Physiological and Gene Expression Response of Interspecific Hybrids of Fraxinus mandshurica × Fraxinus americana to MJ or SNP under Drought

Drought affects the growth and production of Fraxinus tree species, such as the precious woody plant Fraxinus mandshurica. Based on interspecific hybrid F1 combinations, D110 plants of F. mandshurica × F. americana with strong drought resistance were selected for this study. To further reveal their heterosis mechanism under drought, in this study, an analysis was conducted pertaining to the physiological indexes and gene expression of related key gene changes in materials of 5 yr D110 seedlings and their female and male parental controls (D113 and 4–3) in response to drought, as well as to the addition of sodium nitrate (SNP, a donor of nitric oxide) and methyl jasmonate (MJ, a donor of jasmonate) signal molecules after drought. The results showed that under drought stress, hybrid D110 plants performed significantly better than their parents, especially compared to D113, in plant growth (the plant height growth was 29.48% higher), photosynthesis (the net photosynthetic rate was 38.4% higher), peroxidation (the increase in MDA content was 71.77% lower), defense enzyme activity (SOD and POD activities were 36.63% and 65.58% higher), hormone contents (ABA, IAA and GA were 33.9%~51.2% higher) and gene expression (the LHY and TOC1 rhythmic genes were 131.97%~165.81% higher). When an exogenous additive agent (SNP or MJ) was applied after drought, the negative effects of drought on growth were effectively alleviated (the tree height growth of D110 increased from 22.76% to 22.32% in comparison to drought conditions); meanwhile, the height growth of D110 plants was significantly higher than that of their parents. Further results of physiological indexes and the expression of related key gene changes in response to SNP or MJ also indicated that D110 plants can recover faster from drought than their parents after application of SNP or MJ. This article provides new ideas for revealing the heterosis mechanism of the drought resistance of interspecific F1 hybrids and supplies effective measures for improving drought resistance in F. mandshurica.