RNA-Seq reveals the adaptive mechanisms of poplar varieties under different afforestation densities in semiarid regions

Background Planting density is crucial for afforestation. However, the adaptive mechanisms of poplar varieties under different afforestation densities in semiarid regions remain poorly understood. Here, we integrated physiological, biochemical, and transcriptomic data to investigate the adaptive strategies of poplar varieties under high (HD) and low (LD) planting densities. Results These findings indicated that poplar varieties adapt to different planting densities via distinct strategies in semiarid regions. Under HD, ZL suppressed stomatal opening and growth by upregulating ABA synthesis genes, while the accumulation of SS and proline increased the instantaneous water use efficiency (WUEi). Notably, HL exhibited higher ABA catabolism genes expression, which promoted stomatal opening, then improved net photosynthetic rate, transpiration rate, and stomatal conductance (average 10.32% to 224.24% higher than those of ZL and ZX), and enhanced growth through strategies of high photosynthesis/transpiration. Under LD, resource competition is alleviated, and all varieties exhibited lower ABA synthesis genes expression, favoring stomatal opening and growth. Specifically, ZL and HL balance growth from drought resistance through relatively low WUEi and high levels of osmotic adjustment substances. Unlike other varieties, ZX is density insensitive, which achieves a balance between drought resilience and growth by maintaining high WUEi via leaf regulation and increasing carbohydrate metabolism genes expression and soluble protein accumulation. Fifty-eight key genes and four core factors were identified, involving in the regulation of hormones, photosynthesis, proline, and sugar metabolism, which played important roles in adapting to densities and drought stress. Conclusions These findings provide a molecular basis of poplar adaptation to afforestation density and a guide for aligning poplar varieties with optimal afforestation densities in semiarid regions.