The effect of nitrogen levels on appearance traits and cooking and nutritional quality of rice grain under direct seeding in wet bed

Introduction: Nitrogen (N) management is a critical factor influencing rice yield and grain quality, particularly under wet-bed direct seeding, where crop establishment and nutrient dynamics differ from conventional transplanting systems. Cooking and eating quality in rice, including amylose content, gelatinization temperature, and protein content, depends not only on the genetic characteristics of the variety but also on environmental factors, particularly the nitrogen application rate.

Materials and Methods: This study evaluated the effects of five N application rates (0, 60, 90, 120, and 150 kg N ha⁻¹ as urea) on paddy yield, milling quality, grain physical traits, cooking and physicochemical properties, and nutritional composition of three rice cultivars, namely Hashemi, Anam, and Shiroudi. The experiment was conducted at the Rice Research Institute of Iran (Rasht) using a split-plot arrangement in a randomized complete block design with three replications. After harvest, paddy samples were dried, dehulled, milled, and head rice was separated to determine milling and quality-related traits. Data analysis was conducted using SAS software, and treatment means were separated by the Least Significant Difference (LSD) test at the 5% probability level.

Results and Discussion: Paddy yield responded significantly to N rate and cultivar. Maximum yields of Hashemi (4.20 t.ha⁻¹) were obtained at 90 kg N ha⁻¹, whereas Anam and Shiroudi achieved their highest yields at 120 kg N ha⁻¹ (4.79 and 6.56 t.ha⁻¹, respectively), with no significant yield increase at higher N rates. Increasing N generally improved milling recovery and head rice yield, while increasing raw grain length and slenderness. In contrast, cooked grain length and elongation ratio declined with increasing N. Grain protein content increased markedly with N application, while amylose content decreased from approximately 24.8% at 0 N to 21.3% at 150 kg N ha⁻¹, accompanied by an increase in gelatinization temperature. Grain Mn concentration increased with higher N supply, whereas Fe and Zn responses varied depending on cultivar and N rate. Pasting properties were also significantly affected: peak and final viscosity decreased by 12–25% at high N rates, while setback viscosity and pasting temperature increased, indicating less desirable cooking quality under excessive N application.

Conclusion: Overall, the results demonstrate that rice cultivars exhibit genotype-specific and non-linear responses to N under wet-bed direct seeding. Optimized N rates of approximately 90 kg N ha⁻¹ for Hashemi and 120 kg N ha⁻¹ for Anam and Shiroudi provided the best compromise between high paddy yield, milling performance, and acceptable cooking and nutritional quality, whereas excessive N application offered limited yield benefits but adversely affected grain quality.