Tissue-specific induction of resistance to herbivores in a brown seaweed: the importance of direct grazing versus waterborne signals from grazed neighbors

In theory, plants could induce resistance to herbivores after being directly grazed or after sensing cues from the grazing of neighboring plants (i.e., the "talking trees" effect). Despite an extensive literature on induced defenses in plants, the relative importance of direct attack versus cues from neighbors in mediating induction is not well understood; this is especially true for seaweeds, where few investigations of induction due to attacks on neighbors have been conducted. We measured changes in resistance to grazing for stipes and blades of the brown seaweed Sargassum filipendula (Phaeophyta; Fucales) when it was directly grazed by amphipods or when it received potential waterborne cues from the grazing of neighboring plants, relative to plants that experienced neither direct grazing nor cues from grazed neighbors. After 25 days of these treatments in outdoor tanks, the amphipod Ampithoe longimana consumed top stipes from control plants 2.8 times more rapidly than top stipes from directly grazed plants, but did not differentiate between top blades in the same comparison. The amphipod did not discriminate between plants with grazed neighbors and plants with control neighbors when feeding on either stipes or blades. Thus, induction of resistance in S. filipendula was tissue-specific and occurred as a result of direct grazing, but not due to the grazing of neighboring plants. (C) 2002 Elsevier Science B.V. All rights reserved.