Plant physiology - Structure & function of phloem.pptx

Plant physiology - Structure & function of phloem

1. Structure and Function of Phloem V. Dhanush Prabhakar I M.Sc Biotechnology 24BIOB002

2. Introduction Phloem is a complex tissue which forms a part of the vascular bundle. Its components are sieve elements, i.e., sieve tube & sieve cells, companion cells, phloem parenchyma & phloem sclerenchyma or phloem fibres. All the components except phloem fibres are living , yet they form a part of the mechanical tissue system. Phloem is chiefly responsible for the translocation of organic solutes – the elaborated food materials in solution. The elements of phloem originate from the procambium of apical meristem or the vascular cambium. Two terms ‘ Bast’ and ‘Leptome ’ are used to refer to phloem. Bast refers to the phloem fibres while leptome refers to the soft-walled parts of the phloem.

3. Sieve elements They include the sieve cells and sieve tubes . From ontogenetic point of view, sieve cell resembles a tracheid while sieve tube resembles a vessel. Sieve tubes These are long tube-like bodies formed from a row of cells arranged in longitudinal series where the end walls are perforated in a sieve-like manner . The perforated end walls are called sieve plates through which cytoplasmic connections are established between adjacent cells. The sieve areas are comparable to primary pit fields present in the primary wall with plasmodesmata connections. The sieve areas are however more prominent and the connecting strands are wider and more conspicuous.

4. An insoluble substance called callose is impregnated into cellulose or replaces cellulose forming a case around each connecting strand which passes through the sieve area. A sieve area in surface view looks like a depression on the wall having a large number of dots , each dot represents a connecting cytoplasmic strand in cross section and remains surrounded by a case of callose. The sieve plate or the perforated end wall is the primary walls of two cells with the middle lamella in between them. The end walls may be obliquely inclined or transverse. A sieve plate is called simple , if it has only one sieve area; whereas the plate may be compound if there are several sieve areas arranged in scalariform, reticulate or other manners .

5. Sieve cells These are comparable to tracheid and are narrow elongated cells without conspicuous sieve areas. They usually have greatly inclined walls, which overlap in the tissue and there are more sieve areas at the ends. Sieve cells are more primitive than the sieve tubes . Sieve tubes are enucleate at maturity or there is disintegration of the nucleus with the maturity of the sieve elements. The wall of the sieve elements is primary and chiefly composed of cellulose. The tubes cannot withstand the pressure from adjoining cells and often get crushed. Protoplasmic strands pass through the pores of the sieve areas and the strands remain surrounded by callose . With the differentiation of the tube the amount of callose increases and finally forms something like a pad on the sieve plate. The pad is referred to as callus pad. Due to its formation the cell-to-cell communication is considerably cut down or entirely prevented. The callus pad is usually formed with the approach of resting or inactive season; and it disappears with the approach of active season. This type is called seasonal or dormancy callus. In old functionless sieve tubes callus becomes permanent called definitive callus or permanent callus.

6. Companion cells Companion cells remain associated with the sieve tubes of angiosperms both ontogenetically and physiologically. They are small elongated cells having dense cytoplasm and prominent nuclei. Starch grains are absent. They occur along lateral walls of the sieve tubes. A companion cell may be equal in length to the accompanying sieve tube element or the mother cell may be divided transversely forming a series of companion cells. A sieve tube element and a companion cell originate from the same mother cell . Their functional association is evident from the fact that companion cells continue so long the sieve tubes function; and die when the sieve tubes are disorganised. In transverse section it appears as a small triangular, rectangular or polyhedral cell with dense protoplast .

7. Companion cells are absent in pteridophytes and gymnosperms . Here small parenchymatous cells remain associated with sieve cells, which are called albuminous cells . They die when the sieve cells become functionless. Thus, the relation between sieve cells and albuminous cells is similar to that existing between sieve tubes and companion cells, except that they have no common origin. Companion cells occur abundantly in angiosperms particularly in the monocotyledons. They are absent in some primitive dicotyledons and also in the primary phloem of some angiosperms. The wall between the sieve tube and companion cell is thin and provided with primary pit-fields.

8. Phloem parenchyma These are the parenchymatous cells associated with the sieve elements. These are living cells with cellulose walls having primary pit-fields. They are mainly concerned with storage of organic food matters. Tannins, crystals and other materials may also be present.

