Promotion of cold stress tolerance in transplanted Pinus densiflora seedlings after long-term moderate drought hardening

Cold stress is one of the abiotic stresses experienced by trees. Especially, the first winter after transplantation to the forest is critical for the survival of a tree. Drought hardening could improve various abiotic and biotic stress tolerance. Therefore, we applied different drought hardening intensity, from 'moderate' or 'severe,' to Pinus densiflora seedlings and assessed which drought hardening intensity improve cold stress tolerance after transplanting. Seedlings of P. densiflora were grown in three different precipitation conditions (control, moderate, and severe drought) for three years, and then transplanted to another site in colder region. Transcriptome analysis and measurement of physiological parameters were conducted in the first winter and growing seasons after transplanting to compare the hardening effect of drought intensity. In the first winter after transplanting, phytohormone and secondary metabolite biosynthetic genes were upregulated to a greater extent in moderatedrought hardened seedlings than those in unhardened or severe drought-hardened seedlings. Phenylpropanoid pathway seemed to be involved in cold stress tolerance in moderately drought-hardened seedlings. In growing seasons, photosynthesis increased under moderate drought-hardened trees compared to that by other treatments in the second year after transplanting. These results suggested moderate-drought hardening alleviated physiological damage against first cold stress after transplanting by inducing secondary metabolites and hormones.