Abscission zone in plants, how the leaves fall off

how the leaves fall down

1. Abscission zone in plants C003 C004

2. What is an abscission? Abscission, from the Latin words ab (away) and sciendere (to cut), is a tightly controlled cellular progression that occurs at given developmental stages during the lifespan of a plant and ensures that plants can shed organs when they are no longer required – such as flowers after pollination or senescent leaves in the fall – and also as a way of getting rid of infected or damaged organs.

3. Abscission of leaf The term abscission of leaf refers to separation of leaf from stem without causing injury to stem. It is a natural & special phenomenon which is commonly found in leaves of woody dicotyledons & gymnosperms and less commonly found in herbaceous angiosperms.

4. Protective layer The protective layer forms over the exposed region of the plant part after leaf fall. The protective layer of primary origin is formed by transformation of existing parenchyma cells of abscission zone. The protective layer of secondary origin is a typical periderm layer.

5. Separation layer It consists of a few to several layers of cells, which differ from those above & below in shape, by presence of abundant starch grains & dense cytoplasm. Sufficient conduction is maintained by the secondary conducting elements. Due to this, the leaf is kept turgid until it falls.

6. Factors leading to abscission: Removal of leaf blade. Senescence of leaf. Auxin gradient across the abscission zone. Abscission does not occur when gradient is steep. Auxin content is high in young leaf blades. When Auxin content decreases as compared to petiole, the leaves become yellow & abscise.

7. Abscission of floral parts

8. ABSCISSION OF STEMS Maitrayee Pan, C004

9. Cladoptosis Abscission of leaves and stem as a unit i.e. leafy branchlets Characteristic of deciduous conifers. Eg. Taxodium Evergreen species like Salix abscise only a portion of their branchlets The separation zone is formed through the xylem as well as living cells o this region. Those xylem cells in which no separation layer is formed rupture easily as they are weak.  A periderm layer, which is continuous with the main branch, is formed in the living tissue just below the scar

10. Abscission of Woody stems The branch-base xylem constriction would appear to facilitate branch abscission

11. Abscission of Herbaceous stems Tissues are soft & immature There is  no definite zone of abscission Immediately before abscission, a seperation layer is formed E.g. Ailanthus, Morus, Ulmus . In some herbaceous members of families Chenopodiaceae, Amaranthaceae, Asteraceae, the entire above ground portion can abscise on maturity. 

12. Fruit Abscission The fruit may separate at three different places along the peduncle & fruit receptacle., dependent on activation of the abscission zone at these places The structure of the abscission zones & the process of abscission of fruits are similar to those of leaves.

13. Causes of abscission Conditions unfavourable to growth hasten abscission while favorable conditions delay abscission. A critical auxin concentration gradient across the abscission zone is necessary to prevent abscission. Abscissic acid plays a vital role when its concentration is increased, abscission occurs. Carotenoids are known to play a role of precursor in the synthesis of ABA.  ABA maybe formed from xanthophyll, such as violaxanthin which serves as a precursor for the biosynthesis of ABA.

14. Histological defense structures Abscission layer formation Cork layer formation Gum or latex deposition Necrotic structural defense reaction

15. References: Burrows, G. E., Meagher, P. F., & Heady, R. D. (2007). An anatomical assessment of branch abscission and branch-base hydraulic architecture in the endangered Wollemia nobilis.  Annals of botany ,  99 (4), 609–623. https://doi.org/10.1093/aob/mcm003 Hakeem O. E., Ebehi A., Onyemaechi H. O. (2019).  Plants’ innate defence mechanisms against phytopathogens. Journal of microbiology, biotechnology and food sciences , 9(2), 314-319.  https ://doi.org/10.15414/jmbfs.2019.9.2.314-319 https://en.wikipedia.org/wiki/Abscission#In_plants