Stomata will open if there is accumulation of the following element in the guard cells ___________.

Question

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Correct — option (2), Potassium. Stomatal opening and closing is driven by changes in the turgor pressure of the two guard cells that flank each stomatal pore, and that turgor pressure is controlled chiefly by the movement of potassium ions (K+) into and out of the guard cells. When light, low internal carbon dioxide, or other favourable cues are sensed, proton (H+) pumps in the guard-cell membrane actively export hydrogen ions, and the resulting electrochemical gradient drives potassium ions into the guard cells through specific K+ channels. This potassium influx lowers the guard cells' internal water potential, so water follows osmotically by moving in as well, the guard cells swell and become turgid, and because their cell walls are unevenly thickened (thicker on the side facing the pore), the swelling bows the pair of guard cells apart and opens the stomatal pore. Reversing the process — potassium and water moving back out of the guard cells — deflates them and closes the pore, which is exactly what happens under water stress when the plant hormone abscisic acid (ABA) triggers potassium efflux to conserve water.

Why the other options are wrong — (a) Magnesium's best-known physiological role in plants is as the central atom of the chlorophyll molecule, making it essential for photosynthesis and for the green colour of leaves, and its deficiency causes interveinal chlorosis. It has no established role as the ion whose accumulation directly drives guard-cell turgor and stomatal opening; that role belongs specifically to potassium.  (c) Iron is required by plants mainly as a cofactor for enzymes involved in chlorophyll synthesis and electron-transport processes (photosynthesis and respiration), and its deficiency also produces chlorosis in young leaves. It is not the ion that guard cells accumulate to swell and open the stomatal pore.  (d) Zinc is a micronutrient required as a cofactor for several plant enzymes, including those involved in auxin (growth hormone) synthesis, and its deficiency causes symptoms like stunted growth and small, distorted leaves. It has no established role in driving the turgor-pressure changes that open and close stomata; that mechanism is specific to potassium ion movement.

Concept: Each stomatal pore on a leaf's epidermis is flanked by two guard cells whose relative turgidity determines whether the pore is open or closed. Guard cells are unusual among plant cells in having a cell wall that is thicker on the side facing the pore and thinner on the outer side; when the cells become turgid (swollen with water), this uneven thickening makes them bow outward and apart, opening the pore, while loss of turgor lets them relax back together, closing it. The osmotic driver of this turgor change is the active, regulated movement of potassium ions (accompanied by counter-ions such as chloride or malate, and by water following osmotically) into the guard cells to open the stomata, and back out again to close them, a process controlled by light, CO2 concentration, and hormonal signals such as abscisic acid under water stress.

State PCS 2025 Q57 — Science & Technology PYQ with Answer

State PCS 2025 Q57 — Science & Technology PYQ with Answer