Physiological bases of frost mitigation in trees
Due to global climate changes many natural and landrace plant communities will be exposed to shifts in weather extremities that may affect their survival in the near future. For instance, in the semi-arid regions of central California, shifts in winter-rainfall and fog distribution have led to an increased number of frost events in early fall. At this time of year trees are not yet fully cold-hardened and winter-dieback of cold-tolerant species is threatening lowland forest and agriculture ecosystems. Analyses of the impact of early frost on Pistacia orchards suggests that the magnitude of damage depends on attributes of tree physiology (water stress level and stem concentration of nonstructural carbohydrates) and tree biological processes (stem respiration and starch degradation). To study the biological effects of frost we have imitated freeze-thaw events in the laboratory on non-hardened stems of Pistacia trees experiencing different levels of water stress (-0.3, -0.6, and -1.0 MPa).
Stem damage occurred only in plants where stem water crystallized (evidenced by a release of energy) and the level of damage was inversely proportional to water stress. In addition, low water potential reduced the stem-water crystallization temperature by maintaining a super-cooled state, thus providing a potential avoidance mechanism for frost protection. Monitoring stem respiration revealed further information concerning the response of trees to frost. While declining temperature produced an initial drop in respiration that followed the typical Q10 reported for most tree taxa (ca. 1.9), near 0°C a sudden increase in respiration (i.e. negative Q10) resulted in a 50% divergence from predicted values. This phase of excessive respiration was related to the initiation of soluble carbohydrate accumulation in stem tissue. Stem carbohydrates maintain cells hydration while lowering freezing temperatures. The study of the unexpected increase in metabolic activity at near-zero temperatures may further elaborate upon the biological interlinks between time, water, and sugar in cold-hardening, mitigation of frost damage, and plant recovery from frost.