The role of phenolics in goldenrod ball gall resistance and formation

Some genotypes of Solidago altissima have been shown to exhibit a hypersensitive response to the tephritid ball gallmaker Eurosta solidaginis. When the gallmaker attempts to stimulate gall formation in these genotypes, necrotic tissue surrounds the larva and apparently causes larval death Anderson, S. S., et al. (1989) Ecology 70, 1048]. Other genotypes have varying degrees of resistance or susceptibility McCrea, K. D. and Abrahamson, W. G. (1987) Ecology 68, 822]. Studies of other host plants have shown that phenolic compounds can play an important role in this defensive reaction. To determine the role of phenolics in the resistance of S. altissima to ball gallmakers, weekly tissue samples were collected from two resistant and two susceptible clones, both with and without Eurosta attack. These samples were analysed for total phenolics using the Folin-Ciocalteau procedure. Phenolic content increased over time in all clones. In unattacked ramets, phenolic levels were higher in susceptible clones than in resistant clones. No evidence of an increase in phenolics was seen with a hypersensitive response, but a significantly higher concentration was found in plants that had initiated normal gall development. A second study of rapidly growing galls in three additional susceptible clones confirmed this increase in phenolics, demontrated that the higher phenolic concentrations were restricted to gall tissue, and showed that phenolic levels increase as much as five-fold in galls near their peak growth period. Increases in total phenolic levels were clearly not responsible for the hypersensitive reaction for resistance, but phenols could potentially play a role in gall formation by influencing the hormonal control of growth and/or in protection of the gallmaker from its natural enemies.