Question

If one increased the level of carbon dioxide in the room, what result would you expect to happen to the transpiration rate and why?

(one question from my photometer lab)

Answer 1

Stomata are pores or openings on the leaf that allow escape of water from the plants after transpiration. They contain two kidney shaped guard cells(in dicots) and dumbbell shaped guard cells (in monocots) surrounded by subsidiary cells and a pore in the center. The guard cells have thick walls on the pore side (convex) and thin walls on the side (concave). They contain chloroplasts in them and their cuticle are permeable to water. The number of stomata differs in plants living under different conditions. They are very less in xerophytic plants and absent in hydrophytic plants. Stomata closes when the guard cells are flaccid and opens when they are turbid. Increase in the level of the carbon dioxide in the atmosphere allows the formation of carbonic acid in plants i.e., CO2+H2O -> H2CO3 which increases the pH of the guard cells thereby activating Abscisic Acid which is a plant growth hormone inhibitory in nature. It inhibits the entry of the K+ ions from the subsidiary cells into the guard cells and this process decreases the osmotic potential in the guard cells allowing water to move out of the guard cells into the subsidiary cells causing exosmosis reducing the turgor pressure on the guard cells. This makes the guard cells flaccid in nature thereby closing the stomata. Therefore increased levels of CO2 reduces the rate of transpiration in plants.