Silicon-enhanced non-enzymatic antioxidant defense mechanisms in young orange trees under glyphosate-induced stress

Glyphosate is widely used in citrus production, but its overuse can cause oxidative stress and reduced growth in young orange trees. Silicon (Si), a beneficial element, strengthens antioxidant defense pathways and attenuates oxidative damage. However, its role in alleviating glyphosate-induced stress, particularly through the non-enzymatic antioxidant systems, remains unclear. This study examined whether Si application can reduce oxidative stress in young "Valencia" orange trees by enhancing non-enzymatic defenses, reducing oxidative stress indicators, and improving plant growth.

In an 8-month greenhouse experiment using a 4 × 2 factorial design, four glyphosate rates (0, 576, 1008 and 1440 g acid equivalent (a.e.) ha^-1) and two Si treatments (0- and 2-mM Si), were applied to trees via fertigation and foliar sprays. Key parameters were measured nine and sixteen days after the fourth Si application in older and younger leaves, respectively. Trees treated with Si exhibited a 66% Si increase in young leaves and 44% in old leaves. Oxidative stress, measured by malondialdehyde (MDA) levels, was significantly lower in trees treated with Si across all glyphosate rates in old leaves, and in young leaves at higher glyphosate rates (1008 and 1440 g a.e. ha^-1). Proline levels were elevated in control trees exposed to glyphosate, whereas Si treatment increased carotenoid accumulation, particularly in old leaves. Phenolic compounds increased in old leaves where Si was applied across all glyphosate rates, while in young leaves, increases occurred only at lower glyphosate rates (0 and 576 g a.e. ha^-1). Trees treated with Si retained more leaves across most glyphosate rates and showed increased dry matter production, except at 1440 g a.e. ha^-1.

Si application effectively mitigates glyphosate-induced oxidative stress in young orange trees by enhancing non-enzymatic antioxidant defenses, particularly carotenoids and phenolic compounds, while lowering MDA levels. These findings suggest Si as a sustainable strategy to improve herbicide tolerance and strengthen the citrus tree resilience.