Plants rely on microscopic pores, called stomata, to regulate essential life processes. These openings facilitate gas uptake and water regulation. Control of stomatal opening and closing is important for survival.
Stomata: Plant’s Essential Pores
Stomata are specialized structures on the surface of plant leaves, typically found in greater numbers on the underside. Each stoma consists of a pore surrounded by two bean-shaped guard cells. These guard cells can change shape, controlling the stomatal opening. This allows plants to manage gas exchange, taking in carbon dioxide for photosynthesis and releasing oxygen. Stomata also facilitate transpiration, releasing water vapor into the atmosphere.
Nighttime Closure: Conserving Water
Stomata commonly close during the night in most plants. This closure serves a primary purpose: conserving water. Photosynthesis, which requires carbon dioxide uptake, relies on sunlight and does not occur in the dark. Keeping stomata open at night would lead to significant water loss through transpiration without carbon dioxide assimilation.
The mechanism behind this nighttime closure involves changes in the turgor pressure of the guard cells. During the day, light stimulates the uptake of ions, notably potassium ions, into the guard cells, which increases their internal water content. This influx of water causes the guard cells to swell and bow outwards, opening the stomatal pore. At night, the process reverses; potassium ions move out of the guard cells, leading to a decrease in their turgor pressure and causing them to become flaccid and close the pore.
Daytime Closure: Responding to Stress
Stomata can also close during daylight hours, typically as a protective response to environmental stressors. Conditions such as drought, high temperatures, or low humidity trigger this closure. This mechanism helps plants prevent excessive water loss, even though it reduces carbon dioxide intake for photosynthesis. Prioritizing water conservation over photosynthesis becomes necessary for survival under adverse conditions.
The plant hormone abscisic acid (ABA) signals water stress and prompts stomatal closure. When a plant experiences water deficit, ABA levels increase, acting as an internal signal to the guard cells. ABA initiates cellular events within the guard cells, including the efflux of potassium ions and other solutes, leading to a reduction in turgor pressure and stomatal closure. This response helps the plant manage water during limited availability.
The Curious Case of CAM Plants
Certain plants, known as Crassulacean Acid Metabolism (CAM) plants, exhibit a unique stomatal rhythm that deviates from the typical daytime opening and nighttime closure pattern. Examples of CAM plants include cacti, succulents, and pineapples. These plants have evolved a specialized adaptation to thrive in arid and hot environments, where water conservation is essential.
CAM plants open their stomata at night to absorb carbon dioxide from the cooler, more humid air, minimizing water loss. They then store this carbon dioxide as a four-carbon acid, typically malate, within their vacuoles. During the day, when temperatures rise and the risk of water loss is high, CAM plants close their stomata. They then release the stored carbon dioxide internally to fuel photosynthesis, enabling them to produce sugars without losing water. This reverse stomatal behavior is effective for survival in dry climates.