Introduction: Historically, agriculture has primarily focused on maximizing crop yield. However, modern agricultural practices have expanded to encompass environmental conservation, product quality enhancement, and more efficient resource utilization. Various environmental factors, including the type and quantity of applied fertilizers, significantly influence plant growth and yield. Nitrogen, recognized as one of the most critical nutrients for plants, necessitates careful management for sustainable agriculture. Nitrogen deficiency hampers plant growth, while excessive application leads to environmental issues. Studies have demonstrated that the application of complete fertilizers containing nitrogen can substantially impact plant growth and yield. Numerous researchers have shown that the optimal use of chemical fertilizers plays a pivotal role in enhancing yield, reducing production costs, and preserving the environment. Consequently, identifying suitable fertilizers and their efficient application can contribute to improving agricultural product quality and achieving sustainable agriculture. This research investigates the impact of nitrogen fertilizer rates and cultivar type on various plant attributes, including leaf area index, total biomass, grain yield, leaf area index, nitrogen uptake, and nitrogen use efficiency.

Materials and Methods: This experiment was conducted in 2022 at the Khomein Bean Research and Education Farm using a factorial arrangement in a randomized complete block design (RCBD) with three replications. The treatments included five nitrogen application levels (0 (control), 40, 80, 120, and 160 kg N ha-1) and three pinto bean genotypes (a line KS21492 and two cultivars, Kousha and Saleh). Statistical analysis of the data was performed using SAS software version 9.4. Prior to the analysis of variance, a normality test was conducted to ensure the normal distribution of residuals. The General Linear Model (GLM) procedure was used for the analysis of variance. Mean comparisons were carried out using the Least Significant Difference (LSD) test at the 0.05 significance level.

Results and Discussion: The results showed that the highest spad (39.03) was observed at the nitrogen application level of 160 kg N ha-1, representing a 22.01% increase compared to the control. Additionally, plant height at this nitrogen level reached 66.22 cm, which was greater than that observed at other nitrogen levels. Among the genotypes, the Saleh cultivar had the tallest plants, with an average height of 79.20 cm. Moreover, the application of 160 kg N ha-1 resulted in the highest total biomass production (13,273 kg ha-1), which was 39.13% higher than the control. Among the genotypes, Kousha produced the highest biomass at 11,851 kg ha-1. Finally, grain yield was also significantly affected by nitrogen levels. The highest grain yield (4,068 kg ha-1) was recorded at the 160 kg N ha-1 treatment, which was 26.99% higher than the control. The Kousha cultivar also produced the highest grain yield among the genotypes, with 4,067 kg ha-1. The findings of this study clearly demonstrate the significant positive effects of nitrogen fertilization on physiological traits, growth parameters, and yield of pinto bean genotypes. The highest SPAD value observed at 160 kg N ha⁻¹ indicates improved chlorophyll content and, consequently, enhanced photosynthetic capacity under optimal nitrogen availability. This aligns with previous studies suggesting that nitrogen plays a critical role in chlorophyll synthesis and leaf greenness, which are directly linked to biomass accumulation and yield formation. The increase in plant height at the highest nitrogen level reflects the enhanced vegetative growth resulting from improved nitrogen nutrition, which likely promoted cell division and elongation. The superior performance of the Saleh genotype in plant height suggests a genotype-specific potential to utilize nitrogen more efficiently for vertical growth. Total biomass and grain yield also responded positively to nitrogen application, with 160 kg N ha⁻¹ resulting in the highest values. These findings are consistent with earlier research indicating that adequate nitrogen supply enhances assimilate production and partitioning toward economic yield.

Conclusion: This study demonstrated that nitrogen fertilization significantly improves physiological traits, growth, biomass production, and grain yield in pinto bean genotypes. The application of 160 kg N ha⁻¹ was identified as the most effective rate, leading to the highest SPAD values, plant height, total biomass, and grain yield. Among the genotypes, Kousha showed the greatest potential in terms of yield and biomass, while Saleh exhibited superior plant height. These results highlight the critical role of adequate nitrogen supply in enhancing the performance of pinto bean cultivars and suggest that the combination of optimal nitrogen management with high-performing genotypes such as Kousha can be a promising strategy for increasing productivity.