Department of Botany, University of Georgia, Athens, Georgia 30602.
Plant Physiol. 1978 Feb;61(2):283-90. doi: 10.1104/pp.61.2.283.
The movement of (14)C-photosynthate in morning glory (Ipomea nil Roth, cu. Scarlet O'Hara) vines 2 to 5 meters long was followed by labeling a lone mature leaf with (14)CO(2) and monitoring the arrival rate of tracer at expanding sink leaves on branches along the stem. To a first approximation, the kinetic behavior of the translocation profiles resembled that which would be expected from movement at a single velocity ("plug flow") without tracer loss from the translocation stream. There was no consistent indication of a velocity gradient along the vine length. The profile moved along the vine as a distinct asymmetrical peak which changes shape only slowly. The spatial distribution of tracer along the vine reasonably matched that predicted on the basis of the arrival kinetics at a sink, assuming plug flow with no tracer loss. These observations are in marked contrast to the kinetic behavior of any mechanism describable by diffusion equations.However, a progressive change in profile shape (a symmetrical widening) was observed, indicating a range of translocation velocities. A minimum of at least two factors must have contributed to the observed velocity gradient: the exchange of (14)C between sieve elements and companion cells (demonstrated by microautoradiography) and the range of velocities in the several hundred sieve tubes which carried the translocation stream. Possible effects of these two factors on profile spreading were investigated by means of numerical models. The models are necessarily incomplete, due principally to uncertainties about the exchange rate between sieve elements and companion cells and the degree of functional connectivity between sieve tubes of different conductivities. However, most of the observed profile spreading may be reasonably attributed to the combined effects of those two factors.The mass average velocity of translocation (calculated from the mean times of (14)C arrival at successive sink leaves) was about 75% of the maximum velocity (calculated from the times of initial detection at the same sink leaves), which was usually between 0.6 and 1 cm min(-1). Owing to tracer exchange between sieve elements and companion cells, the mass average velocity of tracer in the sieve tubes was probably closer to 86% of the maximum velocity, a figure which agreed with a predicted velocity distribution based on calculated sieve tube conductivities and the size distribution of functional sieve tubes.
对 2 到 5 米长的 morning glory（Ipomea nil Roth，cv. Scarlet O'Hara）藤本植物的（14）C-光合产物的运动进行了跟踪，方法是用（14）CO（2）标记单个成熟叶片，并监测茎上分支处扩展的汇叶中示踪剂的到达率。从第一近似值来看，示踪剂的输运分布的动力学行为类似于在没有示踪剂从输运流中损失的单一速度（“塞流”）下的运动。没有迹象表明在藤本植物的长度上存在速度梯度。该分布沿着藤本植物移动，形成一个独特的不对称峰，其形状仅缓慢变化。示踪剂在藤本植物上的空间分布与基于汇到达动力学的预测基本匹配，假设塞流且没有示踪剂损失。这些观察结果与可用扩散方程描述的任何机制的动力学行为形成鲜明对比。然而，观察到的分布形状（对称变宽）发生了渐进变化，表明存在一系列输运速度。至少有两个因素必须导致观察到的速度梯度：筛管和伴胞之间的（14）C 交换（通过微放射自显影证明）和携带输运流的数百个筛管的速度范围。通过数值模型研究了这两个因素对分布扩展的可能影响。由于筛管和伴胞之间的交换率以及不同电导率的筛管之间的功能连通性程度存在不确定性，因此模型必然是不完整的。然而，大部分观察到的分布扩展可能归因于这两个因素的综合影响。从示踪剂到达连续汇叶的平均时间计算，输运的质量平均速度（calculated from the mean times of 14C arrival at successive sink leaves）约为最大速度（calculated from the times of initial detection at the same sink leaves）的 75%，最大速度通常在 0.6 到 1 cm min（-1）之间。由于筛管和伴胞之间的示踪剂交换，筛管中示踪剂的质量平均速度可能更接近最大速度的 86%，这一数字与基于计算出的筛管电导率和功能筛管的大小分布的预测速度分布一致。
