Greater Biomass Production Under Elevated CO2 Is Attributed to Physiological Optimality, Trade-Offs in Nutrient Allocation, and Oxidative Defense in Drought-Stressed Mulberry

Mulberry (<i>Morus alba</i> L.), a species of significant ecological and economic importance, is widely cultivated for sericulture, soil conservation, and environmental restoration. Despite its remarkable resilience to environmental stresses, the combined impact of elevated CO₂ (eCO₂) and drought stress on aboveground-root-soil interactions remains poorly understood, particularly in the context of global climate change. Here, we investigated the effects of eCO₂ and drought on physiological leaf and root indicators, nutrient absorption and allocation, and soil properties in mulberry seedlings. Mulberry seedlings were grown in environmentally auto-controlled growth chambers under ambient CO₂ (420/470 ppm, day/night) or eCO₂ (710/760 ppm) and well-watered (75-85% soil relative water content, RWC), moderate-drought (55-65% RWC), or severe-drought (35-45% RWC) conditions. Results showed that both above- and below-ground plant biomass production were significantly promoted by eCO₂, particularly by 36% and 15% under severe drought, respectively. This could be attributed to several factors. Firstly, eCO₂ improved leaf photosynthesis by 25-37% and water use efficiency by 104-163% under drought stresses while reducing negative effects of drought on the effective quantum yield of PSII photochemistry and the photochemical quenching coefficient. Secondly, eCO₂ significantly decreased proline accumulation while increasing soluble sugar contents, as well as peroxidase and superoxide dismutase activities, in both leaves and roots under drought stress. Lastly, eCO₂ promoted soil sucrase, urease, and phosphatase activities, as well as plant nitrogen, phosphorus and potassium uptake while facilitating their allocation into roots under drought stress. These findings demonstrate that eCO₂ enhanced the drought tolerance of mulberry plants through improvements in photosystem II efficiency, water use efficiency, antioxidative defense capacity, and nutrient uptake and allocation, providing critical insights for sustainable mulberry plantation management under future climate change scenarios.