ties of movement were too great to be accounted for by diffusion. Furthermore, when plants were double
labeled with^14 C-sugar,^3 H 2 O, and/or^32 p, it was found that no two nuclides moved together as was pre-
dicted for a flow system. Horwitz [211] and Biddulph [212] attempted to explain these data with a flow
model through differential exchange between sieve tubes and adjacent, nontransporting cells. Nonethe-
less, several alternative proposals were put forth during this period. Included was a proposal for electro-
osmotic pumping by removal of Kfrom the downstream side of sieve plates and cycling it to the up-
stream side [213–215], which would require metabolic energy. The resultant potential gradient across a
sleve plate would drive Kthrough small pores in slime plugs, causing a solution flow. Thaine and
coworkers [216,217] reported observing “transcellular strands” of cytoplasm moving in both directions
in each sieve tube. Canny [218] developed a model of assimilate transport based on this system that would
result in bidirectional movement within one sieve tube. Trip and Gorham [219] supplied different leaves
of a squash plants with either^14 CO 2 or^3 H-sugars and found both nuclides in the same sieve tube. They
interpreted their data as supporting bidirectional movement by transcellular strands.
Both the electo-osmotic and transcellular strand models would require metabolic energy along the
translocation pathway. For many plants, such as bean and squash, cooling of only the translocation
stream, but not source or sink, inhibited translocation [220,221] and respiration. However, Swanson and
Geiger [222] demonstrated that chilled (1°C) petioles of sugar beets translocated sugars at a nonchilled
rate after only a few minutes of acclimation. In addition, Sij and Swanson ([223] and personal observa-
tions), demonstrated that squash, a chilling-sensitive plant, translocated carbon through petioles exposed
to an N 2 atmosphere even though such an environment eventually caused tissue death. Furthermore, Pe-
terson and Currier [224], using a fluorescent dye, demonstrated that bidirectional movement within one
sieve tube was unlikely.
Concurrently, data were accumulating regarding the concentration of solutes and the pressure in
sieve tubes [225–227]. Many of the proposed mechanisms could be eliminated by considering specific
mass transport (g dry wt cm^2 hr^1 ) and velocity of translocation (cm hr^1 ). Crafts and Crisp [1] com-
piled values for specific mass transport per unit cross section of phloem that varied from 0.14 to 4.8 (av-
erage 3.6) g dry wt cm^2 hr^1. Phloem is composed of many cells in addition to functional sieve tubes;
therefore, rates must be two to four times those computed values. Assuming that the total solute in the
sieve tubes is 18% w/v [133], probably a low estimate; using the minimum and maximum (and average)
specific mass transfer rates; and assuming that the area of the sieve tubes is one half to one fourth the to-
tal phloem area [228], velocities of 1.6 to 106 cm hr^1 (average 40 to 80) are obtained. Similar velocities
have been obtained using labeled materials. At average velocity, this would result in material moving
from one end of a sieve tube element to the other in 2 sec (sieve tube element length of 0.03 cm measured
from micrograph in Ref. 229).
Cataldo et al. [230,231] explained the differential movement of^14 C-sucrose and^3 H 2 O by differen-
tial lateral exchange. Then Christy and Ferrier [232] presented a mathematical model of phloem translo-
cation by flow that is in agreement with empirical observation, confirming, in principle, the model de-
veloped by Horwitz [211]. Knoblauch and Van Bel [233] were able to observe sieve tubes transporting a
dye in a flow and then observe that damage induced blockage of transport by accumulation of protein on
sieve plates. All of these observations taken together should elevate the pressure flow “hypothesis” to
“theory” status.
Briefly, as understood today, assimilates are loaded into a sieve tube–companion cell complex [ST-
CC] against a free energy gradient using metabolic energy. This causes the sieve tubes to have an osmotic
potential more negative than other cells (except companion cells) in the source. Water follows osmoti-
cally, causing pressure to develop. In sinks, assimilates are unloaded and water follows. These processes
generate a pressure drop from source to sink and flow results.
V. PHLOEM LOADING
A. General Considerations
Phloem loading refers to the transfer of assimilate into the ST-CC from photosynthetic cells or cells in-
volved in temporary storage. It has been a difficult subject to study, for cells and sites involved in trans-
port cannot easily be isolated from portions of the system supplying assimilates. Therefore, it is a rela-
tively new field of study and has been reviewed [234,235].
434 HENDRIX