Nitrogen dynamics in sugarcane: nutritional strategies under elevated CO2 and the spatial distribution along the leaf blade

Abstract

Nitrogen (N) is an essential macronutrient involved in critical plant metabolic processes, including the synthesis of amino acids, proteins, nucleic acids, and pigments. Elevated atmospheric CO2 (eCO2) generally enhances photosynthesis but often reduces leaf N content, a phenomenon known as CO2 acclimation. Adjusting the NO3−:NH4+ ratio in nutrient solutions may help mitigate this decline, particularly in sugarcane (Saccharum spp.), which exhibits genotypic variation in NH4+ uptake and tolerance. Besides this understanding N assimilation, redistribution, and utilization along the leaf can reveal adaptive strategies to optimize N use and photosynthetic capacity, supporting sustainable agronomic practices. This study tested two hypotheses: (i) eCO2 enhances NH4+ assimilation in sugarcane, and increasing NH4+ supply alleviates N decline, potentially mediated by chloroplast positioning due to its role in photosynthetic efficiency; and (ii) total N, NH4+, and NO3− follow a gradient along the sugarcane leaf blade, which is modified under increased NH4+ supply. Sugarcane stem nodes (var. RB 855536) were sprouted and grown in 10 L pots with washed sand under greenhouse conditions (28.8°C, 79% RH) for 45 days. Plants were fertigated every two days with a complete nutrient solution at two NO3−:NH4+ ratios (87.5:12.5 and 50:50), gradually increasing ionic strength during acclimation. Afterward, plants were transferred to open-top chambers (OTCs) for nine days for acclimatation, before exposure to either ambient CO2 (400 μmol mol−1) or elevated CO2 (800 μmol mol−1) for 35 days. CO2 concentrations inside the OTCs were monitored every three hours using an infrared gas analyzer. A 2 × 2 factorial design with nine replicates per treatment was employed. Results show that eCO2 enhanced NH4+ assimilation, with higher NH4+ supply increasing total leaf N. Both eCO2 and elevated NH4+ shifted chloroplasts toward a centripetal arrangement, suggesting enhanced PEPCK activity and photosynthetic efficiency. Biochemical analyses revealed gradients of total N, NH4+, and NO3− along the leaf blade, and increased NH4+ promoted redistribution of inorganic N, potentially optimizing N use and reducing toxicity. These findings provide mechanistic insights into N metabolism under eCO2 and inform strategies to improve N use efficiency in sugarcane under future climate scenarios.