All parts of the plant require a continuous supply of food for their nutrition and development,this is referred to as translocation. Plants differ in their cell arrangement. In simple plants like spirogyra, all cells are green, and each cell can make enough food for its requirement. The higher plants have a great differentiation of structure and division of labor. In higher plants, green cells mainly stick to the leaves, which make up the chief center where synthesis of carbohydrates takes place. The non-green parts of the plant like stem and root get their food supply from leaves. Before food can reach the non-green parts, there must be a proper infrastructure for transferring food. It must travel from leaves through intervening cells and tissues. The food material travels within the plant in the form of a solution in water.
This cell-to-cell movement of food material from one part of the plant to another or through a tissue is the translocation or conductance of solutes. Source is the point where plant synthesis food and sink is the part where plants use food.
Longitudinal translocation in plants
Translocation of food material takes place in plant in different directions. Mainly, food material travels in a downward and upward direction. The path of food flow is not constant throughout the life of the plant. At one time, the flow of the food is upwards, and at other times its direction is downward. The plant manufactures food material in leaves and that material travels downward to the stem and root. The synthesized food nourishes old cells and builds new ones. Plant stores some food in storage organs. In these cases, food is rapidly migrating from the seeds or the vegetative storage organs to the growing tips where they use it rapidly.
This upward movement continues till the seedling or the new shoot develops photosynthetic activity. After that plant reverses the direction of translocation. Upward movement takes place during the development of cereal grains where the ear head is terminal. Another instance of upward movement of food is in the stems of woody plants. The food stored in older parts move upward to the sprouting buds. So we can understand that the direction of transport of food is from the region of higher concentration towards lower concentration.
Lateral translocation in plants
Lateral translocation involves the movement of nutrients both outward to the cortex and inward to the center.
Path of translocation in plants
In simple plants, like thallophyta and Bryophyta, the problem of translocation is simple as the substances move from only small distances. On other hand in high plant, long distances intervene between source and sink, and food material has to travel a long distance even up to several feet in tall trees. In such cases, there must exist adequate channels that carry food. Upward and downward movement of food takes place in the phloem.
Anatomy of phloem
Phloem is a composite tissue comprising five different cells.
- Sieve tubes
- Phloem parenchyma
- Companion cells
- Phloem fibers
- Phloem ray cells
The phloem fibers are dead cells, therefore they are unfit for translocation. They constitute the mechanical tissue of the phloem.
The ray cells assist in lateral translocation, and it is a very slow process. Longitudinal translocation in both directions, occurs mostly in sieve tubes.
Sieve tubes are the most dominant part of the phloem, and are the only living element that are present in vascular plants.
The companion cells are absent in gymnosperms, and phloem parenchyma are absent in monocots. In conifers, the albuminous cells perform the function of companion cells.
Mechanism of phloem conductance
There are three hypotheses or explanations for longitudinal conductance of food substances in sieve tubes, but none is entirely satisfactory.
We know that the direction of the flow of food substance is from the region of higher concentration of solutes to lower concentration. Translocation continues as long as there is positive concentration gradient from supply end to consumption end. Moreover, the rate of translocation increases with the increase in steepness of gradient.
Streaming of protoplasmic theory
According to DeVries (1985), the rapid translocation is due to the streaming movement of protoplasm in sieve tubes. The rapid streaming movement of the protoplasm carry the enclosed solutes from one end to another end.
This hypothesis assumes that the solutes translocate in the sap of sieve tubes, which flow from supply end to consumption end under a turgor pressure gradient.