João Cardoso de Souza Junior, Francisco Antônio Monteiro
Introduction Nitrogen (N) form influences the energetic and metabolic processes underlying nutrient assimilation, yet how these effects regulate sulfur (S)-driven physiological and growth responses in tropical forage grasses remains poorly understood. We investigated how contrasting NO 3 - /NH 4 + ratios interact with S supply to regulate growth, nutrient uptake and use efficiency, photosynthetic performance, oxidative stress, and antioxidant activity in Megathyrsus maximus . Methods The experiment was conducted in a randomized complete block design with four replicates, arranged in a 2 × 3 factorial consisting of two N form ratios (100/0 and 70/30 NO 3 - /NH 4 + ) and three S rates (0.1, 1.0, and 2.0 mmol L –1 ). Plants were grown in pots for 38 days in a nutrient solution under controlled growth chamber conditions. Results and discussion S fertilization enhanced biomass production, N and S uptake, N use efficiency, net photosynthesis, transpiration rate, quantum efficiency of photosystem II, electron transport rate, as well as the activities of glutamine synthetase, guaiacol peroxidase, and ascorbate peroxidase, but only under mixed NO 3 - /NH 4 + nutrition. S use efficiency declined with increasing S concentration in nutrient solution, but was higher under mixed NO 3 - /NH 4 + nutrition than with NO 3 - alone. By contrast, plants grown solely with NO 3 - exhibited low biomass production, photosynthetic activity, nutrient use efficiency, catalase and glutathione reductase activities, and high malondialdehyde concentrations. Our findings indicate that the energetic constraints associated with an exclusive NO 3 - supply do not offset the reduced S utilization in M. maximus , which is severely restricted compared with plants receiving mixed NO 3 - /NH 4 + nutrition. These results provide new insights into managing N forms to improve S and N use efficiency in tropical grasses, thereby enhancing forage productivity and resilience in tropical pasture systems.