This question is about photosynthesis. The photo is of a Red Plum seedling (Prunus prunus) that I have grown from a plum stone. It was stratified in my fridge, germinated on 10 June, planted out the same day. It showed above ground on 19 June, and did what I call, "standing up," when the cotyledons, which have been bent down as the shoot emerges, flip up and open, this morning (20 June). Usually, when this happens with epigeal plants, the cotyledons cast off a brown skin they have been carrying and open up to reveal the vegetative growth inside. With this particular seedling the brown skin stayed in place so this afternoon I performed some surgery and peeled it off with some tweezers. After I wet the cotyledons with a paintbrush dipped in water they opened and as you can see in the picture (taken within minutes of opening) the green leaf is inside.
However, this begs a rather obvious question. Since photosynthesis requires sunlight, how come the shoot, which has seen the sun, is bright white while the plant structure inside the cotyledons, which has not seen sunlight, is green? You will note that the cotyledons, which are normally a bright green are still white because their skin was not discarded.
A few weeks ago I had an anologous situation with an orange pip. Citrus trees are hypogeal, so the cotyledons open below ground, but when I tipped this one out to re-pot it the cotyledons were, unusually, green (normally they are white). These two examples seem to suggest that although sunlight is the energy source the effect can be seen elsewhere.
So the question is: What is the mechanism that controls where, within a plant, photosynthesis changes the colour to green?