Objectives To investigate the regulatory mechanisms of pumpkin root systems on the growth, NO₃⁻ uptake, and nitrogen use efficiency (NUE) of grafted cucumber under reduced nitrogen conditions, and to provide theoretical guidance and technical support for efficient nitrogen management in protected cucumber production.

Methods A hydroponic experiment was conducted using ‘Jinchun No. 4’ cucumber self−rooted seedlings (J), self−grafted cucumber seedlings (J/J), ‘Ribenjingtiantaimu’ pumpkin self−rooted seedlings (R), and pumpkin rootstock−grafted cucumber seedlings (J/R) as experimental materials. These four plant types were grown under normal nitrogen (7 mmol/L) and reduced nitrogen (4 mmol/L) conditions. After 24 days of treatment, plant growth characteristics, root system traits, NO₃⁻ content and accumulation, and nitrogen accumulation were determined. Nitrogen use efficiency was subsequently calculated.

Results Under normal nitrogen conditions, R plants exhibited the highest growth performance, root morphological characteristics, root activity, total NO₃⁻ content, total NO₃⁻ accumulation, total nitrogen accumulation, and NUE, followed by J/R plants. Compared with normal nitrogen treatment, nitrogen reduction decreased NO₃⁻ content and accumulation in all plant types but increased root activity. Nitrogen reduction inhibited plant growth, root development, nitrogen content, and nitrogen accumulation in J and J/J plants, whereas it promoted growth, root characteristics, total nitrogen accumulation, and nitrogen use efficiency in J/R plants, as well as growth and nitrogen use efficiency in R plants. Under reduced nitrogen conditions, R and J/R plants showed superior growth, root morphology and activity, NO₃⁻ content in shoots and roots, total NO₃⁻ accumulation, total nitrogen accumulation, and nitrogen use efficiency compared with J and J/J plants. Correlation analysis revealed that plant growth parameters (dry weight and strong seedling index) were significantly positively correlated with root characteristics (root surface area, root volume, root branching number, and root activity), NO₃⁻ accumulation (in different plant organs and total NO₃⁻ accumulation), nitrogen accumulation (in different plant organs and total nitrogen accumulation), and nitrogen use efficiency indices (NUpE, NUtE, and NUE), except that root NO₃⁻ accumulation showed no significant correlation with root dry weight or strong seedling index. In addition, NO₃⁻ accumulation and nitrogen accumulation were significantly positively correlated with root characteristics. Principal component analysis further demonstrated that both plant genotype and nitrogen concentration significantly affected plant performance, with genotype exerting a stronger effect. Furthermore, R−type plants with pumpkin root systems exhibited significantly greater root development and nitrogen absorption capacity than J−type plants with cucumber root systems.

Conclusions Root genotype is a key determinant of plant growth and nitrogen uptake and utilization. Under reduced nitrogen conditions, the growth of cucumber self−rooted and self−grafted plants was inhibited, whereas pumpkin self−rooted plants showed the best performance due to their superior root characteristics and growth capacity, and this advantage was retained after grafting. Rootstock grafting enhanced NO₃⁻ uptake capacity and nitrogen assimilation efficiency by improving root morphology and structure, thereby enabling cucumber plants to adapt effectively to reduced nitrogen availability (4 mmol/L) while maintaining vigorous growth. Under the background of China’s agricultural “double reduction” strategy, grafting with nitrogen−efficient rootstocks represents an important approach for achieving fertilizer−saving and high−efficiency production in protected vegetable cultivation.