Allelopathy, autotoxicity, and germination

When plants, alive or dead, release chemicals that affect the growth of neighboring plants, allelopathy evolves. Observations in nature may sometimes help to identify the donor, the “aggressive,” as well as the recipient, the “susceptible” plant(s). Molisch (1937) included in the concept of allelopathy (i.e., suffering from the neighbor) negative as well as positive chemical interactions among plants. When allelochemicals that are released from a plant affect growth or germination of individuals of the same species, the term allelopathy is often replaced by the term autotoxicity. Research on allelopathy started mainly in the 1940s (Evenari, 1940, 1949; Evenari et aL, 1942–3; Bonner and Galston, 1944; Bonner, 1946) and was eventually continued in the sixties by Muller (1966). Most of this research has been pursued under controlled or semicontrolled conditions in the laboratory or greenhouse. Germinating seeds were and are still commonly used for evaluating the allelopathic effect. To evaluate the allelopathic potential of a plant, germinating seeds of a test plant are often exposed to whole plants, seeds, plant extracts, or isolated allelochemicals of the “aggressive” species. In this way, the inhibitory effect of many allelochemicals on germination or on root elongation were described (cf. Rice, 1984). Both academic and applicative interests fostered research in the effect of allelochemicals. In the academic level, allelopathy is mostly studied to evaluate its role in shaping the structure of natural plant populations or communities. In the applicative aspect, studies often aim to develop a biological weed control approach (Putnam and Barnes, 1983; Putnam et aL, 1983; Leather, 1983).