It has been suggested that pectic
polysaccharides (or
oligosaccharides cleaved from them) are liberated from the cell wall upon wounding of leaf tissue, and that they act as long-distance
hormones evoking a defence response in neighbouring uninjured leaves (P.D. bishop et al. 1981, Proc. Natl. Acad. Sci. USA 78, 3536-3540, and cited literature). We have tested this hypothesis by infiltration of radioactive pectic fragments (
rhamnogalacturonans and homogalacturonans of degrec of polymerisation down to 6) into
wounds on tomato leaves. No radioactivity was exported from the treated leaf. [(14)C]
Sucrose, applied in the same way, was effectively translocated, probably via the phloem. We suggest that pectic substances are not themselves long-distance
wound hormones. The possibility remains that pectic substances, solubilised on wounding, act in the immediate vicinity of the
wound to stimulate the dispatch of a second messenger, which would be the long-distance
wound hormone.