Stomata are tiny openings found on the surface of leaves and stems in plants. They play a crucial role in regulating gas exchange, allowing for the uptake of carbon dioxide (CO2) needed for photosynthesis and the release of oxygen (O2) and water vapor (H2O). The opening and closing of stomata are controlled by various mechanisms involving both physical and physiological processes.

Guard Cell Structure:

Stomata are surrounded by two specialized cells called guard cells. These cells are kidney-shaped and contain chloroplasts, which enable them to carry out photosynthesis. The inner walls of guard cells are thicker than the outer walls, causing them to buckle when they become turgid (swollen with water) or collapse when they lose turgidity.

Role of Water Potential:

The opening and closing of stomata are primarily regulated by changes in water potential within the guard cells. Water potential is influenced by various factors, including osmotic potential (solute concentration), pressure potential (physical pressure), and matric potential (attraction between water molecules and cell walls).

1. Stomatal Opening:

Stomatal opening occurs when guard cells take up water through osmosis, leading to an increase in turgor pressure within the cells. This process involves several steps:

  • Light Perception: Blue light is essential for triggering stomatal opening. Photoreceptors called phototropins perceive blue light, initiating a signaling cascade that leads to stomatal opening.
  • Proton Pump Activation: The perception of blue light activates proton pumps located on the plasma membrane of guard cells. These pumps actively transport hydrogen ions (H+) out of the guard cells.
  • Potassium Ion Influx: As protons are pumped out, an electrochemical gradient is established across the plasma membrane, causing potassium ions (K+) to enter the guard cells through specific channels.
  • Water Influx: The entry of potassium ions and other solutes increases the osmotic potential within the guard cells, leading to the influx of water by osmosis. As a result, the guard cells become turgid and bulge outwards, creating an opening between them called the stomatal pore.

2. Stomatal Closure:

Stomatal closure occurs when there is a need to conserve water or prevent excessive transpiration. It can be triggered by various factors, including high temperatures, low humidity, water stress, and the presence of certain hormones. The closure process involves the following steps:

  • Abscisic Acid (ABA) Release: During stressful conditions, such as drought or high salinity, the hormone abscisic acid (ABA) is synthesized and released from different plant tissues. ABA plays a crucial role in triggering stomatal closure.
  • Ion Efflux: ABA stimulates the efflux of potassium ions from guard cells through specific channels. This reduces the osmotic potential within the cells and causes water to move out by osmosis.
  • Loss of Turgor Pressure: As water leaves the guard cells, their turgor pressure decreases, causing them to become flaccid. The reduction in turgor pressure leads to the collapse of guard cells and closure of the stomatal pore.

Other Factors Influencing Stomatal Opening and Closure:

In addition to light and ABA, several other factors can influence stomatal opening and closure:

  • Carbon Dioxide (CO2) Concentration: Elevated levels of CO2 generally lead to stomatal closure as plants try to conserve water.
  • Humidity: Low humidity promotes stomatal closure as it reduces the vapor pressure deficit between the leaf surface and the atmosphere.
  • Temperature: High temperatures can induce stomatal closure to prevent excessive water loss through transpiration.
  • Other Hormones: Besides ABA, other hormones like cytokinins, gibberellins, and auxins can also regulate stomatal opening and closure.

Circadian Rhythm

Stomatal opening and closing are also influenced by the plant’s internal clock or circadian rhythm. The circadian clock controls the sensitivity of stomata to environmental cues such as light and temperature. It ensures that stomata open during the day when photosynthesis occurs and close at night to prevent excessive water loss.