Our review of published studies on volatile interactions between plants demonstrates that plant communication is a general event not limited to damaged plants, and that it is more a phenomenon of volatile eavesdropping rather than signalling. Empirical studies in this thesis show that barley plants perceive the growth pattern of their neighbours via volatiles and adapt their own growth strategy accordingly, which can be seen as a preparation for future competition. Field experiments show different trait responses of cultivars grown in mixtures, and that these responses depend on the neighbouring cultivar identity. Plants responded to their neighbours with adaptive and maladaptive growth responses, with increased nitrogen accumulation efficiency, biomass production and grain yield as a result. Aphid populations were generally supressed in these cultivar mixtures; not due to aphid colonization pattern or natural enemy abundance, but possibly induced by volatile interactions between neighbouring plants of different cultivars. Aphid responses to one cultivar in a mixture were neighbour specific and pest suppression was most pronounced when both cultivars in a mixture showed a reduced aphid-plant acceptance after reciprocal volatile exposure in the laboratory. Such individual neighbour responses mediated by volatiles can explain the inconsistent effects of cultivar mixtures in previous research. The findings of this thesis establish a better understanding of volatile communication between plants adding a new dimension to plant behaviour and community processes. Combining cultivars in mixtures based on how they interact with each other is a promising strategy for productive and sustainable agriculture.
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