Ans : (d) Potassium influx and efflux
Explanation:
Stomata are small adjustable pores on the surface of aerial plant tissues. The pores open to facilitate uptake of carbon dioxide and close to limit the loss of water. Consistent with this function, stomata open in the light to enable photosynthesis and close during drought. Stomatal movements are forced by changes in the volume of two guard cells that are positioned on either side of the pore. During stomatal opening, guard cells accumulate potassium salts, causing an osmotically driven uptake of water. Owing to the influx of water the guard cells swell and bend, thereby pushing each other apart and creating an open pore in the middle. Various environmental signals, such as light, carbon dioxide and humidity, influence the ion transport machinery within the guard cell and thus, alter stomatal movement. Because of their role in regulating carbon dioxide uptake and transpiration of water, stomata are likely to play a key role in the adaption of plants to the anticipated global warming.