Nanosilica increases tolerance to Cu toxicity in soybean seeds by reducing Cu uptake and improving nitrogen use efficiency

The treatment of soybean seeds with copper (Cu) can induce toxicity in seedlings depending on the applied dose and seed storage time after treatment. Silicon (Si) has the potential to mitigate Cu toxicity; however, the underlying mechanisms remain poorly understood. This study provides provides new evidence on soybean responses to toxic Cu doses in seed treatment and investigates whether nanosilica application via fertigation after sowing can alleviate this toxicity. Two experiments were conducted under controlled conditions using soybean grown in Oxisol. The first experiment assessed the nutritional responses of seedlings subjected to seed treatment with increasing Cu doses (0, 1, 4, 7, 15, and 29 g kg⁻¹ of seed as CuO) combined with two storage periods (48 and 96 h). The second experiment evaluated the same Cu doses in combination with the absence or presence of Si (2 mmol L⁻¹) applied via fertigation. Seed treatment with Cu was feasible up to 4 g kg⁻¹ when seed storage did not exceed 48 h. Higher Cu doses induced oxidative stress and reduced nitrogen use efficiency. Nanosilica application mitigated Cu toxicity at doses up to 7.4 g kg⁻¹ by reducing Cu accumulation in shoots and oxidative stress. This study establishes reference limits for Cu seed treatment and strategies to prevent toxicity in soybean.