Worked example 1
A student observes a plant wilting on a hot, sunny afternoon. Explain the homeostatic mechanisms the plant would employ to try and recover or minimise further water loss, referring to the role of a specific plant hormone.
Show solution outline
- Detection of Water Stress: The high temperature and sunlight increase transpiration, leading to significant water loss and a decrease in the plant's water potential (specifically, a reduction in guard cell turgor and overall leaf cell turgor, causing wilting).
- Abscisic Acid (ABA) Production: In response to reduced water potential in the roots (and potentially leaves), the plant increases the production of the stress hormone, abscisic acid (ABA).
- ABA Transport and Action: ABA is transported to the guard cells of the stomata.
- Stomatal Closure: ABA triggers an efflux of potassium ions (K+) from the guard cells. This raises the water potential inside the guard cells. Consequently, water moves out of the guard cells by osmosis, causing them to become flaccid. The flaccid guard cells 'collapse' towards each other, closing the stomatal pore.
- Reduced Transpiration: Stomatal closure significantly reduces the rate of transpiration, thereby minimising further water loss through evaporation from the leaves.
- Other Adaptations (Implicit): In the longer term, the plant might orient its leaves away from direct sun, and if conditions persist, root growth might be stimulated to seek more water, or existing leaves might senesce to reduce transpirational area. The immediate effect of wilting itself also reduces the surface area exposed to direct sunlight.