Question

How is a plant experiencing a water shortage connected to the way a guard cell works?

Answer 1

Guard cells are kidney shaped or dumbbell shaped green epidermal cells which surround the stomata. Stomata are the tiny pores found on the epidermis of leaves and other green aerial parts of a plant.

Complete answer:
Stomata are involved in gaseous exchange and transpiration. Gaseous exchange helps in the process of photosynthesis. Transpiration is the loss of water in the form of vapours from the exposed parts to the plant. Guard cells control the opening and closing of stomatal pores through the turgor changes in guard cells. The concavo-convex curvature of two guard cells is variable and causes stomatal pores to open and close. The walls of guard cells are thickened on the inner side and thinner and more elastic on the outer side. When the stomata are to open, these guard cells can swell up on the outer side by the development of high turgor pressure. The inner concave sides also bend out slightly to create a pore in between two guard cells. During closure movements, the reverse changes occur.

The stomatal opening and closing are governed by various factors: light, pH, carbon dioxide and availability of water. During photosynthesis, the production of sugars increases the osmotic concentration of the guard cells, which leads to absorption of water from the neighbouring epidermal cells. The turgid guard cells bend outwards and create a pore between them. When plants undergo water stress i.e., the availability of water is less than the rate of transpiration, it brings about the closure of stomatal cells due to ABA (Abscisic acid) and rise in Diffusion pressure deficit (DPD) of epidermal cells. This closure of stomata prevents loss of water through transpiration but also affects photosynthesis.

Note: The rate of transpiration depends upon the rate of absorption of soil water by roots. A decrease in uptake of water by the roots causes partial dehydration of leaf cells resulting in closure of stomata and wilting.