Allelopathy—Chemical Warfare Between Aquatic Plants

8 Think about your first reaction if you saw a poster at your local pet shop announcing “New Ornamental Plant Kills All Algae in Your Tank Within 24 Hours—Money Back Guarantee!” How much would you be willing to spend on such a plant? Unfortunately, no such plant has yet been identified. But it is a scientific fact that some plants secrete chemicals that are toxic to other plants. Can we use such plants in algae control in our aquaria or does it explain why some plants do poorly in our aquaria? In 1999, Diana Walstad published perhaps the best book ever written on planted aquaria. In her book Ecology of the Planted Aquarium, she devoted an entire chapter to the phenomenon of allelopathy and this has inspired me to go deeper into this fascinating subject (Walstad, 1999). I must also say that my search through the scientific literature has made me more skeptical about the importance of this phenomenon in planted aquaria. I am sure that Diana Walstad in her book exaggerates the prevalence of true allelopathy in planted aquaria. By definition, allelopathy is the negative influence on other organisms—including higher plants and algae—by excretion of toxic chemicals (Rice, 1984). The ecological benefits of allelopathic behavior are evident. It provides the plant with great competitive advantage with a limited investment in toxic chemicals that are harmless to the plant itself. Allelopathy is rather well documented for a variety of terrestrial plants, but the information on how widespread this phenomenon is among aquatic plants is scanty (Gopal & Goel, 1993).