Carbon–Nitrogen Metabolism in Mulberry (Morus alba L.) and its Influence on Leaf Protein Quality for Silkworm Nutrition

Abstract

Mulberry (Morus alba L.) is the sole food plant of the domesticated silkworm (Bombyx mori L.), and the nutritional quality of its leaves directly determines larval growth, cocoon weight, shell ratio, and silk yield. Among the physiological factors governing leaf quality, carbon–nitrogen (C–N) metabolism plays a pivotal role by coordinating photosynthetic carbon assimilation with nitrogen uptake, reduction, and assimilation into amino acids and proteins. The balance between carbon skeleton availability and reduced nitrogen largely determines both the quantity and quality of leaf proteins accessible to the silkworm. This review synthesizes current knowledge on carbon–nitrogen interactions in mulberry, with particular emphasis on photosynthetic carbon metabolism, enzymatic pathways of nitrogen assimilation, and their regulation by nitrogen form, rate, and timing of application. The influence of environmental factors such as light, temperature, water availability, and soil fertility on C–N metabolism and leaf biochemical composition is also discussed. Special attention is given to how variations in leaf protein content, amino acid profile, soluble carbohydrates, moisture retention, and secondary metabolites affect silkworm digestion, metabolism, silk gland development, and silk protein synthesis. An improved understanding of the physiological basis of C–N metabolism in mulberry provides a scientific foundation for precision nutrient and water management, enhanced leaf protein quality, and efficient silkworm performance. Integrating plant physiological insights with agronomic and sericultural practices is essential for developing climate-resilient, resource-efficient, and sustainable sericulture systems capable of improving silk productivity and quality.