PHLOEM TRANSPORT

PHLOEM TRANSPORT

3.     PHLOEM TRANSPORT

The movement of substances in phloem tissue is called translocation. The main moving substances are sucrose and amino acids, which are in solution in water. These substances have been made by the plant and are called assimilates. Translocation by phloem is a bidirectional process.
Girdling experiment demonstrates that translocation of organic solutes occurs through the phloem. In this classic experiment, the bark of a tree was removed in a ring around the trunk (called girdling). Bark describes all tissues external to the vascular cambium consisting mainly of the secondary phloem and layers of periderm. With this experiment, it has been observed that girdling has no immediate effect on transpiration. However, transport of organic solutes in the trunk is blocked at the site where the bark has been removed the organic solutes accumulate about the girdle. Eventually, the bark below the girdle dies while the bark above swells and remains healthy.

3.1.    Phloem tissue
3.1.1.    Sieve elements
Cells of the phloem through which photosynthesis assimilates, move throughout the plant. Sieve elements include both the highly specialized since tube elements typical of the angiosperms and the relatively less specialized sieve cells of gymnosperms.

Sieve tube elements are elongated living cells. As they mature, they undergo a series of progressive chains that result in the breakdown and loss of the nucleus, the tonoplast, oil bodies and ribosomes. The sieve tube elements of most angiosperms also contain a protein called P-protein. P-protein seals off damaged since elements by plugging up the sieve plate pores. Sieve elements are joined to the end with pore-filled sieve plates to make a sieve tube.
In gymnosperms, sieve elements are called sieve cells. In sieve cells, there is no sieve plate and all sieve areas are similar. Pores in sieve areas appear blocked with membranes.
3.1.2.    Companion cells 
Alongside each sieve-tube element is a nonaddicting cell called a companion cell which is connected to the sieve-tube elements by numerous channels, plasmodesmata.
Companion cells (albuminous cells in gymnosperms) have a dense cytoplasm, mitochondria, nucleus, Golgi, endoplasmic reticulum and chromoplasts.
Although their function is not well understood, they can be considered Russe cell to the sieve tube elements. These cells are derived from the same cambial initial cell as the sieve tube elements. There are 3 different types of companion cells
(i)    Ordinary companion cells – have smooth walls and few or no plasmodesmata connections to cells other than the sieve tube.
(ii)    Transfer cells – are like ordinary companion cells but have much-folded walls that are adjacent to non-sieve cells, allowing for large areas of transfer.
(iii)    Intermediary cells – have smooth walls and numerous plasmodesmata connecting them to other cells.
Transfer cells transfer sugars from the apoplast to the simplest of the sieve.
Intermediate cells function in the symplastic transport of sugars from mesophyll cells to sieve elements in plants.
3.2.    Pattern of translocation
Materials are translocated in the phloem from source to sink. An organ or tissue that produces more assimilate than it requires for its own metabolism and growth, are known as the source. Mature leaves, exporting storage organ such as beet or carrot roots in second year and cotyledons and endosperm cells of seed are sources. On the other hand on organ or tissue that imports photoassimilate is a sink. Any growing, storing or metabolizing tissues or organs such as root, developing fruits act as a sink. Since the source-sink relationship is variable, the direction of movement in the phloem is bidirectional.
3.3.    Mechanism of translocation
The pressure flow hypothesis first proposed by E. Munch in 1926 is the most accepted mechanism of phloem translocation. It states that the flow of solution in the sieve–elements are driven by an osmotically generated pressure gradient between source and sink tissue. The gradient is a consequence of phloem loading at the source and phloem unloading at the sink. According to the pressure-flow hypothesis, translocation of solute is a passive process.

Phloem loading and unloading
Transfer of photoassimilates into the sieve elements at the source end is called phloem loading. Similarly, the transfer from sieve elements into the target cells at the sink end is called unloading. Phloem loading and unloading occurs through both symplastic as well as apoplastic pathways.
In symplastic phloem loading, photo assimilates are translocated into the sieve tubes through specialized companion cells, called intermediatory cells by a large number of plasmodesmata. In contrast, in apoplastic phloem loading, photoassimilates are first transported from the mesophyll cells via bundle sheath cells to the extracellular apoplastic region and then by active transport into the specialized companion cells called transfer cells. From transfer, cells photo assimilates subsequently enter into the sieve tube elements via plasmodesmata. The loading of sucrose and amino acids from the apoplast into the sieve elements proceeds via a proton symport. This is driven by a proton gradient. The proton gradient is generated by a H+ – ATPase present in the plasma membrane. There are again two possibilities of phloem unloading. In symplastic unloading, the phloem sap reaches the cells of the sink organs from the sieve tube element – companion cell complex via plasmodesmata. In apoplastic unloading, the substances are first transported from the sieve tube element - companion cell complex to the extracellular apoplastic region and are then taken up into the cells of the sink organs. Unloading through apoplastic pathway is an active process and requires metabolic energy.


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