Question

The substance that triggers the fall of mature leaves and fruits from plants is
A. Auxin
B. Gibberellin
C. Abscisic acid
D. Cytokinin

Answer 1

Hint: A plant hormone, which can trigger the fall of mature leaves and fruits from plants, should be responsible for seed-dormancy. The same hormone should also be able to control the closure of stomata, inhibit shoot growth

Complete answer:
Auxin- Auxin is a plant hormone that controls various growth and developmental processes in plants.
Gibberellin- These plant hormones promote stem-elongation, flowering, dormancy, germination, etc.
Abscisic acid- is a growth-inhibiting plant hormone. It is responsible for seed-dormancy and also inhibition of seeds. ABA (abscisic acid) also stimulates the closing of the stomata, inhibits shoot growth, and stimulates the protein storage in seeds.
Cytokinin- These are growth promoters and hormones which stimulate cell division and cytokinesis in cells of root and shoots.

Additional information: Chemical formula is $C_15H_20O_4$ and
The melting point is 163 °C (325 °F; 436 K)
Effects of ABA are as follows:
1. Antitranspirant - Induces stomatal closure, causes decreasing transpiration rate to prevent water loss.
2. Inhibits fruit ripening process.
3. Cause for seed dormancy by inhibiting cell growth – inhibits seed germination.
4. Inhibits the fusion of Kinetin nucleotides.
5. Downregulates enzymes required for photosynthesis.
6. Functions on endodermis to prevent the growth of roots when exposed to salty conditions, delays cell division
6. Dormancy inducer - It is accountable to induce dormancy in the seeds.
7. Used as anti - transpirant - In drought-prone areas, water stress is a major problem in agriculture production, so sprays of ABA are used that cause partial closure of stomata for few days, to reduce the transpirational loss of water

So, the correct option is C. Abscisic acid

Note: ABA is synthesized by leaves and then transferred to stem apex via phloem tissues. Almost 50% of growth inhibition can also be triggered by it. Even gibberellin stimulates the growth of plants and can also be inhibited.