Influence of nitrate - ammonium ratio on the growth, nutrition, and metabolism of sugarcane

Although ammonium (NH4+) has been claimed as the preferential N source for sugarcane (Saccharum spp.), the intense uptake of this mineral form by plants can impair metabolic processes and crop yield. We aimed to assess the growth, nutrition, and metabolic responses of sugarcane grown under different amounts of nitrate (NO3-) and NH4+. Sugarcane setts were grown in nutrient solution at a total concentration of 15 mM N using different NO3-/NH4+ ratios (100/0, 75/25, 50/50, 25/75, and 0/100, respectively) for 163 d under controlled conditions. The pH of the medium was daily adjusted to 5.8 +/- 0.1, with replacement of the hydroponic solution every 10 d. NH4+-only fed plants showed lower dry biomass yield, nutrient content, leaf surface area, and leaf gas exchange than those under sole NO3- supply, in addition to favoring the development of brown rust (Puccinia melanocephala). However, there was no indication that NH4+ is directly related to oxidative stress in sugarcane. On the other hand, the highest N utilization efficiency was obtained with NO3- -only fed plants, which also resulted in the highest biomass yield, leaf surface area, nutrient content, leaf gas exchange, and root growth. Since NO3- was not stored in plant tissues, we therefore suggested that most of this N form is assimilated following its uptake. Despite the well-known preference of the crop for NH4+, the optimal growth response of sugarcane plants to NO3-/NH4+ ratios was observed under NO3- supply.