Interaction of seed coat color and seed hardness: An effective relationship which can be exploited to enhance resistance to the safflower fly (Acanthiophilus helianthi) in Carthamus spp.

A choice test study was conducted in the field from 2014 to 2016 to determine the mechanism(s) involved in resistance to the safflower fly (Acanthiophilus helianthi, Diptera, Tephritidae) in germplasm of the wild safflower species with pigmented (Carthamus oxyacanthus, C. lanatus, and C. glaucus) and white (C. palaestinus) seed coats, some white-seeded cultivated genotypes, and a novel breeding line (A82) with black seeds obtained through interspecific hybridization of C. tinctorius × C. oxyacanthus. The choice test in 2014 revealed the significantly higher values of infested heads per plant and seed yield loss per plant of the white-seeded cultivated safflower (26.3-48.7% and 37.3-73.2%, respectively) compared with those recorded for the color-seeded wild accessions (3.0-10.9% and 3.8-11.4%, respectively) and line A82 (15.9% and 19.3%, respectively). Similar results were obtained in the two consecutive years of 2015 and 2016 as the black seed coated genotype A82 exhibited significantly lower seed loss per head and seed yield loss per plant than the other four cultivated genotypes. The highly significant correlations found between resistance to the safflower fly and each of the two parameters of seed hardness and seed coat color indicated that these latter parameters strongly affected plant response to the safflower fly; that is, harder seeds with darker seed coat colors made greater contributions to plant's resistance against the safflower fly while the white-seeded lines were generally found to be more susceptible to the fly. In short, the color-seeded wild species and the black-seeded line A82 exhibited a higher resistance while the white-seeded cultivars showed little or no resistance at all. Scanning electron microscopy was also used to investigate the morphological differences in seed coat among the color-seeded wild genotypes, black-seeded line A82 (resistant), and white-seeded cultivars (susceptible). The color-seeded genotypes and the white-seeded cultivars exhibited differences in their seed coat surface structure and thickness, suggesting that morphological structure and seed coat color concomitantly contribute to the fly resistance in some safflower genotypes. Based on the results obtained, genotype A82 may be suggested as a preferable and superior genotype for cultivation in areas under infestation by the safflower fly.