Translocation of Solutes and Mass flow hypothesis

a simple explanation of Translocation of Solutes through phloem and Mass flow hypothesis

1. WELCOME

2. T ranslocation of solutes SUBMITTED BY ANANDHALAKSHMI A. L BSc Botany 2016-19 GOVERNMENT COLLEGE KOTTAYAM

3. Translocation Long distance transport of materials through xylem and phloem. Xylem translocation involves the transport of absorbed water and minerals from roots to leaves . Phloem transport involves the transport of the organic food from green parts to non-green parts for consumption or storage. Translocation of photosynthates , translocation of photoassimilates or translocation of organic solutes. It is polar and occurs from source to sink .

4. Translocation of organic solutes

5. Direction of Translocation Downward translocation Occurs from leaves to stem and roots The food synthesised in leaves are transported to sinks in the stems and roots. Upward translocation Occurs from leaves to developing buds, flowers and fruits for consumption and storage. In germinating seeds , stored food is transported upward from cotyledons or endosperm to the growing apex Transport of organic food during the appearance of new foliage During the renewal of growth on deciduous trees in spring. Radial(lateral) translocation Occurs from the cells of pith to those of cortex and epidermis It is common during the development of fruits and seeds

6. Bidirectional translocation Occurs near stem apices or at the growing points of stem nodes . The food received from mature leaves is transported upward to growing points and downward to storage regions or consumption regions.

7. Pathway of translocation

8. Phloem translocation Active Physiological process Passive Physical process

9. Mechanism of translocation Phloem loading Phloem unloading

10. PHLOEM LOADING The initial transfer of photosynthate from leaf mesophyll to the seive elements of minor veins. It is the transfer of sugars from mesophyll cells to phloem cells. It occurs against the concentration gradient. Hence it involves energy expenditure Thus, its an active process Selective process that occurs at the source Amino acids, reducing sugars and organic ions are most usually taken up. Phloem loading symplastic apoplastic

11. Symplastic loading Seive tubes are connected to adjacent cells through plamodesmata . Primitive and less efficient mechanism Plants with symplastic loading have low relative growth rate. Van Bel and Gamalet (1992) postulated that the mode of phloem loading depends on the degree of plasmodesmatal connectivity i.e , connectivity, symplastic translocation.

12. Apoplastic loading Takesplace in the absence of plasmodesmata connections. Acts against environmental adversaries, such as drought and low temperature. Plants with apoplastic phloem loading have relatively higher growth rate and better adaptations to environmental stresses.

13. PHLOEM UNLOADING Reverse of phloem loading Transfer of solutes from sieve tubes to the sink Occurs in a concentration gradient Can be symplastic or apoplastic

14. Theories about phloem translocation Short-distance translocation Protoplasmic streaming Contractile protein hypothesis Diffusion hypothesis Activated diffusion theory Long-distance translocation Munch’s mass flow hypothesis

15. Theories

16. Pressure flow theory (Munch Hypothesis) Proposed by Munch (1926) Osmotic mechanism controls downward movement of organic substances in phloem. During photosynthesis the cell sap concentration of mesophyll cells at the top is maintained high. As a result osmotic pressure increases and causes absorption of water. These two factors combine to produce a high turgor pressure in mesophyll cells. The mesophyll cells remain interconnected through small pores known as plasmodesmata . These connections occur from cell wall of mesophyll to sieve tubes. Through these connections some of the solution is forced down to sieve tubes due to torgur pressure. The loss of solutes from mesophyll cells would then be compensated by newly formed photosynthetic products and movement of water from xylem The movement is always from a region of higher to lower turgor pressure. A turgor pressure gradient is established between source and sink.

17. Diagrammatic representation

18. Mesophyll cells

19. Evidence Widely accepted It is supported by the fact that phloem sap, containing high sugar content, exudates from the cut end of a stem. Maskell and Zimmermann have revealed that in phloem there exists a positive concentration gradient for the mass movement of solutes from higher to lower concentration

20. Objections against pressure flow hypothesis Explains about unidirectional flow whereas in plants its usually bidirectional Does not account for an active role of cytoplasm and strongly holds that phloem translocation is absolutely a physical process. Swanson proposed and demonstrated that phloem loading and phloem translocation are active process. Munch’s hypothesis holds that phloem translocation is a passive physical process It is generally held that sieve pores remain plugged with dense protoplasm preventing the mass flow of solute molecules According to this hypothesis high turgor pressure is needed to overcome the resistance at the cross wall plasmodesmata which is not possible.

21. QUESTIONS Define Source Sink Downward translocation Upward translocation Radial translocation Bidirectional translocation Explain mechanism of translocation Write a short note on theories about translocation Explain Munch’s mass flow or pressure flow hypothesis What are the objections against pressure flow hypothesis? Reference : Plant Physiology by S. N Pandey and B K Sinha , Plant Physiology and Biochemistry