What happens to plants if they are exposed to "sunlight" 24h a day?
This review (from 2011)1 states that it depends on the plant species and cultivar.


  Some continuous light (CL)-grown photosynthetic organisms show
  increased productivity. However, CL also induces negative effects in
  several plant species. The most visible CL-induced negative effects
  are leaf chlorosis and necrosis. CL also lowers photosynthetic
  parameters ... The CL-sensitive species include eggplant, geranium,
  some onion species (Allium fistulosum), peanut, some cultivars of
  potato, tomato, and even lichens (Xanthoria parietina) and mosses
  (Pleurozium schreberi and Ceratodon purpureous).


Among the plants that have better yields under continuous light are lettuce, roses, some potato cultivars, some algae.

The reasons why some plants can be damaged under continuous light aren't completely clear (at least they weren't in 2011). One of the problems is carbon imbalance. Leaves produce too much sucrose and other organs such as roots are not able to process it fast enough. This leads to high sugar concentrations and can cause leaf senescence. Continuous photosynthesis also results in more photo-oxidative damage and plants that better detoxify reactive oxygen species have less negative effects from continuous light.

So look for research on the particular species you want to grow or just experiment with different light conditions.



 1. Velez-Ramirez, A.I., van Ieperen, W., Vreugdenhil, D. and Millenaar, F.F. 2011. Plants under continuous light. Trends in plant science, 16(6):310-318.