In this article, we will discuss translocation and the role of phloem in the process in detail. So, let us get started.

Introduction

Plant cells, tissues, and organs have adaptations that enable them to perform specific functions. The roots, stems, and leaves of the plants collectively form an organ system that carries substances into, around, and outside the plant. The leaves of the plants are the primary organ of photosynthesis. Photosynthesis refers to the process by which plants make their food using light energy.

A green substance known as chlorophyll is essential for photosynthesis, therefore this process occurs only in the green parts of the plant. Plants basically produce glucose as a result of photosynthesis. They produce glucose from simple inorganic molecules such as carbon dioxide and water. Some of the glucose produced by the plants during photosynthesis is employed for respiration. The glucose produced by photosynthesis is converted into sucrose which is transported around the pant in phloem vessels. It must reach all cells in the plant so that sucrose can be transformed back into glucose and used for respiration. Translocation refers to the movement of sucrose and other substances such as amino acids around the plant. Hence, we can define translocation as:

The movement of sugar made as a result of photosynthesis to the rest of the plant for respiration and other processes is known as translocation.

We already know that translocation occurs in phloem vessels. Generally, it happens between the sources and the sinks. Sources are where these substances are created whereas the sinks are where they are employed or stored:

  • In the early springtime, from sources in the root to sinks in the leaves
  • In the summer, from sources in the leaves to sinks in the roots

In the next section, we will compare xylem and phloem vessels in the plants.

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Xylem and Phloem – A Comparison

In this section, we will compare two vascular bundles: xylem and phloem.

  • Xylem: Xylem vessels refer to the columns of hollow and dead reinforced cells that play a fundamental role in transpiration. The main purpose of the xylem vessels is to transport water and minerals from roots to leaves.
  • Phloem: Phloem refers to the columns of living cells that play a fundamental role in translocation. The main purpose of phloem vessels is to transport food substances from the leaves to the remaining parts of the plants and from stores like in the roots.

In the next section of the article, we will summarize and discuss the entire process of translocation in detail.

Translocation Process Summary

  • As a result of photosynthesis, plants produce soluble products like sugars (generally sucrose) and amino acids. These products are collectively referred to as cell sap.
  • Together these substances are moved around the plant in phloem vessels that are composed of living, elongated cells (unlike xylem vessels that are composed of dead cells).
  • Phloem tissue carries dissolved sugars from the leaves to the remaining parts of the plant so that they can be immediately utilized or stored.
  • The cells are attached end to end and have pores in the end cell walls known as sieve plates. These plates enable the easy flow of substances from one cell to another.
  • The movement of sucrose and amino acids in the phloem, from areas of production to the areas of storage and utilization, is referred to as translocation.
  • The transportation in the phloem occurs in several different directions. This transportation depends on the stage of plant development or time of year, but it should be kept in mind that the dissolved food is always moved from the source (where it is produced) to the sink ( where it is utilized or stored).

In the next section of the article, we will discuss how the transportation of food substances occurs in different stages of the plant life cycle and at different times of the year.

How does transportation occur in different stages and times of the year?

  • Winter: During winter, when the majority of the plants have no leaves, the phloem vessels move dissolved sucrose and amino acids from storage organs to the rest of the plants so that plant can continue to respire.
  • Growth period: During the growth period, for instance, springtime, the roots which are storage organs act as a source and many growing parts of the plants will act as sinks.
  • Growth completed: After the completion of plant growth, the leaves photosynthesize and produce a large amount of sugar mainly during the summertime. Hence, the leaves become the source and roots become sinks to store sucrose as a starch so that it can be utilized when needed.

So far, we have already discussed that translocation occurs in phloem tubes. In the next section of the article, we will discuss where the phloem is located and how does it play an important role in translocation

Role of Phloem in Translocation

The phloem transports food substances that are produced by the plant as a result of photosynthesis to where they are required for certain processes like:

  • Growing areas of the plant where they are used immediately
  • Storage organs of the plant, for instance, tubers and bulbs
  • Developing seeds

Movement in phloem occurs both up and down the stem, unlike movement in xylem vessels which only occurs upwards. Phloem is made up of living cells and the cells that make up phloem have the following adaptations to perform certain functions:

  • Sieve tubes: These tubes are specialized to transport substances and do not contain nuclei. There is a perforated end in each sieve tube so that its cytoplasm attaches one cell to another.
  • Companion cells: Companion cells move substances in the phloem and need energy. One r more companion cells attached to each sieve tube are responsible for providing energy. A sieve tube is entirely dependent on its companion cells.

Both vascular bundles xylem and phloem are distributed differently in the stems and roots. Xylem creates a central column in the roots and creates solid support. The phloem is located towards the centre, outside the xylem. In the stems, the transport tissues of the xylem and phloem are grouped into vascular bundles.

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Rafia Shabbir