Evaluation of abiotic stress tolerance in transplanted Pinus densiflora seedlings grown under different water availability

Abstract

Successful adaptation of seedlings depends on nursery conditions after the transplanted seedlings are exposed to various stresses such as drought, cold and waterlogging. Stress conditions inhibit plant growth and survival, but stress reponse can enhance adaptation in subsequent stress by regulating gene expression. Hardening is able to exert more tolerance to subsequent condition and affect crossstress condition. However, very few studies have reported the hardening effect on transplant stress in woody plants. Korean red pine (Pinus densiflora) is the most important species in Korean forest lands, and mass dieback has been reported due to climate change. This study was conducted to identify a cross-stress hardening effect in P. densiflora after transplanting. The aim of this study are (1) to identify the drought hardening effect on transplant stress, (2) to determine the drought hardening effect on transplanted plants subjected to cold stress, and (3) to identify the waterlogging hardening effect on transplant stress. For the first study, threeyears-old P. densiflora seedlings were grown under different drought hardening intensity for three years, and then transplanted to another site. In order to identify the drought hardening effect, transcriptome responses were compared according to drought intensity before and after transplanting (growing season and winter). Measurement of physiological characteristics was conducted after transplanting (growing season and winter). After transplanting in growing season, moderate drought hardening promoted cell wall organization and defense responses to repair the damaged tissue due to transplanting. In addition, water conservation capacity also improved in moderate drought hardening compared to non-hardening. In first winter after transplanting, moderate drought hardening showed a greater upregulation of gene expression linked to cold stress tolerance such as cyroprotectants, secondary metabolites, and phytohormones compared to severe drought- and non-hardened conditions. Photosynthesis and root collar diameter growth improved under moderate drought hardening after transplanting due to upregulation of the defense response. For the second study, three-years-old P. densiflora seedlings were grown under waterlogging and control conditions for three years, respectively. After three years of waterlogging, hardened seedlings were transplanted to another site. Transcriptional analysis was conducted before and after transplanting and compared between waterlogging- and non-hardened seedlings. Physiological characteristics were measured after transplanting. After transplanting, waterlogging hardening upregulated genes related to cell wall formation and secondary metabolites along with the upregulation of phenylpropanoid biosynthesis genes to alleviate wounding stress caused by root cutting. However, degradation of auxin caused the decreasing tendency in growth under waterlogging hardened seedlings. This study showed that moderate drought hardening enhances cross-stress tolerance after transplanting, and suggests the nursery management strategy to relieve transplant stress and improve various stress tolerances