Abstract
The “ Salvia phenomenon” is one of the most famous examples of allelopathic interaction between higher plants. The Salvia thickets are surrounded by zones of bare soil (“bare zone”, 1-3 m in width), which merge into areas of inhibited grassland (“zone of inhibition”) and finally undisturbed grassland at a distance of 3-9 m. This characteristic vegetation pattern was attributed to monoterpenes, especially 1,8-cineole and camphor, which volatilized from S. leucophylla leaves, got adsorbed in the soil around the Salvia thickets, and inhibited germination and seedling growth of annual herbs. Initially, continuity of hydrophobic environment (clay soil particles – cuticular waxes on the seed/seedling surfaces – plasmodesmata - plasma membrane) was regarded to be important for the lipophilic compounds to enter the target cells. However, monoterpenes can reach the target cells via aqueous route as well. Because monoterpenes produced by S. leucophylla all induce similar symptoms in the seedlings of target plants, their mode of action appears to be essentially common. They exert various deteriorating effects on the cells of target plants, which might be totally explained if the primary point of action resides in mitochondrial function (respiratory ATP synthesis) and/or generation of reactive oxygen species. In contrast to the previous belief that cuticular waxes act as the pathway of lipophilic monoterpene to enter the site of action or reservoir of the inhibitors, they may act as “adsorptive barrier” to prevent the entering of monoterpenes inside the cell wall.
Highlights
INTRODUCTIONSalvia leucophylla (purple sage, gray sage, or wild Carifornia sage) is an aromatic shrub dominant in the coastal sage scrub in southern California
Salvia leucophylla is an aromatic shrub dominant in the coastal sage scrub in southern California
How are terpenes accumulating in Salvia leaves released into the environment? Basic mechanism of monoterpene volatilization has been studied in black sage (S. mellifera), which, like S. leucophylla, is an aromatic shrub growing in coastal Southern California [33, 34]
Summary
Salvia leucophylla (purple sage, gray sage, or wild Carifornia sage) is an aromatic shrub dominant in the coastal sage scrub in southern California. (iii) The soil-bound monoterpenes inhibit the seed germination and seedling growth of annual grasses and forbs, contributing to the above-mentioned characteristic “bare zones”. The zones of contact between Salvia thickets and grassland characteristically exhibit “bare zones” extending 1 to 2 meters beyond the Salvia crowns The soil within this zone is completely barren of herbs or exhibits only sparsely scattered and stunted seedlings of several selective annual species (Erodium, Festuca, and Bromus mollis); these seedlings cannot grow to maturity. (2)), and found that volatile materials emanating from crushed leaves of S. leucophylla could severely inhibited root growth of seedlings of cucumber (a model test plant) and A. fatua (one of the herbs efficiently excluded from the vicinity of S. leucophylla stands in the field), in a dose-dependent manner Muller and colleagues [1, 4] developed a new bioassay system to test the atmospheric transfer of the volatile toxins from Salvia leaves (Fig. (2)), and found that volatile materials emanating from crushed leaves of S. leucophylla could severely inhibited root growth of seedlings of cucumber (a model test plant) and A. fatua (one of the herbs efficiently excluded from the vicinity of S. leucophylla stands in the field), in a dose-dependent manner
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