Several studies have assessed the function and significance of the presence of dead, hardened husks on germination and seedling growth in several grass species and reached to inconsistent results. Here, we assess the roles of husks (dead lemma and palea) and an inner membrane surrounding the grains on germination behaviour and seedling growth of Brachypodium hybridum, one of three species of the genetic model B. distachyon complex, in an arid mountain of Arabia. The interactive effects between temperature and the incubation light were assessed on germination of husked and dehusked-demembraned grains. Germination and seedling growth were assessed for different combinations of grain treatments (soaked and non-soaked husked, dehusked-membraned and dehusked-demembraned). Dehusked-demembraned grains were also germinated in different dormancy regulating compounds (DRCs) and light qualities (light, dark and different red: far red [R: FR] ratios). The results indicated an insignificant difference between husked and dehusked-membraned grains on final germination and the germination rate index (GRI), with the former producing significantly bigger seedlings. Removal of the inner-membrane resulted in a significant reduction in all traits. Soaking grains in water resulted in significant enhancements in germination and seedling growth of only husked grains. Husked-membraned and demembraned grains germinated more significantly and faster at lower rather than higher temperatures. None of different concentrations of several DRCs succeeded in enhancing final germination of dehusked-demembraned grains. Red-rich light significantly enhanced germination of dehusked-membraned grains in comparison to other light qualities. It could be concluded that the role of husks is to mainly enhance seedling growth, while the major role of the membrane is to increase final germination. The ability of red-rich light in enhancing the germination of dehusked-membraned but not dehusked-demembraned grains suggest a role for the inner membrane in regulating dormancy through differential filtering of light properties.

Seed dormancy and germination have been studied in the genetic model Brachypodium species complex in cooler, moist higher latitudes. Studying environmental factors in arid mountains affecting dormancy and germination in Brachypodium complex could determine the factors controlling these processes. This study assesses the impacts of temperature during seed maturation, seed after-ripening, drought, photoperiod, and thermoperiod on final germination and germination rate index of B. hybridum in the Arabian Desert. Seeds were germinated under dark or light conditions and under different ratios of red:far-red light, with three diurnal thermoperiods. The final germination percentage was significantly greater at 15/25 °C and 20/30 °C than at 25/35 °C and in light rather than in darkness. Seeds that reached maturity at 15/25 °C attained greater germination rates and faster germination than those that reached maturity at 20/30 °C. One-year after-ripening enhanced the final germination percentage and reduced photoperiod requirements. Light quality did not affect final germination percentage. The seeds tolerated drought of up to –0.8 MPa polyethylene glycol. The tolerance of B. hybridum seeds produced at higher thermoperiods to moderate levels of osmotic stress and their higher dormancy indicate that this species has the potential to survive the projected global warming in its native and introduced ranges.