Freezing tolerance and recovery of arbuscular-mycorrhizal and non-mycorrhizal Thuja occidentalis.

Mycorrhizal symbiosis increases nutrient uptake of the host plant, and it can also improve their stress tolerance. Roots are considered the most frost-sensitive plant parts. However, the freezing tolerance of mycorrhizas, and especially arbuscular mycorrhizas (AM), is poorly understood. Here, we studied the freezing tolerance and recovery of AM and non-mycorrhizal (NM) Thuja occidentalis (L.). After a simulated summer and autumn, whole-plant freezing tests were done using seven exposure temperatures from 5 °C to -45 °C. Then freezing damage of needles was assessed by relative electrolyte leakage. The seedlings were kept for 2 weeks in long-day recovery conditions with day temperature either 10 °C or 22 °C, and then visual damage, shoot and root mass, nutrient concentrations and mycorrhizal colonization were analyzed. Before the frost exposure, AM plants had higher phosphorus (P) concentrations and similar growth to NM plants. Needle freezing tolerance was -23 °C (corresponding to lethal temperature for 50% of specimens) and was not affected by AM. Visual investigation after the recovery period showed a similar result. Lower foliar nitrogen (N) concentration and root mass in seedlings exposed to -18 °C in both AM and NM plants suggests that fine-root damage had taken place already before -18 °C. Recovery in 22 °C increased nutrient uptake and growth only in seedlings exposed to +5 °C and -5 °C, but specific root length increased also after -18 °C. The AM plant shoots grew less than NM ones during the recovery period. Arbuscular mycorrhizas increased foliar N during recovery in all non-lethally exposed seedlings, and P concentrations in seedlings exposed to +5 °C and -5 °C. This was due to a concentration effect by the lower dry mass increment. These results suggest that the functioning of AM fungi can be limited by short growing seasons or in cold soil conditions, which may affect their distribution in cold regions.