Give an example of a physiological activity and how the hormone affects that activity in plants.

1. Auxins
 Apical dominance - Apical dominance in plants is regulated by auxin hormones,
particularly indole-3-acetic acid (IAA), produced in the terminal bud. Auxins inhibit the
growth of lateral buds along the stem, suppressing the development of side branches.
This ensures upward growth of the main stem, directing resources for efficient energy
utilization. Environmental factors, such as light conditions, can influence apical
dominance. Manipulating auxin levels or removing the apical bud can be employed in
horticulture to control plant architecture and promote branching for desired shapes or
productivity.
2. Abscisic acid (ABA)
 Regulation of stomatal closure - Abscisic acid (ABA), a crucial plant hormone, regulates
stomatal closure as a key physiological activity in response to environmental stress,
particularly water scarcity. When plants experience drought conditions, ABA levels rise,
initiating a signal cascade. ABA binds to receptors on guard cell membranes, triggering
the efflux of potassium ions, reducing turgor pressure, and leading to stomatal closure.
This response minimizes water loss through transpiration, aiding plants in water
conservation during stress.
3. Gibberellins (GA)
 Elongation of the Internodes - The most significant impact of gibberellins on plant
growth is the elongation of internodes, to the extent that they can overcome genetic
dwarfism in certain plants like dwarf pea and dwarf maize. For example, light grown
dwarf pea plants exhibit short internodes and expanded leaves. However, when treated
with gibberellin, the internodes undergo substantial elongation, resembling tall plants.

In dwarf plants, this response is attributed to either the absence of the gene responsible
for gibberellin production or higher concentrations of natural inhibitors. External
application of gibberellin compensates for the deficiency of endogenous gibberellins or
counteracts the effects of insufficient natural inhibitors.
4. Cytokinins (CK)
 Promotion of Cell Division in Shoot Meristems - Cytokinins enhance cell division in the
shoot apical meristem, a region with actively dividing cells at the growing shoot tip.
Elevated cytokinin levels stimulate meristematic cell division, fostering the emergence of
new shoots and branches crucial for overall plant growth. This effect involves the
interaction of cytokinins with the cell cycle machinery, specifically activating cyclin-
dependent kinases (CDKs) that regulate cell cycle progression from G1 to S phase. The
increased cell division leads to the development of more branches, leaves, and lateral
shoots, contributing to the plant's architecture and size. Additionally, cytokinins play a
role in preventing the aging (senescence) of plant tissues, ensuring sustained growth and
vitality.

5. Ethylene (ET)
 Fruit Ripening - Ethylene, a gaseous plant hormone, is pivotal in initiating the ripening of
fruits along with other hormonal and signaling factors. Initially, unripe fruits exhibit low
 ethylene levels, but as they mature, ethylene production increases, serving as a signal
for ripening. Post-harvest, ethylene production continues, reducing fruit shelf-life and
making it more susceptible to pathogens. Hence, it is crucial to monitor and manage
ethylene levels to prevent over-ripening, ensuring fruits remain marketable and
profitable both on the tree and during storage.

References

Cutler, S. R., Rodriguez, P. L., Finkelstein, R. R., & Abrams, S. R. (2010). Abscisic acid: emergence of a core
signaling network. Annual Review of Plant Biology, 61, 651-679.

Davies, P. J. (Ed.). (2010). Plant Hormones: Biosynthesis, Signal Transduction, Action. Springer.

Hauser, F., & Waadt, R. (2005). The role of abscisic acid in stomatal regulation. Current Opinion in Plant
Biology, 8(5), 509-514.

Leyser, O. (2005). Auxin Signaling: A Marriage of Perception and Transcription. Current Biology, 15(10),
R384–R387. doi: 10.1016/j.cub.2005.05.047.

Taiz, L., & Zeiger, E. (2010). Plant Physiology (5th ed.). Sinauer Associates.

University of Maryland Extension (2018). https://extension.umd.edu/resource/ethylene-and-regulation