Stomata open when the guard cells possess

Stomata open when the guard cells possess more K⁺.
Guard cells accumulate potassium (K⁺), which results in stomatal opening and release K⁺, which results in stomatal closing.
Guard cells use osmotic pressure to open and close stomata, allowing plants to regulate the amount of water and solutes within them. For plants to produce energy and maintain cellular function, their cells undergo the highly intricate process of photosynthesis.