Question

What two conditions does the closing of the stomata prevent in a leaf?

I know that one is transpiration.

Answer 1

massive amount of gaseous exchange takes place through stomata which is responsible for its closing and opening.

Answer 2

The two condition that the closing of stomata prevent from the leaf is the transpiration and respiration.  

Respiration:

Respiration is a process of exchange of ‘gases’ between the ‘leaf’ and the ‘atmosphere’, during day time the plants take in carbon di oxide and give out oxygen, so as to enable photosynthesis.  

Transpiration:

The transpiration is a process that causes the evaporation of excess water in the leaves as water vapour. Example of transpiration is a plants absorb water from its roots.