Abscisic Acid (ABA) Regulation of Stomatal Closure
📌 ABA plays a crucial role in regulating stomatal opening and closing, preventing water evaporation during high temperatures.
💧 ABA signaling is initiated when it binds to the ABA receptor (ABAR) on guard cells, activating phospholipase C to produce IP₃.
📉 This process leads to a primary $\text{Ca}^{2+}$ influx (aided by $\text{ROS}$ generating secondary influx), which deactivates $\text{K}^{+}$ influx carriers and inhibits the proton pump.
🌊 The resulting efflux of $\text{K}^{+}$ and $\text{Cl}^{-}$ ions creates an osmotic imbalance, causing water to move out of the guard cells, making them flaccid and closing the stomata.

Light Regulation of Stomatal Opening
☀️ During the daytime, the absence of ABA allows sunlight (specifically blue light) to activate the proton pump.
🔋 Activated proton pumps hydrolyze ATP, pumping $\text{H}^{+}$ out of the guard cell, which establishes an electrochemical gradient.
⏫ This gradient activates $\text{K}^{+}$ influx carriers, causing $\text{K}^{+}$ to rush inside, along with sugars (glucose/sucrose) produced during photosynthesis.
💧 The resulting increase in solute concentration inside the guard cell drives water influx, making the guard cells turgid and causing the stomata to open for $\text{CO}_{2}$ intake.

Key Components in Guard Cell Signaling
🧪 Key cellular components involved include specific $\text{Ca}^{2+}$ storage vacuoles, $\text{K}^{+}$ and $\text{Ca}^{2+}$ ion channels, $\text{Cl}^{-}$ export channels, and the proton pump located on the guard cell membrane.
🔄 The proton pump typically moves $\text{H}^{+}$ outside, while $\text{K}^{+}$ transporters facilitate both influx (daytime) and efflux (ABA presence).
⚛️ Activation of the $\text{ABA}$ receptor also triggers Reactive Oxygen Species ($\text{ROS}$) like $\text{H}_{2}\text{O}_{2}$, which further contribute to $\text{Ca}^{2+}$ influx.

Key Points & Insights
➡️ Stomatal opening requires sunlight/blue light activation of the proton pump and influx of $\text{K}^{+}$ and sugars.
➡️ Abscisic Acid ($\text{ABA}$) acts as the signal to close stomata, primarily by inducing $\text{Ca}^{2+}$ spikes that halt solute influx and promote ion efflux.
➡️ Stomatal closure is achieved when guard cells become flaccid due to water loss following the rapid outflow of $\text{K}^{+}$ and $\text{Cl}^{-}$ ions.

📸 Video summarized with SummaryTube.com on Dec 02, 2025, 02:08 UTC