Frost tolerance and ice formation in Pinus radiata needles: ice management by the endodermis and transfusion tissues

Conifers are among the most frost tolerant tree species. Cryo-scanning electron microscopy (cryo-SEM) was used to visualise ice formation in pine needles to better understand how conifer leaves manage extracellular ice. Acclimated and unacclimated needles of Pinus radiata (D.Don) were subjected to freezing treatments (at a rate of 2°C h^-1), tested for electrolyte leakage and sampled for cryo-SEM analysis. Half maximal electrolyte leakage occurred at -4 and -12°C for unacclimated and acclimated needles, respectively. Ice nucleation occurred at similar temperatures (-3°C) in both acclimated and unacclimated pine needles, indicating that frost tolerance did not increase supercooling. During freezing and thawing, the tissues outside and inside the endodermis shrank and swelled independently, with little or no transfer of water between the two regions. During freezing, mesophyll cells shrank, exhibiting cytorrhysis, and extracellular ice accumulated in gas spaces of the mesophyll tissue. Mesophyll cells from acclimated needles recovered their structure after thawing, and unacclimated mesophyll showed significant damage. In the vascular cylinder, ice accumulated in transfusion tracheids which expanded to occupy areas made vacant by shrinkage of transfusion parenchyma, Strasburger cells and the endodermis. This behaviour was reversible in acclimated tissue, and may play an important role in the management of ice during freeze/thaw events.