9. In primary phloem the parenchyma cells are somewhat elongate and occur with the sieve elements along the long axis. In secondary phloem, parenchyma is of two types, those which occur in vertical series are called phloem parenchyma and others occurring in horizontal planes are known as ray cells . The cell wall is primary, composed of cellulose. Parenchyma is absent in the phloem of monocotyledons .

10. Phloem sclerenchyma or phloem fibres Sclerenchymatous fibres constitute a part of phloem in a large number of seed plants, though they are rare in pteridophytes. They occur both in primary as well as secondary phloem. These are typical elongated cells having inter-locked ends, lignified walls with simple pits. The fibres of primary phloem are essentially similar to those occurring in cortex and secondary phloem. They are of considerable commercial importance, as these fibres are abundantly used for the manufacture of ropes and cords. Sclerotic cells are also often present in primary phloem. They probably develop from parenchyma with the age of the tissue. (Secondary sclerosis) .

11. Function of Phloem The primary function of the phloem is to transport organic compounds, primarily sugars, produced during photosynthesis in the leaves to other parts of the plant. This process is called translocation and relies on both active and passive mechanisms. Companion cells play a vital role in maintaining the pressure gradient required for the movement of nutrients through the phloem. P hotosynthetic products ( photoassimilates ) is facilitated by phloem elements. Two important terms in phloem transport are Source :  It is the part of plant that synthesizes food. Sink :  The part of plant that needs or stores the food. Sucrose is basically transported by the vascular tissues phloem from the source to the sink. Storage or photosynthesizing organs, which have surplus sugars, can either metabolize or export them. These are known as  source . On the contrary, actively metabolizing organs or the ones which store carbohydrates need to import them. These plant parts are known as  sinks.

12. Phloem loading and unloading Phloem loading refers to the transfer of sugar from mesophyll cells (source) to sieve tube elements, and phloem unloading refers to the transfer of sugar from sieve tube elements to roots or other storage cell s. There are two different types of phloem loading mechanisms : Active Phloem Loading: It is also known as the sucrose-H+ cotransporter mechanism. This method involves an energy-driven movement of sugars from apoplast, or cell walls outside the plasma membrane, to sieve phloem tubes. Passive Phloem Loading: Organic solutes move freely from mesophyll cells through symplast (i.e. from cell to cell) to sieve tubes of phloem element via companion cells through plasmodesmata.

13. Phloem unloading occurs similar to phloem loading, either by symplast or apoplast. When sugar arrives at the receiving end, it is unloaded from the filter tube into the cells or sink. There are three types of phloem unloading mechanisms. Sieve Element Unloading: In this procedure, imported sugars leave sink tissue sieve components. Short Distance Transport: A short-distance pathway, also known as post-sieve element transfer, is now being used to transport the sugars to the cells in the sink. Storage and Metabolism: Carbohydrates are finally stored or metabolised in the cells of the sink. Generally, when sucrose consumption rates are very high and sink cells are metabolically very active, as in the meristematic tissue of developing roots, fruits, leaves, etc., symplast is used for phloem unloading. When storage organs like fruits (grapes, oranges, etc.) and roots have sink cells, sucrose unloading happens through the apoplast.

14. Mass Flow Hypothesis

15. Girdling and Its Impacts Girdling is a practice that removes a ring of bark (the phloem layer) from around the entire circumference of a tree or plant stem. This disrupts the downward transportation of sugars and other metabolites from the leaves through the phloem. Girdling can cause the death of a tree because it interrupts the supply of food from leaves to the roots, essentially starving the plant. However, girdling also has a deliberate use in horticulture . It encourages the plant to produce larger fruits or to direct the plant’s energy towards certain branches. By disrupting the flow of nutrients, the plant overcompensates in the remaining portions, often leading to increased yield or size of the produce.

16. References Lucas, W.J., Groover, A., Lichtenberger, R., Furuta, K. , et al., (2013), The Plant Vascular System: Evolution, Development and FunctionsF. Journal of Integrative Plant Biology, 55: 294-388. https://doi.org/10.1111/jipb.12041 https://www.geeksforgeeks.org/phloem-function-structure/ https://sciencenotes.org/xylem-and-phloem-plant-vascular-system/ Slideshare - PHLOEM https://www.slideshare.net/slideshow/phloempptx/254512086