Variation in the number of capitate glandular trichomes in wild and cultivated sunflower germplasm and its potential for use in host plant resistance

The capitate glandular trichomes of wild sunflowers ( Helianthus spp.) are considered to be effective defence components that act against some herbivorous insects, but cultivated sunflowers are reportedly deficient in glandular trichomes. To investigate whether glandular trichomes have a role in the protection of cultivated sunflowers against insects, in the present study, Helianthus annuus L. accessions were grown to quantify glandular trichome density in wild and cultivated germplasm types and assess potential anti-insect effects of terpenoids in the glandular trichomes of cultivated sunflowers. Evaluation revealed that capitate glandular trichomes are often abundant in cultivated sunflowers; relative to wild H. annuus , inbred maintainer (HA) lines have similar numbers of glandular trichomes per floret, while commercial hybrids have only ≈ 20% fewer trichomes when compared with wild sunflowers. In the laboratory assay, it was found that glandular trichome extracts increased the mortality rates of sunflower moth, Homoeosoma electellum (Hulst), larvae exposed from the neonatal stage to 9 d. In the surviving larvae, the extracts significantly reduced larval mass and head capsule width. Though there are limitations to the value of glandular trichomes for host plant resistance, the feeding deterrent or toxic effects of sesquiterpene lactones and diterpenes in sunflower glandular trichomes are not limited to sunflower moth larvae, suggesting a potential for resistance to other sunflower insect pests. Additional research is required to understand the inheritance and value of glandular trichomes in commercial sunflower germplasm and how the composition of terpenoids in the glandular trichomes of wild H. annuus may differ from that in cultivated material.