14.2.2—Daily rhythms of stomatal opening and closing

Many plants show a daily rhythm of stomatal opening and closing. Stomata often open during daylight, when carbon dioxide is needed for photosynthesis, and close at night, when photosynthesis cannot use incoming carbon dioxide.

The rhythm is not just a direct reaction to the current light level:

• A daily pattern can continue when a plant is kept in constant light or constant darkness, showing an endogenous timing component.
• Light and other environmental cues can reset or adjust the timing, so the rhythm remains aligned with the day.

Opening around the day’s photosynthetic period supports carbon-dioxide entry but also permits water-vapour loss. Closing at night reduces unnecessary transpiration and conserves water. Water stress, high temperature, low humidity or changed carbon-dioxide conditions can override or shift the expected pattern when opening becomes too costly.

A daily pattern is not proof that light is the only cause, and it is not identical in every plant or condition. This card separates timing from the guard-cell micro-mechanism and from ABA signalling, which are taught separately.