It is a reactant of photosynthesis that comes from the plant’s root system.

The reactant of photosynthesis that comes from the plant's root system is water. Here are the steps of how it is used:

1. The plant's root system absorbs water from the surrounding soil.
2. This water is then transported up through the plant's stem to the leaves.
3. In the leaves, water is one of the reactants used in the photosynthesis process. The other reactant is carbon dioxide, which the plant absorbs from the air.
4. Inside the chloroplasts (the parts of the plant cells where photosynthesis happens), water and carbon dioxide undergo a chemical reaction under the influence of sunlight.
5. This reaction produces glucose (a type of sugar that the plant uses for energy) and oxygen (which the plant releases back into the air).