Plants, like animals, undergo cellular respiration whereby they metabolize sugars and release CO₂. The metabolic reaction to break down sugar requires Oxygen. Underground where roots are growing and respiring is a place of low oxygen as there's not much of it in gaseous form (like above ground) and there's no photosynthesis going on down there. Thus plant roots can be oxygen limited underground and prefer oxygenated conditions.

Plants respire (burn sugars and oxygen to produce carbon dioxide and water - just as we animals do) when they aren't actively photosynthesizing. If a plant is in darkness (e.g. at night) and can't get oxygen, it will drown, just as an animal would.