Development of an indoor freezing tolerance test for switchgrass

Winter survival is the most important limiting factor for switchgrass cultivation in northern climate. Field selection for that trait is a tedious process which relies on unpredictable winter conditions to identify more winter hardy genotypes. Winter survival is a complex trait depending on plant capacity to withstand multiple abiotic and biotic stresses, among which freezing tolerance (FT) has a critical role. We therefore developed an indoor screening method to assess FT of switchgrass populations. This method relies on plant cold hardening followed by the application of successive freezing stresses to identify the temperature at which 50% of plants die (LT50).The cold hardening period is essential for the full expression of the freezing tolerance potential. We used 432 genotypes of 3 switchgrass cultivars (Kanlow, Cave-in-Rock, and Dacotah) originating from different latitudes (25°N to 45°N) in USA. Plants were either cold harden (induced by changes in photoperiods and temperatures) or not before the freezing test. The lowland ecotype had a similar LT50 in both treatments. Significant LT50 differences were observed between the cold harden and the non-cold harden upland populations. Within the non-cold harden populations; Cave-in-Rock had a similar LT50 than Kanlow, whereas Dacotah had a higher LT50 than both populations. Within the cold harden populations; Kanlow had a LT50 of -8.5, Cave-in-Rock of -12.7 and Dacotah of -16.7°C. Based on these results, the three cultivars responded as expected based on the literature and the geographic origin of each cultivar. The hardening period, which is essential for the full freezing tolerance expression, was effective for the upland cultivars whereas the lowland cultivar Kanlow does not seem to respond to our hardening conditions. More research on lowland ecotypes cold hardening process is needed to increase switchgrass adaptation to northern areas.