Elevated CO2 alleviates decreased freezing tolerance under high nitrogen in the grass, Poa pratensis

Abstract

Freezing stress, which can greatly reduce the growth, reproduction, and survival of plants, is predicted to increase for herbaceous species in northern temperate ecosystems as a result of climate change-induced reductions in snow cover. Increased N availability and elevated CO2 are other factors that may alter responses of plants to freezing, but studies regarding their interactive effects on the freezing tolerance of herbaceous plants are limited. We investigated the interactive effects of N and CO2 on the freezing stress response of the grass, Poa pratensis. Tillers were grown in controlled environment greenhouses in London, Ontario under ambient (400 ppm) or elevated (750 ppm) CO2 for 11 weeks in combination with low or high N fertilization. Tillers were then subjected to either 0, −2, −4, −6, −8, or −10 °C for 18 h (controls were harvested prior to freezing), and relative electrolyte leakage was measured to assess leaf freezing damage. We also determined leaf sucrose, glucose, and fructose concentrations before freezing, which helped explore the potential mechanisms underlying variation in leaf freezing tolerance. Electrolyte leakage began to increase for some of the treatments at −4 °C, and all treatments exhibited increased electrolyte leakage by −10 °C. There was a significant interaction between CO2 and N, whereby elevated CO2 alleviated the decrease in leaf freezing tolerance observed under high N. Treatment effects on freezing tolerance were not associated with changes in total sugar concentrations. Our results highlight that there are important interactions among global change factors that can alter plant freezing tolerance.