Drought hardening effect on improving transplant stress tolerance in Pinus densiflora

Drought stress inhibits plant growth. However, plants adapt to prolonged drought by remodeling their tran-scriptomes. Drought hardening mitigates the negative effects of future drought events. In this study, we exam-ined whether long-term moderate and severe drought hardening enhance drought and transplant stress tolerance in Pinus densiflora. Transcriptomes were compared among trees under three different water availability condi-tions after three years: control, 100% natural precipitation; moderate drought, 40% precipitation blocking; and severe drought, 80% precipitation blocking. Trees were then transplanted and their transcriptomes and physi-ological characteristics were compared against those under their previous growth conditions. Genes and tran-scription factors related to drought tolerance were more highly expressed under severe than moderate drought stress. Contrary to what was expected, drought and transplant stress had common nine pathways and wounding due to root cutting was main cause in transplant stress rather than water stress. After transplanting, however, phenylpropanoid biosynthesis, lignin and cell wall organization were upregulated under moderate drought hardening. Moreover, the water potential was higher in transplanted trees subjected to moderate drought than those under control conditions after transplanting. Moderate drought hardening enhanced transplant stress tolerance through improved wounding resistance and water conservation ability more effectively than severe or no drought hardening.