Re: Why can rushes live in toxic water?

Dear Doreen -

It is the environment in which the rush is located that makes it less 
susceptible to toxicity from hydrocarbons and other toxic compounds, more 
than the structure of the plant itself.  So what is the environment in 
which a rush lives?  It is a wetland environment, and two of the most 
important characteristics of wetlands are: soil that is saturated with 
water, and a high level of organic matter in that soil. 

In saturated soil, the amount of oxygen is lower than in soil that is not 
saturated.  This is due to the fact that water holds less oxygen than air. 
Plants, including rushes (Juncus spp.), respond to the low levels of 
oxygen in the soil by decreasing their metabolism.  Oxygen is needed for 
aerobic metabolism, and so with less oxygen the overall metabolism of the 
plant decreases.  With lower metabolism, the plants take up less water and 
therefore they also take up fewer hydrocarbons, and therefore are exposed 
to a lower amount of hydrocarbons than a terrestrial plant - a plant that 
does not live in wetlands - would be.  

The organic matter in the wetland - which is made up of partly decomposed 
plant leaves and stems, insects and other stuff - has the ability to grab 
onto (or sorb) hydrocarbons.  Therefore the soil itself may grab onto the 
hydrocarbons, taking the hydrocarbons out of the water, and making the 
hydrocarbons unavailable to the plant.  This is not the case as much in 
soils that do not have a high organic matter content.  

You can look at the book "Wetlands" by William J. Mitsch and James G. 
Gosselink (John Wiley and Sons, Inc., New York, 1993) if you want more 
information on plant adaptations to wetland environments.