Factors affecting freezing tolerance: a comparative transcriptomics study between field and artificial cold acclimations in overwintering evergreens.

Cold acclimation (CA) is a well-known strategy employed by plants to enhance freezing tolerance (FT) in winter. Global warming could disturb CA and increase the potential for winter freeze-injury. Thus, developing robust FT through complete CA is essential. To explore the molecular mechanisms of CA in woody perennials, we compared field and artificial CAs. Transcriptomic data showed that photosynthesis/photoprotection and fatty acid metabolism pathways were specifically enriched in field CA; carbohydrate metabolism, secondary metabolism and circadian rhythm pathways were commonly enriched in both field and artificial CAs. When compared with plants in vegetative growth in the chamber, we found that the light signals with warm air temperatures in the fall might induce the accumulation of leaf abscisic acid (ABA) and jasmonic acid (JA) concentrations, and activate Ca2+ , ABA and JA signaling transductions in plants. With the gradual cooling occurrence in winter, more accumulation of anthocyanin, chlorophyll degradation, closure/degradation of photosystem II reaction centers, and substantial accumulation of glucose and fructose contributed to obtaining robust FT during field CA. Moreover, we observed that in Rhododendron 'Elsie Lee', ABA and JA decreased in winter, which may be due to the strong requirement of zeaxanthin for rapid thermal dissipation and unsaturated fatty acids for membrane fluidity. Taken together, our results indicate that artificial CA has limitations to understand the field CA and field light signals (like short photoperiod, light intensity and/or light quality) before the low temperature in fall might be essential for complete CA.