Re: Does Caffiene affect the growth of plants?

Many student projects water plants with coffee or soda pop containing 
caffeine to see if caffeine reduces plant growth. The problem with those 
experiments is that both coffee and soda pop contain a lot of other 
ingredients which may also affect plant growth. Therefore, you cannot tell 
if caffeine alone is affecting plant growth. Some students find that 
plants watered with coffee grow better than control plants receiving 
water. A possible explanation is that mineral nutrients in coffee, such as 
potassium or phosphorus improve plant growth.

To determine if caffeine is toxic to plants, a pure source of caffeine 
should be used instead of coffee or soda pop. Several caffeine 
concentrations should be used if possible.

There are at least two ideas of the role of caffeine in plants. One 
hypothesis is that caffeine protects plants from insect or fungal attacks 
(Frischknecht et al., 1985). Another hypothesis is that caffeine is 
excreted by the plant into the soil and reduces growth of competing plants 
and bacteria (Friedman and Waller,1983a and b). Plants producing chemicals 
to inhibit growth of competing plants is called allelopathy. Alleopathy is 
often difficult to demonstrate under natural conditions but easy to 
demonstrate under laboratory conditions. The dose under lab conditions is 
often much higher than under natural conditions. Also, under natural 
conditions, chemicals released by the plant may be quickly inactivated by 
the soil.

References

The effects of 
caffeine in Coffea arabica L.

Friedman, J., and Waller, G.R.. 1983a. Caffeine hazards and their 
prevention in germinating seeds of coffee (Coffea arabica L.). J. Chem. 
Ecol. 9:1099-1106.

Friedman, J., and Waller, G.R.. 1983b. Seeds as allelopathic agents. J. 
Chem. Ecol. 9:1107-1115.

Frischknecht, P.M., et al. 1985. Purine alkaloid formation in buds and 
developing leaflets of Coffea arabica: expression of an optimal defense 
strategy?? Phytochemistry. 3:613-616.