Abstract:

Objective: This study systematically investigated the growth performance, physiological characteristics, and molecular mechanisms of Populus × tomentosa ‘Qinbaiyang 3’, an elite variety, in response to water-nitrogen coupling treatments under field conditions. The purpose of this study is to address the issues of extensive water and fertilizer management and low yield per unit area in poplar plantation cultivation in northwest China, thereby providing theoretical basis and practical support for efficient poplar cultivation. Method: Two-year-old P. × tomentosa ‘Qinbaiyang 3’ seedlings with generally consistent growth vigor were used as experimental materials. A two-factor completely randomized block design was adopted, with four irrigation levels and four nitrogen application levels. Irrigation treatments: during the growing season, irrigation 3 times, with once in early June, July, and August (W3); irrigation twice, with once in early June and early July (W2); irrigation once in early June (W1); and no irrigation served as control (W0). Nitrogen treatments: fertilization three times (F3), twice (F2), once (F1) during the growing season, and no fertilization as control (F0). Furrow irrigation was applied to reach 100% field water capacity. Nitrogen fertilizer was applied in holes before irrigation (single dose: 250 g per plant). Treatments were continuously applied for two years, and growth indicators were measured at the end of the growing season. Result: Water-nitrogen coupling significantly promoted tree growth. The increment of diameter at breast height (DBH) was greatest observed under the W3F2 treatment, the tree height growth was greatest under W2F3 treatment, and the volume accumulation was also greatest under W3F2 treatment. Based on a comprehensive evaluation of growth indicators and photosynthetic performance, W3F2 was identified as the optimal treatment combination. Both irrigation and nitrogen fertilization significantly promoted secondary xylem development, and increased vessel area and fiber length, and their coupling effect was superior to that of single-factor treatments. Compared with the drought control (W0F0), the optimized water-nitrogen treatment (W3F2) significantly upregulated the expression of key genes involved in lignin and cellulose synthesis in the phloem and primary xylem, such as PaPAL2, PaCesA4, and PaCesA7A, indicating that water-nitrogen synergy promotes wood formation by enhancing cell wall anabolic metabolism. Conclusion: The W3F2 treatment (irrigation three times during the growing season and nitrogen applications twice per year) shows the most outstanding performance in promoting DBH growth and volume accumulation. This regime effectively coordinates tree growth and wood formation by enhancing photosynthetic capacity, optimizing vessel structure, and driving the expression of cell wall synthesis genes.

Key words: Populus × tomentosa ‘Qinbaiyang 3’, water-nitrogen coupling, growth indicators, photosynthetic characteristics, vascular tissue