Physiological responses associated with improved yield performance under heat stress in proso millet: the role of rice bran-coated urea in re-programming growth and partitioning

Objective: Heat stress disrupts the photosynthetic machinery and shortens developmental phases, leading to yield loss in cereals. This study investigated the potential of rice bran-coated urea (RBCU) to ameliorate heat-induced constraints on growth, phenology, and dry matter partitioning in proso millet (Panicum miliaceum L.).Methods: A two-year field experiment evaluated uncoated urea (UCU), RBCU, gypsum-coated urea (GCU), and cement-coated urea (CCU) at four nitrogen rates (۰, ۶۰, ۸۰, and ۱۲۰ kg urea ha-¹) under optimal (spring) and heat stress (summer) conditions. The experiment was conducted as a split-plot factorial arrangment based on a randomized complete block design with three replications. The main plots included planting season (spring and summer seasons), while the subplots consisted of the factorial combination of coating type and urea-N rate. In this study, several traits, including days to heading, days to physiological maturity, chlorophyll a, total carotenoids, leaf area, panicle length, biomass, grain yield, and harvest index, were measured.Results: The results indicated that RBCU at ۸۰ kg urea ha-¹ generally ranked among the best-performing treatments, particularly under summer conditions. Under summer heat stress, RBCU maintained significantly higher chlorophyll a and carotenoid contents, increasing chlorophyll a and carotenoids by ۲۵.۴% and nearly twofold, respectively, compared with the stressed UCU control. It concurrently improved canopy architecture, increasing leaf area and panicle length by ۱۴.۹% and ۱۵.۲%, respectively. RBCU also modulated crop phenology by delaying heading by ۱۱.۵%, and was associated with an improved dry matter partitioning efficiency, reflected in a higher harvest index. It increased total biomass by ۱۷.۳% and, most importantly, enhanced the harvest index by ۶.۳% under severe stress, indicating a more efficient allocation of assimilates to grains. Consequently, grain yield with RBCU at ۸۰ kg urea ha-¹ was ۱۷.۵% higher than that with UCU under summer conditions.Conclusion: RBCU was associated with improved physiological performance and higher yield under summer heat conditions, likely through coordinated effects on canopy traits, phenology, and harvest index.

کلیدواژه ها:

Biomass partitioning ، Canopy architecture ، Harvest Index ، Phenology ، Photosynthetic pigments ، Proso millet ، Slow-release nitrogen