Such a chemical would allow the proton gradient to run down without coupling it to the production of ATP, wasting the energy. If this continued, it could kill the plant by preventing the Calvin cycle from operating.
Work Step by Step
This is a similar question to several in Ch. 9, and all of these go to the question of chemiosmosis: how does it work and how important is it (very). Several herbicides work in the same way as the chemical described in the question.