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 fr eezing temperatures has been pointed out as a major factor responsible for minter damages, little information exists on freezing tolerance and cold ha rdening of green-type annual bluegrass. This study was conducted to assess freezing tolerance and carbohydrate changes occurring during cold acclimati on of green-type annual bluegrass ecotypes cold hardened under both environ mentally controlled and simulated winter conditions in an unheated greenhou se. The 50% killing temperatures (LT50), levels of fructans, and mono and d isaccharides were determined during cold acclimation in three annual bluegr ass ecotypes originating from Western Pennsylvania (OK), Coastal Margland ( CO) and Central Quebec (CR), The ecotypes differed significantly with regar d to their freezing tolerance (LT50 ranking: OK < CO < CR) and maintained t heir relative ranking under both environmentally controlled and simulated-n atural winter conditions, Maximum freezing tolerance was observed after exp osure to nonlethal subfreezing temperatures and annual bluegrass achieved h igh levels of freezing tolerance with LT50 of -31.2 degreesC for OK, -24.6 degreesC for CO, and -22.8 degreesC for CR. High molecular weight fructans (DP >6) were the most abundant carbohydrates found in plants cold-acclimate d under low, nonfreezing temperature with levels up to 170 mg g(-1) dry wei ght as compared with 60 to 70 mg g(-1) dry weight in nonacclimated plants. Sucrose levels in crowns of annual bluegrass markedly increased at temperat ures below freezing and maximum sucrose concentration coincided with maximu m freezing tolerance of annual bluegrass, However, variations in fructan an d sucrose levels were not related to differential freezing tolerance among the three annual bluegrass ecotypes tested.