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 seedlings and assessed which drought hardening intensity improve cold stress tolerance after transplanting. Seedlings of 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 moderate-drought 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.

中文翻译：

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长期中度干旱硬化后移栽赤花松幼苗耐冷胁迫能力的提升

冷胁迫是树木所经历的非生物胁迫之一。尤其是移植到森林后的第一个冬天对于树木的生存至关重要。干旱硬化可以提高各种非生物和生物胁迫的耐受性。因此，我们对幼苗应用不同的干旱硬化强度（从“中度”到“严重”），并评估哪种干旱硬化强度可以提高移栽后的冷胁迫耐受性。幼苗在三种不同的降水条件（对照、中度和严重干旱）下生长三年，然后移植到寒冷地区的另一个地点。在移栽后第一个冬季和生长季进行转录组分析和生理参数测量，比较干旱强度的硬化效果。移栽后第一个冬季，中度干旱炼苗的植物激素和次生代谢物生物合成基因比未炼苗或重度干旱炼苗的上调程度更大。苯丙素途径似乎与中度干旱硬化幼苗的冷胁迫耐受性有关。在生长季节，与其他处理相比，移植后第二年，中度干旱硬化树的光合作用有所增加。这些结果表明，中等干旱硬化通过诱导次生代谢产物和激素减轻了移植后针对首次冷应激的生理损伤。