What Happens to the Plant?

When the growing medium is allowed to over-dry, the plant is not be able to absorb enough water, so the plants cells lose turgor due to transpiration which causes the leaves to wilt or droop. With prolonged dryness of the media, some leaves (usually the lower older leaves, but not always) will stop functioning, turn yellow and die. This is natural. The older leaves are abscising so the younger leaves will receive the water. The solution is to water the plant more frequently, so as not to allow the soil to over-dry.

When the plant is being over-watered, the same effect is taking place, but in a somewhat different manner. Firstly, a plants roots must have oxygen for both the roots and the plant to be healthy and survive. If the growing medium has poor structure and is kept constantly wet, the plants roots are harmed due to a build up of poisonous gases (carbon dioxide, ethylene, etc.) in the soil given off by the plants roots as they transpire. The damaged, and possibly decayed, roots can no longer absorb enough water to meet the plants demands, and the same effect occurs as with an over-dry soil: the leaves turn yellow and droop. The difference here is that a plant that wilts from drought will usually bounce back if it is watered, whereas, an over-watered plant that suffers from root decay will only get worse and eventually collapse and die if it continues to be over-watered.