Plants must maintain a balance between their carbon (C) supply and utilization during the day–night cycle for continuous growth since C starvation often causes irreversible damage to crop production.  It is not well known how C fixation and allocation in the leaves of crops such as maize adapt to sudden environmental changes.  Here, to quantify primary C fixation and partitioning in photosynthetic maize leaves under extended darkness and to relate these factors to plant growth, maize seedlings were subjected to extended darkness (ED) for three successive days at the 6th leaf fully expanded stage (V6).  ED reduced plant growth and leaf chlorophyll levels but not the rate of net CO₂ exchange.  As a result of the reduction in photoassimilates, the accumulation of starch and total soluble carbohydrates (TSC) in mature leaves also decreased under ED.  However, the percentage of the daily C fixation reserved in mature leaves increased.  These transient C pools were largely composed of TSC and were mainly used for consumption by increased nocturnal respiration rather than for transport.  As the days went on, both the amount of C accumulated and the percentage of the daily fixed C that was reserved in leaves decreased, which could be largely accounted for by the attenuated starch synthesis in all treatments.  The activities of ADP-glucose pyrophosphorylase and soluble starch synthase decreased significantly over time.  Therefore, this study concluded that both starch and TSC are involved in the coordination of the C supply and plant growth under a sudden C shortage but that they may be involved in different ways.  While the ratio of reserved C to daily fixed C increased to maintain blade function under acute C starvation, both the amount and the proportion of C reserved in mature leaves decreased as plant growth continued in order to meet the growth demands of the plant.