It depends on the plant; some have modifications that allow different parts of the plant to do most of the photosynthesis. Typically, the leaves do the most photosynthesis. They are broad and flat that allows increased surface area for reactions.

The union made modifications to the contract that the membership protested.

Root modifications are modifications of the roots that help them to survive in extreme conditions

Depends on the motorycle, number of cyclinders, idle speed, engine modifications, exhaust modifications, intake modifications etc... not much

The root word for "modifications" is "modify." "Modify" means to change or alter something, so "modifications" refers to the changes or alterations made to something.