What causes the guard cells to become swollen and relaxed?

Guard cells become swollen and turgid when water enters them through osmosis, usually in response to favorable environmental conditions like high humidity or sufficient soil moisture. This swelling causes the stomata to open, facilitating gas exchange for photosynthesis. Conversely, when conditions are dry or when the plant needs to conserve water, guard cells lose water and become flaccid, causing the stomata to close. This regulation helps the plant maintain homeostasis and optimize water use.