No it is not. It still persists, just that it is not in perfect syncrony with the outside world. The plant has it’s own bioritm, which is not exactly 24 hours. So put in complete darkness, the circadian rythm of the plant is still keeping perfect time, but it may run, for example, at 26 hours/cycle. Keeping the plant in light adjusts the cycle at 24 hours/cycle. This is done through specific receptors, that depend on the plant. Some plants use red light receptors(Phytocrome), others use blue light receptors(cryptocromes, Zeitlupe protein and sometimes phototropin) and still other plants use both.

Cyrcadian rythms in plants are still a topic of hot research. I find them particularily interesting. I hope this post helps you in your understanding of plant physiology