Question

what will happen if vaselline is applied on the lower surface of the leave of plant ​

Answer 1

It will impede the photosynthesis of the plant.

Answer 2

The lower surface of the leaf contains pores called as stomata, which is responsible for the exchange of gases(O2 and CO2) as well as water vapours. If you apply vaseline it will not be able to perform it’s basic function and will eventually die.