Our work and that of others has shown that the spread of induced responses through the plant is spatially and temporally heterogeneous. One reason for this is that the movement of materials and signals within the plant is constrained by the pattern of vascular connections. Leaves with direct vascular connections respond together while others lag or may not respond at all. We have mapped the patterns of response within poplar trees and Arabidopsis using chemical and isotope tracers (Ferrieri et al. 2015).
Another major influence on the pattern of defense response is the relative strength and position of photosynthate sinks relative to sources. We have shown that insect feeding creates strong local sinks, drawing photosynthate to the point of attack, which is incorporated into defense compounds (Arnold and Schultz 2002; Arnold et al. 2012). These locally formed sinks can compete with other sinks in the plant such as buds or fruits, and some tissues such as old leaves cannot respond with increased sink strength. We also found that chilling roots changes the pattern of photosynthate movement above ground among sinks and sources (Ferrieri et al. 2013).
The heterogeneity caused by differential vascularization and sink-source relationships creates a highly variable chemical environment for foraging insects. Not all parts of a plant are equally defended at a given point of time, and the resulting pattern is dynamic over the course of hours or even minutes. As we theorized a long time ago, the behavior of insects can be influenced by this, as they move from points of declining (induced) food quality to better sites or even different plants. This can result in elevated mortality arising from increased encounters with predators, parasites and pathogens (Schultz 1983a; Schultz 1983b).
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