Freezing Tolerance and Carbohydrate Changes during Cold Acclimation of Green-Type Annual Bluegrass (Poa annua L.) Ecotypes

Winterkill is recurrently observed on annual bluegrass (Poa annua L.) golf greens in northern climates. Although annual bluegrass susceptibility to freezing temperatures has been pointed out as a major factor responsible for winter damages, little information exists on freezing tolerance and cold hardening of green-type annual bluegrass. This study was conducted to assess freezing tolerance and carbohydrate changes occurring during cold acclimation of green-type annual bluegrass ecotypes cold hardened under both environmentally controlled and simulated winter conditions in an unheated greenhouse. The 50% killing temperatures (LT₅₀), levels of fructans, and mono and disaccharides were determined during cold acclimation in three annual bluegrass ecotypes originating from Western Pennsylvania (OK), Coastal Maryland (CO) and Central Québec (CR). The ecotypes differed significantly with regard to their freezing tolerance (LT₅₀ ranking: OK < CO < CR) and maintained their relative ranking under both environmentally controlled and simulated-natural winter conditions. Maximum freezing tolerance was observed after exposure to nonlethal subfreezing temperatures and annual bluegrass achieved high levels of freezing tolerance with LT₅₀ of −31.2°C for OK, −24.6°C for CO, and −22.8°C for CR. High molecular weight fructans (DP>6) were the most abundant carbohydrates found in plants cold-acclimated under low, nonfreezing temperature with levels up to 170 mg g⁻¹ dry weight as compared with 60 to 70 mg g⁻¹ dry weight in nonacclimated plants. Sucrose levels in crowns of annual bluegrass markedly increased at temperatures below freezing and maximum sucrose concentration coincided with maximum freezing tolerance of annual bluegrass. However, variations in fructan and sucrose levels were not related to differential freezing tolerance among the three annual bluegrass ecotypes tested.