Genotype‐Dependent Modulation of Stomatal Function and Carbon Isotope Fractionation by Silicon in Common Bean Under Water Deficit

ABSTRACT Silicon (Si) modulates plant responses to abiotic stress, but its role in legumes remains unclear. We investigated the effects of Si supply on drought responses in common bean genotypes contrasting in drought tolerance (BAT 477, tolerant; IAC Carioca 80 SH, susceptible), focusing on leaf gas exchange, carbon isotope discrimination (δ 13 C), leaf nitrogen (N), and C/N. Plants were grown under greenhouse upon different water regimes (80% and 40% of soil water holding capacity) and Si supply (0 and 200 mg dm −3 ). Silicon application increased intercellular CO 2 concentration (Ci) in both genotypes, regardless of water regime, suggesting changes in the balance between stomatal and non‐stomatal limitations to photosynthesis. Effects on water‐use efficiency (WUE) were genotype and stage‐dependent, with increases observed only in the susceptible genotype under well‐watered conditions. Leaf N increased with Si supply, particularly under drought in the tolerant genotype, while changes in the C/N reflected shifts in carbon‐nitrogen balance associated with improved physiological performance. Despite higher Si accumulation in IAC Carioca 80 SH, this genotype did not exhibit superior drought tolerance, highlighting that Si accumulation per se does not determine physiological responsiveness. Multivariate analysis revealed coordinated associations among Si, WUE, δ 13 C, and C/N under drought, suggesting that Si modulates the balance between carbon assimilation, nutrient status, and water use. Our findings demonstrate that Si regulates integrated physiological responses rather than directly enhancing individual traits, with effects dependent on genotype and environment. These results highlight the potential of Si as a strategy to improve the physiological performance of legumes under drought.