In search of allelopathy: an eco-historical view of the investigation of chemical inhibition in California coastal sage scrub and chamise chaparral

The Results. With Haines collecting in the field, Muller finalized laboratory tests in the greenhouses at UCSB. The experimental design was not the same as his effort in 1953 with desert plants because he was hypothesizing a different delivery system for toxic agents; evaporates into the air rather than rain-drip off leaves.

To set up appropriate germination tests or bio-assays to measure possible toxic effects, leaves of both Salvia and Artemisia were crushed and placed in separate dishes. Moist filter paper resting on a wire mesh an inch and a half above the leaves was sprinkled with cucumber seeds. The entire arrangement was then covered and sealed. Two days later Muller et al. (1964) observed, "In every instance volatile materials emanating from the crushed leaves radically inhibited seedling root growth." Bioassays using sliced instead of whole leaves on Cucumis sativus (cucumber) and Avena fatua (wild oat) seeds showed similar results.

Muller recognized sealed containers in a lab could not replicate natural conditions in the field, so he attempted to design a delivery pathway more closely resembling the natural distribution of volatile materials into the soil. He suggested dew formation.

By using cooling coils, dew was condensed from an atmosphere in which Salvia was growing. The dew was then collected and used to soak cucumber seeds. After several unsuccessful trials, Muller was able to show reduced root growth. He concluded, "Although field experimentation is still to be performed, it appears that whole plants of Salvia leucophylla release a volatile substance that condenses in dew and that significantly inhibits the growth of cucumber roots."

The effect Muller was classifying as an allelopathic response in the field was dramatic. He suggested that the volatile substances from these aromatic species appeared to be responsible for inhibiting growth of plants as far as 27 feet away. If true, allelopathy in coastal sage scrub would be considered one of the most remarkable ecological interactions on earth.

The coastal sage scrub "bare zone" model was presented in Science under the title, "Volatile Growth Inhibitors Produced by Aromatic Shrubs." The paper was considered important enough to be the subject of the magazine's cover photo (Fig. 1). The picture is an aerial shot of coastal sage scrub stands of Salvia leucophylla and Artemisia californica bordering grassland. Surrounding the clumps of sage were the characteristic "halos" of bare ground. The photo's title read, "Chemical Plant Competition."

Along with Walter Millier, Bruce Haines was listed as a coauthor to the 1964 paper, a significant accomplishment for a young undergraduate.

Cowpaths. It was not long before others began examining the data and making their own conclusions, as Muller had done with Gray and Bonner's work several years before. In May, four months after publication, Philip Wells (1964) from the University of Kansas wrote a critique listing several alternative explanations for the bare zones, using details of the Science cover photo as a focal point. "The interconnecting and in part rectilinear character of the network of white lines does not suggest origin by chemical inhibition," Wells suggested. "An equally good explanation is that most of them are cattle trails."