I'l bet you are wondering that because you already know that plants can make oxygen. You probably already know that in photosynthesis, plants take CO₂ from the air, water (H₂O) from their roots, and energy from the sun, and make sugar (C₆H₁₂O₆).

What a lot of people don't realize is that when there's little or no light, plants do the same thing we do. The break down the sugar to release CO₂, water, and energy. This requires oxygen. The reason is pretty complex, but basically, electrons get passed around, and oxygen has to pick them up at the end of the process.

If you measured the amount of oxygen and CO₂ dissolved in a lake, how do you think the daytime levels would compare to the nighttime levels? Would a plant need oxygen if it were under lights 24 hours a day?

Plants respire, just like we do. When a plant doesn't have access to light, it burns sugar to make energy, consuming energy. It's just that plants use sugars to build their bodies as well as an energy storage, so over the course of a plant's life, as it grows, it makes more sugar than it burns, and so releases more oxygen than it consumes.

Plants need oxygen for the same reason you and Ido -- without oxygen we can't convert the carbohydrates, fats, and proteins we eat into energy. We call this process respiration, and the formula for this sort of reaction is like this:

sugar + oxygen --> carbon dioxide + water + energy

So we breathe in oxygen and eat food, and we exhale carbon dioxide and excrete water.

This exact same reaction goes on in every living cell, including all plant cells. But of course plants don't have to eat food, because they make their own food using photosynthesis.

The formula for photosynthesis is basically this:

carbon dioxide + water + sunlight --> sugar + oxygen

You can see that this is basically the reverse of respiration, but plants convert the energy in sunlight into the chemical bonds of the sugar. When cells respire, they break those bonds and get the energy out of them. Anyway, you can see that photosynthesis produces oxygen as a waste product, so for the most part plants don't have to breathe in extra oxygen -- they can just use the oxygen that they produce during photosynthesis. However, plants only perform photosynthesis in the green parts, like leaves and stems, but all plant cells need oxygen to respire. Cells in the leaves get plenty of oxygen from photosynthesis, but cells in the roots often need to get oxygen from the environment to stay alive. Even though roots are buried, they can absorb oxygen from the small air spaces in soil. This is why it's possible to 'drown' plants by watering them too much.

If the soil is way too wet, the roots are smothered, the roots can't get any oxygen from the air, and the cells in the roots die. Without those root cells, the rest of the plant dies. Some plants have evolved adaptations to deal with extremely wet soil.

Mangroves are trees that live in swampy environments along the coast in the tropics. The roots of mangroves are often entirely under saltwater, so they have special structures called pneumatophores (Greek for "air carrier") that act like snorkels, sticking up out of the water to get a oxygen for the roots.