Plants suppress their emission of volatiles when growing with conspecifics.

Plant volatiles mediate interactions with herbivores, herbivore enemies, and abiotic stresses, but these interactions mostly have been studied with individual isolated plants. It is not yet known how intra- and interspecific plant competition influence volatile emission. In a greenhouse experiment, we investigated the volatile emission by red clover (Trifolium pratense) growing alone, with a conspecific, or with an individual of the naturally co-occurring orchard grass, Dactylis glomerata. The individual and combined effects of above- and below ground plant contact were investigated. When T. pratense grew together with a conspecific, both total and herbivore-induced emission of volatiles was significantly reduced as compared to T. pratense growing with D. glomerata or growing alone. This reduction in emission occurred despite the fact that there was a significant reduction in T. pratense biomass due to competition with D. glomerata. The suppression of T. pratense volatile emission growing next to a conspecific was a general pattern observed for all major herbivore-induced volatiles and independent of whether plants were in contact above ground, below ground, or both above- and below ground. The reduction in volatile emission from plants growing with conspecifics may serve to reduce attack by specialist herbivores and minimize exploitation of herbivore attack information by neighbors.