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

"Oh yeah, I remember him," Reed Gray said when reflecting upon his career in 2003. After a brief pause, he let out a soft laugh. "Yes, he was the guy who said we were all wet."

Unfortunately neither Gray nor Bonner published a response to Muller, but both men vigorously disagreed with his conclusions (Reed Gray, pers. com. July 03). Bonner's opinion is especially significant because of his reputation for being open minded and willing to change if presented with convincing evidence. Frank Salisbury (1998), who went on to become a successful plant physiologist himself wrote, "Some scientists develop a hypothesis and defend it to their deaths without flinching and in spite of any contrary evidence that might appear. James was not cast in that mold. For him, only the truth mattered."

C. H. Muller published a follow up manuscript on desert allelopathy with a close, but unrelated colleague, Walter Muller (Muller and Muller 1956). Their paper described further field research and experimental results confirming the presence of toxins in desert plants but also their ineffectiveness as growth inhibitors in the field. Refining the statements Muller made in 1953 about the importance of understanding the complexity of nature they wrote,

It should be emphasized that the natural habitat is a complex of physical and biological factors that influence growth. Even though distributions may give the impression of an antibiotic effect by some of the individuals, careful investigation may indicate that the situation cannot be explained in such a fashion. Environmental influences and the metabolic activities of organisms are complex factors which are variously intermingled, and in most cases it is doubtful whether any one factor would be distinguishable as the primary causative influence.

So it appeared allelopathy was once again relegated to its status as a rejected hypothesis. It would be a very short period of exile.

A New Set of Circumstances. Between 1957-58 inquisitive students from the University of California, Santa Barbara kept asking their botany professor about unusual bare zones they saw around stands of Salvia leucophylla (purple sage) during field trips to a coastal sage scrub plant community in the Santa Ynez Valley, northwest of campus. The "halos" of naked ground were so striking it was as if they had been created by herbicide. Bare areas were observed around Artemisia californica (California sagebrush) as well. What was preventing plants from growing in these zones? Could Salvia or Artemisia themselves be responsible?

The students' professor was in a unique position to evaluate the possibility because he was already familiar with allelopathy. He had just completed a major paper on the subject concluding it was not responsible for vegetation patterns elsewhere. But this was different. It was a new setting, a different group of plants. So demonstrating a remarkable level of intellectual flexibility, he decided to re-examine the possibility and let the data speak for itself. The professor's name was C. H. Muller.