Early-stage detection of root freezing injuries of Scots pine (Pinus sylvestris L.) seedlings by impedance loss factor and hydraulic conductance.

The condition of the root system affects the quality of seedlings in forestry and horticulture. Previously, the electrical impedance loss factor (δ) and the reverse-flow hydraulic conductance (K ) of the roots of Scots pine seedlings were found to increase when assessed a few days after frost damage. How these variables change with time after the root damage is unknown. We arranged an experiment with 1.5-year-old Scots pine seedlings exposed to -5°C or - 30°C, with the control seedlings kept at 3°C. Then, δ and K of roots were monitored for 5 weeks in favorable growing conditions. The properties of the roots were observed to be in a dynamic state after the damage. A significant difference in δ was found between the test temperatures -30°C versus -5°C and 3°C (p = 0.004 and p < 0.001, respectively). The clearest effect of freezing injuries on δ of roots was observed in the first measurement 1 week after the freezing test. The temperature significantly affected K too, with a significant difference between the low-temperature treated plants -30°C versus -5°C and control (p < 0.001, respectively). The difference in K between -30°C and the other two temperatures increased with time and was the largest in the last samples, taken after 5 weeks. We conclude that the impedance loss factor may detect root damage if the measurements occur early enough after the damage, but a longer time difference (3-5 weeks) is needed according to the reverse-flow hydraulic conductance.