Question

To avoid excessive water loss during severe drought stress, the closure of stomata is signallised by the production of ?? a)IAA b)NAA c)ABA d) IBA​

Answer 1

Answer:

ABA

Explanation:

the concentration of ABA increases in the leaves of the plant during dessication and other stressful conditions .as a result stomata present in the epidermis of the leaves close to prevent loss of water ie: transpiration .therefore it can also be known as anti transpirant . thus synthesis of ABA is stimulated by drought , water logging and other adverse  environmental conditions . ∴ it is known as stress hormone.

Answer 2

Answer is ABA

Guard cells are specialized plant cells in the epidermis of leaves, stems and other organs that are used to control gas exchange. They are produced in pairs with a gap between them that forms a stomatal pore. The stomatal pores are largest when water is freely available and the guard cells turgid, and closed when water availability is critically low and the guard cells become flaccid. Photosynthesis depends on the diffusion of carbon dioxide (CO2) from the air through the stomata into the mesophyll tissues. Oxygen (O2), produced as a byproduct of photosynthesis, exits the plant via the stomata. When the stomata are open, water is lost by evaporation and must be replaced via the transpiration stream, with water taken up by the roots. Plants must balance the amount of CO2 absorbed from the air with the water loss through the stomatal pores, and this is achieved by both active and passive control of guard cell turgor pressure and stomatal pore size.