1. Plant growth and commercial uses of hormones

2. Learning Outcomes
Outline the role of hormones in leaf loss in deciduous plants.

3. Leaf Abscission
Trees in temperate countries shed their leaves in autumn.
Survival advantage
Reduces water loss through leaf surfaces
Avoids frost damage
Avoid fungal infections through damp, cold leaf surfaces
Plants have limited photosynthesis in winter

4. Abscission and hormones
Two different plant hormones control abscission
Auxin
Inhibits abscission
Ethene (gas)
Increase in ethene production inhibits auxin production

5. Stages in leaf abscission
As leaves age, rate of auxin production declines
Leaf is more sensitive to ethene production
More ethene produced, inhibits auxin production
Abscission layer begins to grow at the base of the leaf stalk.

6. Leaf Abscission

7. Abscission Layer The abscission layer is made of thin-walled cells
Weakened by enzymes that hydrolyse polysaccharides in their walls
Layer is so weak that the petiole breaks
Leaf falls off
Tree grows a protective layer where the leaf will break off
Cell walls contain suberin
Leaves a scar which prevents the entry of pathogens

8. Learning Outcomes
Describe how plant hormones are used commercially.

9. Commercial use of Auxins
Sprayed onto developing fruits to prevent abscission
Sprayed onto flowers to initiate fruit growth without fertilisation
Parthenocarpy – promotes the growth of seedless fruits

10. Commercial use of Auxins
Applied to the cut end of a shoot to stimulate root production
Synthetic auxins are used as selective herbicides killing broad leaved plants

11. Commercial use of Ethene
Fruits harvested before they are ripe allows them to be transported without deteriorating.
Scientists have developed 2-chloroethylphosphonic acid which can be sprayed on and releases ethene inside the plant. This promotes ripening at the sale point.
E.g. bananas from the Caribbean

12. Commercial use of cytokinins
Delay leaf senescence – can be sprayed on lettuce leaves to prevent them from yellowing
Can be used in tissue culture to mass produce plants

13. Commercial use of Gibberellin
Sprayed onto fruit crops to promote growth e.g. apples elongate
Sprayed onto citrus trees to allow fruit to stay on the trees longer
Sprayed onto sugar cane to elongate stems and therefore increase the yield of sucrose.
Sprayed on to grapes to allow stalks to elongate and therefore the grapes get bigger

14. Commercial use of Gibberellin
Plant breeding – in conifers gibberellin can induce seed formation in young trees
Used in brewing, where GA is sprayed onto barley seeds to make them germinate early, amylase is produced, starch is broken down into maltose, the action of yeast on the maltose produces alcohol.
Spraying with gibberellin inhibitors can keep flowers short and stocky