Final answer:
The lettuce plants wilted due to loss of turgor pressure during drought (plasmolysis) and swelled up when rain provided water which entered the plant cells, restoring turgor pressure (osmoregulation), leading to a stiffening of the plant structures.
Step-by-step explanation:
The cellular process that caused the lettuce plants in Felicia's garden to wilt and then swell after a downpour is known as osmoregulation and involves turgor pressure. When the lettuce plants experienced drought, the lack of water in the soil led to a hypertonic environment around the plant cells, causing water to leave the cells in an attempt to balance the concentration of solutes inside and outside the cell, a process called plasmolysis. This resulted in loss of turgor pressure, causing the plants to wilt. After the downpour of rain, the environment around the plant cells became hypotonic, and water entered the cells, resulting in the restoration of turgor pressure which supports and stiffens the plant structures, thereby causing the leaves to swell back up.
Osmoregulation in plants operates via the interactions between the cell wall, cell membrane, and the diffusion of water across the membrane. During drought, cells lose water and become flaccid, but upon rehydration, water reenters the cells and expands the vacuoles, generating turgor pressure which is opposed by the cell wall, enabling the plant parts to regain their firm, upright form.