Energetic Constraints and Carbon Efficiency During Sucrose Storage in Sugarcane Culms

Sugarcane stores sucrose in a living culm for extended periods, yet the respiratory cost of maintaining this storage tissue remains poorly quantified. We quantified growth and maintenance respiration along the culm (internodes 1 to 12) in three genotypes at mid-season (rapid growth) and end-season (maturation) using a composition-based carbon accounting framework derived from measurements of biomass accumulation and composition. Growth respiration was highest in elongating internodes (3 to 6) and declined with maturation, whereas maintenance respiration increased progressively and dominated in mature storage internodes (10 to 12). Consequently, total sink demand remained substantial even after structural growth slowed, indicating that mature internodes continue to require significant metabolic input despite limited biomass production. To evaluate the potential impact of energetic constraints, we simulated reduced mitochondrial energy contribution to assess the sensitivity of respiratory carbon demand to decreased energetic efficiency. These simulations predicted an increase in glucose requirement for respiration across all internodes, with the largest proportional effect in mature tissue where maintenance costs dominated. Despite this predicted increase in respiratory demand, sucrose accumulation was maintained in mature culms, indicating that respiratory carbon loss remains constrained during storage. This suggests that storage tissue operates with relatively high carbon-use efficiency during maintenance-dominated metabolism. We interpret this pattern as consistent with metabolic configurations that reduce ATP demand, potentially involving partial substitution of ATP-dependent reactions by pyrophosphate (PPi)-dependent pathways, although this mechanism was not directly measured. These findings highlight the importance of maintenance respiration and energetic efficiency in determining sink strength and sucrose yield, and they provide a physiological framework for understanding carbon conservation in long-lived storage organs.