Everybody knows that you need to water plants. This water is taken in from the roots and ends up transpiring through the leaves. Why would giving a plant too much water kill it?

Also, my wife tells me that you can tell when a plant has been over-watered because its leaves turn yellow. If this is true do you know why? I can't fathom it.

Thanks once again for you standard excellent help!

This link is more relevant to cacti but answers your question well:

http://www.cactus-art.biz/note-book/Dic … tering.htm

Plants die from over-watering in your house, or being inundated or flooded in the environment, for a number of reasons. For plants that are not used to having their roots submerged they quite rapidly drown - root tissue needs oxygen just like the rest of the plant. Species which grow in water or waterlogged environments have a range of adaptations to get oxygen down to the roots. A second cause of mortality is indirectly related to over-watering; keeping root tissue wet all of the time softens the tissues and allows soil fungi to infect the root tissue, causing root rot and subsequent death. Again, plant species which routinely grow in wet places are resistant to this. Cacti, as in the link above, are particularly susceptible to over-watering, but plants from much wetter habitats can also be killed by too much water.