Proof 23 Long-Distance Signal Transmission in Trees Stefano Mancuso, Sergio Mugnai Abstract Long-distance transmission of signals is a critical event in the life of trees. Many physiological studies have deduced that hormone-like substances together with hy- draulic/electrical signals mediate this important function of the plant life. However the nature of the complex network of signalling in trees has remained essentially unexplored. Recent molecular and genetic studies offer new approaches to understanding the mecha- nisms underlying the transmission of signals. 23.1 Introduction The ability to sense and respond to environmental parameters is one com- mon feature of living being. This talent is particularly significant in land plants because of their necessity to manage all the environmental informa- tion and stimuli without escaping. Among plants, trees, for their necessity to respond in a short time to environmental stimuli despite the often big dimensions, are the most obvious and interesting subjects of study for the long-distance signal transmission. Trees live in a continuously changing environment and although not all parts of the tree are exposed to the same stimuli at the same time, different parts of it respond in a coordinated fashion, demonstrating the existence of communication between various regions of the tree. For years, researchers have concentrated their efforts on the study of chemical (hormonal) signals in trees, neglecting the fact that plants also use and rely on electrical and hydraulic signals. In addition, it is especially in big plants such as trees that the need for signals other than hormones becomes more important. Considering the large leaf area of a tree, enormous amounts of chemicals would need to be synthesized and transported in order to respond to a signal coming from the roots. In contrast, both electrical and hydraulic signals consume no chemicals in their propagation. Here, “long distance” will refer to signals running for distances that can- not be covered in reasonable times (a few hours) by diffusion in the aqueous phase, and that often have to cover several metres as in the cases of root-to- shoot communication. For a long time, plant physiologists have presumed that signalling systems in plants must involve transport of “something” through the vascular system, effectively ignoring any other kind of signal Communication in Plants F. Baluška, S. Manusco, D. Volkmann (Eds.) © Springer-Verlag Berlin Heidelberg 2006