Induced Responses of Common Annual Sunflower Helianthus annuus L. from Geographically Diverse Populations and Deterrence to Feeding by Sunflower Beetle

Abstract

The coumarins ayapin and scopoletin, which are expressed in leaves and other tissues of Helianthus annuus L., have a variety of effects on fungi and insects. The relative importance of these coumarins in the defenses of H. annuus against the sunflower beetle was investigated. Unstimulated leaves of 107 populations of wild annual sunflower from various states of the United States were analyzed for coumarin expression. Contralateral leaves were stressed with cytotoxic HgCl2 to provoke a typical wounding response (including the synthesis of defensive chemicals) and then were also analyzed. Some populations showed almost no increase in scopoletin content after stimulation, and others showed up to a 56-fold increase. The average increase of scopoletin content was about nine fold. Populations collected from the region where H. annuus ssp. texanus was prevalent had the highest concentration of scopoletin after stress. A subselection of those populations that had high coumarin content or that had been stimulated to high levels of coumarin was assayed for feeding deterrence against the sunflower beetle (Zygogramma exclamationis F.). A two-choice feeding bioassay was accomplished within a standard arena over 20 hr. When preferences for leaves of each sunflower population were tested against a commercial cultivar without leaf stress, only one of 11 populations was found to show significant deterrence when total leaf area eaten was analyzed. However, after leaf stress, eight of 11 populations became significantly deterrent, including the one that had been deterrent without stimulation. Differences between scopoletin content and between feeding deterrence before and after stimulation were displayed graphically to determine if a correlation could be made between coumarin content and feeding behavior. The plot showed that in six of 11 populations increased scopoletin content was indeed correlated with decreased feeding, but that in another two, these factors were not correlated; another two populations showed no difference in feeding from the control scores after stress and could not be used in this graphical correlation procedure. One population showed a decrease in both coumarins after stimulation along with decreased feeding, suggesting that additional unidentified deterrents were formed. From the analysis of coumarin distributions and the feeding results, we conclude that coumarins are widely occurring, inducible constituents in the wild common annual sunflower. Coumarins may be of general importance in H. annuus populations throughout their range either in deterring sunflower beetle or in additional effects on other organisms. Coumarins appear to be deployed typically in induced defenses and may not be important in constitutive defenses.