Influence of Biotic and Abiotic Factors on Dark Discoloration of Durum Wheat Kernels

Black point (BP, discoloration restricted to the germ end of cereal seed) and dark smudge (DS, discoloration mostly along the crease) are believed to be caused by fungal infection in grain exposed to high humidity, although it has also been reported that BP might result from abiotic stresses causing physiological changes in grain. The objective of this growth-chamber study was to determine the effects of abiotic (temperature and high humidity) and biotic (infection by Cochliobolus sativus [Ito and Kurib.] Drechs. ex Dast. (anamorph Bipolaris sorokiniana [Sacc.] Shoemaker); or Alternaria alternata [Fr.] Keissl.) factors on the development of BP and DS in durum wheat (Triticum turgidum L. ssp. durum [Desf.] Husn.). Five treatments under two temperature regimes (Low T, 17°C day and 12°C night; or High T, 26°C day and 18°C night) were conducted: inoculation with A. alternata or C. sativus at mid-milk with 30-h incubation at 100% humidity; one exposure to 100% humidity at mid-milk (HUMo); multiple exposures to 100% humidity from heading to maturity (HUMm); and no exposure to 100% humidity or fungal inoculum (DRY). Kernels were evaluated for incidence and extent of discoloration. The highest incidences of discoloration occurred in the C. sativus treatment, followed by the A. alternata treatment, with HUMm producing low incidences, and HUMo and DRY only occasional discoloration. In general, High T favored BP and Low T favored DS. In the C. sativus treatment, Low T also favored extent of discoloration. Whether fungal infection was promoted or unhindered by a primary effect of high humidity on the physiology of the kernel and its defenses could not be determined given that exposure to high humidity is a requirement for fungal infection.