High nitrogen (N) is often associated with decreased freezing tolerance in grasses, possibly as a result of delayed cold acclimation. Furthermore, elevated carbon dioxide (CO2) can influence the freezing tolerance of grasses by altering ice nucleation temperatures and increasing the concentration of compatible solutes, which includes various soluble sugars. Both CO2 and N may interact to determine the freezing tolerance of plants, but such interactions have not been examined for herbaceous species. We investigated the interactive effect of CO2 and N on the freezing tolerance of Kentucky bluegrass (Poa pratensis). We also examined the relationship between the treatment responses and variation in glucose, sucrose and fructose concentrations to explore the possible mechanism for variation in freezing tolerance. We grew plants at high CO2 (750 ppm) and low CO2 (400 ppm) in the Biotron Experimental Climate Change Research Centre in the summer, and fertilized plants weekly with Hoagland’s solution high or low in NH4NO3. Plants were then exposed to 5, 0, -2, -4, -6, -8 or -10 °C for 18 h in freezing chambers in the fall, and we subsequently assessed electrolyte leakage for leaves. We determined sugar content in the plants prior to freezing using the phenol-sulfuric acid method.
Based on the electrolyte leakage results, N fertilization increased leaf freezing damage, whereas elevated CO2 concentrations reduced leaf freezing damage. There was a significant interaction between CO2 and N, whereby elevated CO2 reduced freezing damage under high N fertilization. However, this interaction was not correlated with increased glucose, sucrose and fructose concentrations in leaves under elevated CO2 and N. In a broader context, exposure to extreme freezing is expected to increase for herbaceous plants in northern temperate regions over the next century as a result of decreased snow cover. Our study suggests that concomitant increases in atmospheric CO2 concentrations over this time may help alleviate decreases in freezing tolerance for grasses grown under high N.