Bioassays to establish mechanism of allelochemical transport

A series of bioassays established the credibility of the route of volatile terpenes produced by the plants to the soil. Toxin production from roots was eliminated in the preliminary experiments, but leaf material, in contrast, was highly toxic when assayed in direct contact with germinating seeds and also when crushed leaves were present in a sealed bioassay chamber in which only vapor transport of toxins to seeds was possible. Avena fatua root growth was reduced 50% by volatiles from 0.5 g of Artemisia califomica, and 64% by 0.5 g of S. leucophylla (Muller et al. 1964). Volatile inhibitors were also indicated by observations that the bare and inhibition zones were similar in extent on both the uphill and downhill sides of S, leucophylla stands (Muller 1966). 

Several monoterpenes were identified in the leaves of three Salvia species. They included a-pinene, 1,8-cineole, dipentene, and camphor (Muller Muller 1964). Camphor and cineole were the most toxic in bioassays with cucumber. Analyses of the atmospheres in sealed containers of crushed Salvia leaves showed significant quantities of monoterpenes. It is important to note that although monoterpenes are typically thought of as volatile compounds, they are also 

It was shown that soil colloids could adsorb volatile terpenes from the atmosphere (Muller del Moral 1966). Fresh soil exposed to volatile terpenes and also soil that had been stored several months were both toxic when bioassayed with a grass, Bromus rigidus. Other work showed that cineole and camphor could be recovered from soil in bare zones next to Salvia, but not in adjacent grassland (C.H. Muller 1965). Growth of the grass in surface soils of the bare zone was reduced compared to controls from nearly artificial bare areas. By 4-6 weeks into the growing season, terpenes were no longer detectable in the soil (Muller Chou 1972). 

Tyson et al. (1974) took the work a step further when they conducted studies of the rate of terpene volatilization from foliage of S. mellifera. The calculated rate of volatilization was 13.3 Lig/dm per day. Total loss of photosynthate was estimated at 0.06%. On an area basis, it was calculated that 3.1 kg/km were released daily. The total terpene content of the leaves was more than 1000 times the amount volatilized. 

Several studies by Muller and Hauge (1967) and W.H. Muller et al. (1968, 1969) showed that S. leucophylla terpenes affected development of cucumber seedlings. Root and hypocotyl growth were reduced, lipid globules built up within cells, outer cell walls had deposits of excess cutin in the epidermis, fewer lateral roots were produced (Muller Hauge 1967), respiration of roots diminished (W.H. Muller et al. 1968), and uptake of oxygen by mitochondria was reduced (W.H. Muller et al. 1969). Unfortunately, similar studies were not conducted with chaparral and grassland species. 

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