Asked by: Preston Shumack, US
All plants can survive for short periods without light. Obviously, they need to be able to last through the night, but they can also cope with a longer darkness in an emergency. If you leave a tent pitched on the same patch of lawn, the grass underneath turns yellow and spindly. This is an adaptation, called etiolation, which focuses the plant’s remaining resources into growing as far as possible to try and reach sunlight again.
There are also some plants that have lost the power of photosynthesis altogether. The genus Orobanche (commonly known as ‘broomrape’) is an example. The plants have no chlorophyll and get all their nutrients by parasitically attaching to the roots of nearby plants instead. Although broomrape doesn’t harness sunlight itself, it is still indirectly reliant on the Sun to provide energy to its host plant.
Some other parasitic plants, called mycoheterotrophs, feed on fungi and these could theoretically survive in complete darkness for months or even years. But of course, those fungi in turn get their energy by digesting dead plants, and in a permanently dark world, this food source would eventually run out. No plant can live without sunlight forever.