Dose-Dependent Role of Silicon in Alleviating Copper Toxicity in In Vitro Olive (Olea europaea L. cv. Moraiolo) Shoots

Although copper (Cu) is one of the essential micronutrients for plant metabolism, excess Cu induces oxidative stress and growth inhibition. In contrast, silicon (Si), despite not being considered an essential nutrient, plays a role in plant growth improvement and stress alleviation. This study aimed to assess the protective role of Si against Cu toxicity in olive shoots cv. Moraiolo cultured in vitro under different Cu and Si concentrations. Shoots were cultured on OM medium supplemented with 200 and 300 µM CuSO4, with or without 2 and 4 mM K2SiO3 for eight weeks. Cu stress significantly reduced the survival rate, biometric parameters, chlorophyll content, and macronutrient accumulation, while increasing Cu accumulation, oxidative stress markers, and antioxidant metabolites. Si supplementation alleviated these adverse effects, particularly at 2 mM under 200 µM Cu, by improving growth, chlorophyll content, nutrient balance, and antioxidant responses. Interestingly, Si alone promoted growth and physiological performance beyond control levels. However, the Si effect at 300 µM Cu was less pronounced, suggesting that Si-mediated amelioration depends on both Si dose and Cu concentration. These findings indicate that Si improves the physiological performance of olive microshoots under Cu stress. The protective effect of Si was dose-dependent, with 2 mM Si generally providing greater beneficial effects under moderate Cu stress.