Since the source-sink relationship is variable, the direction of movement in the phloem can be upwards or downwards, i.e.,
bi-directional. This contrasts with that of the xylem where the movement is always unidirectional, i.e., upwards.
Hence, unlike one-way flow of water in transpiration, food in phloem sap can be transported in any required
direction so long as there is a source of sugar and a sink able to use, store or remove the sugar.
direction of translocation of solutes
Solutes are translocated in the plants in downward, upward or radial direction. Path of upward and downward translocation
is phloem, while path of radial translocation is medullary rays.
Downward translocation : It is of most important type, i.e., from leaves to stem and roots.
Upward translocation : From leaves to developing flowers, buds, fruits and also during germination of seeds and
tubers, etc.
Radial translocation : From pith to cortex and epidermis.
Evidences in favour of downward conduction of solutes by phloem
Downward conduction of solutes takes place through phloem is evident from the following points.
Suitability : Phloem has long tubes (most suitable for long distance transport) placed one above the other. Thus, phloem
seems to be the only suitable tissue for downward conduction of solutes.
Chemical analysis of phloem sap : Chemical analysis of sieve tube sap proves that concentrated solution of sucrose
is translocated from the place of synthesis to other parts of the plant body. Glucose and fructose are sometimes found
in traces only. The amount of sucrose is more in phloem sap during the day and less in night.
Isotopic studies : If leaf of a potted plant is illuminated in the presence of radioactive C^14 O 2 , it forms radioactive
products of photosynthesis which are then transported to stem. It was detected by autoradiographic studies that these
substances are translocated through phloem particularly sieve tubes. Radioactivity is found below and above the nodes
of the leaf to which radioactive carbon was provided.
Ringing or girdling experiment : In this experiment, all the tissues of the stem
outside the vascular cambium are removed in a ring. The upper part of the plant
is attached to lower part only by external xylem cylinder and pith. Accumulation
of food occurs in the form of swelling just above the ring, which suggests that in
absence of phloem, downward translocation of food is stopped. In a girdled plant,
roots die first and ultimately shoot dies. This is because the upper part of stem
gets ample amount of water supply (as transport of water and minerals occurs
through xylem). But as the roots die due to starvation, the upper part of stem also
dies in the course of time.
mechanism of translocation of solutes
Several theories have been put forward to explain the mechanism of translocation of organic nutrients through phloem,
but Munch mass flow hypothesis or pressure flow hypothesis is most accepted one. It was given by Munch
(1930) and supported by Crafts.
According to this theory, translocation of solutes takes place in bulk, through phloem along a gradient of turgor pressure
from higher concentration of its soluble form (source) to lower concentration of its soluble form (sink).
Because in leaves (source), osmotic concentration remains always high (due to photosynthesis) and in roots (sink),
osmotic concentration remains low, so mass flow of organic food continues from leaves to roots (i.e., along a gradient
of turgor pressure).