Light Requirement - Seed Dormancy
The light requirement for germination of many seeds is presumably a mechanism that prevents the germination of small seeds buried deep underground, where they would exhaust their reserves before reaching the surface ad becoming autotrophic. However, after a fire many seeds germinate at once, and forest regeneration begins.
This mechanism prevents overcrowding in a mature forest and ensures rapid re-growth after a disaster. Not all seeds require light for germination; some are unaffected and a few are inhibited by light. Blue light at quite high intensity has some effect on the germ ination of certain seeds, but it is not clear whether this is mediated through the absorption of blue light by phytochrome or by some other pigment.