Allelochemicals influence the growth of nearby plants, typically reducing competition, or alerting nearby plants to dangerous biota in time for them to start synthesis of phytoalexins. Evidence suggests that plants in monocultures are likely to produce more volatile alleochemicals [
117], and that plants show greater response to BVOCs from genetically identical wounded plants, and hence are capable of self-recognition and kin-recognition [
118]. A greater response to constitutively produced volatiles was observed from the same cultivar [
119], implying a species level adaption of sesquiterpene lactone composition and detection, and a strong genetic component in kin-recognition. The response would appear to get stronger as the genetic similarity increases with self recognition of clonal plants eliciting the strongest response [
120] and showing a continuum of responses as similarity of the detected volatile profile approaches that of the plant’s own. This is further supported by the fact that volatile emission is very variable between species, even within the same genus. Many sesquiterpene lactones are highly characteristic of the genus or class in which they are found, for example the guaianolides in the
Lactuca subclass are known to be bound to glucose, and lack C-3 oxysubstitution in comparison with the guaianolides of the Ixeris subclass which do not [
30], as well as being unique, to our knowledge, in having sesquiterpene lactone oxalates and sulfates [
121]. A plant’s priming response is a mechanism by which it can prepare to defend itself against attacking microbes or insects, and is exhibited in many stress responses. Such an effect can be seen in a study by Karban
et al. [
114] in which tobacco plants (
Nicotiana attenuate Torr.) reacted with antifeedant production in reaction to the sensing of methyl jasmonate produced by cut sagebrush (
Artemisia tridentata Nutt.). This response was seen only when tobacco plants were in the airflow of injured sagebrush, and not when airflow was cut off, consequently it was suggested that the signal was transmitted by air. The study however only looked at levels of methyl jasmonate, which increased upon injury, but does not test this theory with direct application of pure methyl jasmonate, however such a method has been successfully attempted by others [
122]; and although studies have shown allelopathic interactions with methyl jasmonate [
123], there has been to our knowledge, a dearth of conclusive evidence for the role of methyl jasmonate in inducing phytoalexins. Another study [
119] showed that the growth rate of aphids was affected by exposure of a plant to volatiles of nearby plants of the same species. The study also showed that the acceptance of the plant as an aphid host is reduced. This was interpreted by the experimenters to be the consequence of insecticidal terpenoids being produced by the host plant in response to allelopathic signals from a nearby plant. The mechanism for distinguishing different volatile complexes has not been characterized.
Despite this capacity to react to the stress responses of nearby plants, neighboring plants are one of the main sources of stress, in terms of competition for light, nutrients, and water [
124], and hence the alleochemicals produced which typically slow growth may be a selfish response to competing plants of the same species. It can be argued that plants of the same species represent greater competition, as they will compete for exactly the same resources as the stressed plant. A study by Ninkovic [
115] showed that the Kara cultivar of barley (
Hordeum vulgare) responded more greatly to volatile emissions from the cultivar Alva than Kara cultivar, increasing the ratio of root to shoot growth. However, both of the Ninkovic studies reviewed specifically chose Kara as a cultivar which is known to be relatively inert to self-induction, potentially as a result of domestication; hence Kara would be expected to respond unusually weakly to its own volatiles. The author also states that the benefit to either participant remains to be seen, as increased root growth results in less leaf growth, potentially resulting in a competitive disadvantage for light in exchange for greater nutrient acquisition. Results of a follow up study [
119] suggest a large amount of variation in reaction to volatiles to other plants within the species. The accepted hypothesis is that BVOCs are produced to hinder the growth of competitor plants, especially during times of other stresses; however proponents of the “selfish gene theory” may well put forth the concept that such volatiles are produced in order to aid nearby plants of the same species, thus causing a proliferation of identical genetic material on a species level. Reduction of growth, in addition to inhibition of seed germination, would make sense in terms of nutrient conservation in order to avoid wasted resources being directed to organs under threat of attack, and prevent germination until the microbial threat has subsided. Consequently, BVOCs should be seen as compounds produced for the benefit of the species as a whole, and not just for the individual producing plant.