[Spike]

Spike asked the Naked Scientists:
   
Why does my house plant die when it is over-watered?

What do you think?

[Earthling]

Excessive moisture promote the growth of diseases that are harmful to the roots of the plants. Too much water compacts the growing medium and deprived the plants of oxygen.

[Don_1]

Earthling is quite right, too much water can also cause the roots to rot & will also flush out nutrients from the limited soil.

House plants, by & large, should be well watered, then left until virtually dry, or until they show signs of wilting before watering again.

[chris]

But what about aquatic plants - elodea, or water lilies for example? Clearly they don't seem to suffer these problems, so what adaptations do they possess to enable them to tolerate such an environment?

[Don_1]

Good question Chris.

Most terrestrial plants have a protective epidermis which restricts water loss through evaporation. This is very much decreased in marginal & floating aquatic plants, allowing them to evapotranspirate to a much higher level. So water taken in through the roots evaporates through the leaves much quicker.

The protective epidermis of terrestrial plants, through it's restriction of evaporation, would result in the plant becoming waterlogged if in a substrate which was kept continually wet. The waterlogged cells would eventually rupture, killing the plant. It's the plant world's equivalent of drowning.

[Earthling]

Aquatic plants have no soil to be compacted they grow in water. However, even non-aquatic plants can adopt to water medium provided the water is nutrient rich and  well oxygenated.Take for example the hydroponics growing like lettuce, pepper, tomatoes, cucumber,eggplants and almost all of soil growing vegetables do well with their roots submerged in nutrient rich solution continuously. They grow even  better than they were planted in the soil.Maybe the adaptation of water plants is their ability to extract better the nutrients and oxygen from water.What do you think,Chris?

[Don_1]

Fully submerged aquatic plants use their roots mainly for anchorage and as runners for proliferation. Since the leaves are submerged, they do not have the ability to evpotranspirate, therefore, the plants rely on foliar feeding.

Floating plants, such as Water Lilly, Water Hyacinth and Duckweed do feed through their roots and have stomata on both sides of the leaf, unlike terrestrial plants where the stomata are found only on the underside. This allows for breathing and evaporation.

I don’t know much about hydroponics, but from what I gather, in hydroponics, the water is kept aerated by the continuous movement of the water or by an air pump and diffuser (as in an aquarium) this may ensure that the roots are partially protected from the water by air bubbles forming on the root system. In static systems I think the plant has to be removed from the water occasionally for oxygen to get to the roots. If a terrestrial plant was to be kept in waterlogged soil, the weight and compaction of the soil coupled with the stagnant water would deprive the root system of oxygen. I wonder if a good supply of air in the water, which might protect the roots from being too wet, might also prevent the plant cells from becoming waterlogged.

[Earthling]

You don't need to remove totally the plants from the water in hydroponics. Some plants will survive even if no active aeration is done i.e. pumps,agitation, etc. as long as there is a water free space between the bottom of the container where the plant is anchored and the water.That goes to show that the water is not the problem when you put too much of it in the soil. It is the effect of "choking" the roots and deprived them of the 02 and other elements that is necessary for the plant's growth.Perhaps oxygen does not stay too long in the water and need to be replenish by active agitation by different means.The water plants whether partial or totally submerged is a totally different adaptation as mentioned by don-1.

[tangoblue]

when you over water a plant the water that is not used by the plant just sits in the soil if adequate drainage is not there and the plant eventually rots and dies.