Controlled freezing as an indirect selection method for field winterhardiness in turf-type perennial ryegrass

Winterhardiness is a complex trait that has been difficult to select for in many Poaceae species. Controlled freezing has been investigated as a selection method to improve winterhardiness; however, most previous reports in perennial ryegrass (Lolium perenne L.) have not provided estimates of genetic parameters. This research was conducted to determine the potential of controlled freezing as an indirect selection method for winterhardiness improvement in perennial ryegrass. Seedlings of perennial ryegrass half-sib families were frozen to -6°C under controlled conditions with freezing tolerance evaluated after 3 wk recovery as visual injury and wet and dry weight shoot regrowth. The same seedlings were subjected to a second freezing treatment of -9°C with freezing tolerance reevaluated. The combination freezing treatment of -6°followed by -9°C was more highly correlated with field winterhardiness than the single freezing treatment of -6°C. Measuring freezing tolerance as wet weight after the combination freezing treatment resulted in the highest genetic correlation (r(A) = 0.91) indicating that this trait and field winterhardiness were under similar genetic control. Selection using the combination freezing treatment (-6°C followed by -9°C) dry weight evaluation was 58% as efficient as field selection for improving winterhardiness. Ten of the top 15 winter-hardy families were selected by regrowth evaluation following the combined freezing temperature treatment using a selection intensity of 35%. These favorable results, plus the additional benefit of short cycle time, indicate that controlled freezing can be successfully used as an indirect selection method for increasing winterhardiness in perennial ryegrass.