Phloem Transport: Flow from Source to Sink – Have you ever wondered how plants transport their food from leaves to any other part of a plant without having any specific circulatory system, as seen in animals? Plants transport water and minerals over longer distances using vascular tissues (the xylem and phloem). In rooted plants, the transport is unidirectional and occurs through the xylem, which runs from roots to stems.

Whereas, phloem is a complex living tissue present in vascular plants which transports the organic compounds made by photosynthesis called photosynthates in a bidirectional manner, i.e., upward and downward or from source to sink. Plant leaves produce glucose through photosynthesis, which gets converted into sucrose for transport and finally stored as starch. Read this article to know more about Phloem Transport: From Source To Sink.

Significance of Transport of Substances in Plants

Translocation in Phloem

The movement of various molecules, like sucrose, amino acids, etc., through phloem in a plant, is called translocation in the phloem. The translocation in the phloem is not affected due to gravity. Sucrose is instead translocated from the point of supply (leaf) to the point of metabolism or storage, referred to as a sink. These source and sink points can be reversed depending on the plant’s need.

For example, e.g., in deciduous trees, sugar moves from root to the growing buds in early spring and summer from photosynthesizing leaves to roots, showing the bidirectional movement of sap in the phloem. In contrast, the movement in the xylem is unidirectional, i.e., always upwards. Hence, the food in phloem sap can be transported in any required direction depending upon the need.

Composition of Phloem Sap

The organic matter which is translocated through phloem is known as phloem sap. It contains sucrose and water, hormones (auxin, gibberellins, cytokinins, and abscisic acid), amino acids, and other sugars.

Pathways of Translocation

1. Sugars and other organic materials travel in the plant’s phloem cells by means of sieve elements.
2. The term sieve element encompasses both the highly differentiated sieve cells of gymnosperms as well as the relatively unspecialized sieve cells of angiosperms.
3. Furthermore, the phloem tissue has companion cells and parenchyma cells in addition to sieve elements.
4. Additionally, fibres and sclereids (for protection and strengthening of the tissue) and laticifers (latex-containing cells) are present in phloem tissue. However, only sieve cells directly participate in translocation.

Mechanism of Translocation In The Phloem

The process of translocation of sugars from source to sink in plants is best explained by the mass flow hypothesis or pressure flow hypothesis, given by German physiologist Ernst Munch in 1930 and elaborated by Grafts. According to this hypothesis-

1. The movement of organic matter (sucrose) moves in solution form from source to sink due to the osmotic pressure gradient developed between them.
2. Glucose is produced in the mesophyll cells of the leaves by photosynthesis, which has been converted into sucrose (disaccharide sugar) for transportation.
3. Sucrose moves from the mesophyll cell to sieve-tube companion cells by active transport. This process is known as phloem loading.
4. The loading of sucrose into the phloem produces hypertonic conditions and negative osmotic potential. Hence, water from the adjacent xylem moves into the phloem by osmosis generating a high-pressure potential.
5. As the osmotic pressure builds up, the phloem sap moves towards the region of low osmotic pressure, which is maintained at the sink region.
6. At the sink region, the sucrose moves out from the phloem sap through an active process. As sucrose is removed, osmotic pressure decreases, and water moves out of the phloem, making the sieve cells flaccid. This process is known as phloem unloading.

Experiment to Prove Phloem Transport – Girdling Experiment

Malpighi gave this experiment to demonstrate the translocation pathway of food and identify the tissues involved in it. The following steps are involved in this experiment:
1. To remove the phloem, a ring of bark is removed from the trunk of the woody plant.
2. It was observed that the bark above the bark ring was swollen after a few weeks.
3. It does not cause the leaf to wilt, but growth below the ring was reduced.          
4. The swelling of bark above the ring is due to the accumulation of food in that region as the translocation of food was stopped; on the other hand, the upward movement of water was not affected.
5. This experiment proves that the phloem performs the translocation of food.

Fig: Girdling Experiment/ Ringing Experiment

Summary

At the source, glucose is produced by photosynthesis, converted to sucrose (sugar), and transported to the different parts of the plant depending on their needs. This sucrose is then moved into sieve tube cells by active transport. It produces hypertonic conditions in the phloem. Osmosis moves water from the adjacent xylem into the phloem.

As a result of high osmotic (turgor) pressure, phloem sap moves to the lower-pressured areas. Osmotic pressure decreases at the sink. Phloem actively transports sugar out when it enters. The osmotic pressure decreases upon removing the sugar, leading to water movement out of the phloem and into the xylem.

Learn Means of Transport in Plants

Frequently Asked Questions (FAQs)

Q.1. Who proposed the mass flow hypothesis?
Ans: German physiologist Ernst Munch proposed the mass flow hypothesis.

Q.2. What is the direction of flow in phloem?
Ans: The movement in phloem is bidirectional.

Q.3. Name the form of carbohydrates that are transported in plants as food.
Ans: In plants, food is transported in the form of sucrose.

Q.4. What are the main components of phloem sap?
Ans: The main components of phloem sap are sugars, amino acids, vitamins, organic and inorganic acids.

Q.5. Which plant tissue is responsible for food transport?
Ans: Food is transported from the source to the sink by phloem. Such translocation is bidirectional as the source-sink relationship is variable.

We hope this detailed article on phloem transport helped you in your studies. If you have any doubts, queries or suggestions regarding this article, feel to ask us in the comment section and we will be more than happy to assist you. Happy learning!