Concentrations of sunflower phenolics appear insufficient to explain resistance to floret- and seed-feeding caterpillars

Authors: Prasifka, Jarrad; Wallis, Christopher

Submitted to: Arthropod-Plant Interactions
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 7/17/2019
Publication Date: 7/26/2019
Citation: Prasifka, J.R., Wallis, C.M. 2019. Concentrations of sunflower phenolics appear insufficient to explain resistance to floret- and seed-feeding caterpillars. Arthropod-Plant Interactions. 13:915-921. https://doi.org/10.1007/s11829-019-09706-y.
DOI: https://doi.org/10.1007/s11829-019-09706-y

Interpretive Summary: Specific varieties of cultivated sunflowers can be more or less damaged by insect pests, but it is not clear what plant traits determine apparent resistance to insects. In other wild and cultivated plants, a specific group of similar chemicals, called phenolics, often provides resistance to insects or diseases. Because there are several phenolic compounds known from sunflowers, information was collected on the amounts of phenolics in many different wild and cultivated sunflowers, and tests were conducted to determine if the variation in phenolic conncentration is important to development and survival of sunflower insect pests. The amount of phenolics in sunflowers varied, with about a four-fold difference (˜18–72 mg/g) between the highest and lowest varieties. When diets with a specific phenolic, chlorogenic acid, were used to feed larvae of one sunflower pest, the sunflower moth, small effects on size and survival were sometimes seen at the highest doses, though this effect was inconsistent. When information on sunflowers with resistance to another pest, the banded sunflower moth, was compared to phenolic concentrations in the same varieties, the resistant varieties did not match the lines with more phenolics. The information collected shows that the variation in sunflower phenolic compounds is not a primary tool to manage floret- and seed-feeding insect pests, though there are other possible uses for variation in phenolics, including improved stability of sunflower oil.

Technical Abstract: Cultivated sunflowers, Helianthus annuus L., show significant variation in susceptibility to insect pests, though specific mechanisms of resistance are not clear. Plant secondary compounds, including phenolics, are often associated with resistance to insects and pathogens. Because several phenolics are present in sunflower florets, wild sunflowers and cultivated inbred lines were sampled to document natural variation in free phenolics and evaluate their potential effects on floret-feeding pests. Four di-O-caffeoylquinic acid isomers were the largest contributors to total phenolic concentration in disc florets. When identified phenolics were combined with unidentified flavonoid glycosides, total phenolic concentration in inbred lines varied four-fold (˜18–72 mg/g), and was significantly greater than that in wild sunflowers. Two lab assays with 50 or 100 mg/g chlorogenic acid showed inconsistent reductions in mass of sunflower moth (Homoeosoma electellum Hulst) larvae after 7 d, and tests using floret tissue with low or high levels of total phenolics did not show an effect of tissue type on development. Tests for correlation between total phenolics and field data on susceptibility of sunflowers to another floret-feeding pest, banded sunflower moth (Cochylis hospes Walsingham), did not show any significant association. Cumulatively, results suggest there may be minor effects of phenolic compounds on floret-feeding insects, but these alone are insufficient for plant defense. However, variation in concentration of phenolic compounds may remain valuable, both as a component of plant defense and a source of oxidative stability in sunflower oil.