When the seed is shed it contains very less moisture content, hence when it receives appropriate temperature before germination, seed first absorbs water which activates the enzyme inside it, now these activated enzymes and hormones start the digestion of food stored in endosperm mostly in the form of starch. Once starch is mobilized, root and shoot start appearing and light is needed when chloroplast is developed in leaves. Because till that time seed has used all its stored food and now it will prepare food using sunlight.
