When does photosynthesis start in a plant?

I'm doing an experiment on factors affecting germination of green bean seeds. On the internet many say green beans do not need light to germinate. But I've already set up the experiment with green beans as the only type of seeds I have, so I'm thinking of changing the factor from light intensity to temperature. However, I also see a pattern of different qualitative results regarding the physical strength in each seed with different light intensity (initially I used light bulb to control the light intensity but later I used it for temperature instead aftering realizing that green beans don't need light to germinate); the root of seeds with higher light intensity (closer to the light bulb) has a strengthened shape standing straighter, whereas the root of those with lower light intensity has a weaker shape in contrast. In this case, is light intensity playing a role of helping germination or photosynthesis, causing the root to grow healthier? Thanks!!