j) = 16 mm and J(max) = 70 mug C/min dm(2) or 490 nmoles sucrose/min.dm(2). Rates for phloem loading and translocation of exogenous sucrose are equal to or greater than those observed for compounds derived from photosynthetically fixed CO(2). The data indicate that a free space sucrose concentration in the region of the minor vein phloem of approximately 20 mm can support translocation at the rates commonly observed for photosynthetically produced sugars.
对甜菜（Beta vulgaris, L.）源叶中的韧皮部装载进行了研究，以确定其对能量代谢的依赖程度以及载体系统的参与情况。浓度为4 mM的二硝基苯酚解偶联呼吸作用，使源叶ATP降至对照叶水平的约40%，并将外源供应的（14）C - 蔗糖的转运抑制至对照的约20%。浓度为8 mM的二硝基苯酚抑制而非促进CO₂产生，表明存在除呼吸解偶联之外的抑制机制。8 mM的二硝基苯酚也将ATP降至对照源叶水平的约40%，并将外源蔗糖的转运降至对照的约10%。向未处理的源叶施加4 mM ATP使转运速率比对照提高了约80%，而在用4 mM二硝基苯酚处理的叶片中，4 mM ATP将转运恢复到对照水平。当向用8 mM二硝基苯酚处理的叶片施加ATP时，未观察到转运的恢复。结果表明源叶中韧皮部装载和转运都需要能量。通过源叶上表面施加一系列浓度的（14）C - 蔗糖溶液，产生了一个用于从喂食区域转运出去的双相等温线。对于漂浮在（14）C - 蔗糖溶液上的叶盘进行韧皮部装载也获得了类似的双等温线。第一阶段以及可能的第二阶段归因于小叶脉韧皮部中由载体介导的主动积累。组织的放射自显影证实大部分蔗糖位于小叶脉中。通过完整叶片磨损表面摄取的数据（最可靠的方法），采用霍夫斯泰方法进行分析。计算出类似于酶研究中的Km和V(max)的动力学参数为：K(j) = 16 mM，J(max) = 70 μg C/min dm² 或490 nmoles蔗糖/min.dm²。外源蔗糖的韧皮部装载和转运速率等于或大于光合固定CO₂衍生化合物所观察到的速率。数据表明小叶脉韧皮部区域中约20 mM的自由空间蔗糖浓度可以支持以光合产生的糖通常观察到的速率进行转运。
