Abstract

Although photosynthetic carbon assimilation occurs only in the light, growth and respiration requiring carbon also occur in the dark. Starch reserve enables this continuous growth throughout the day-night cycle. Starch is stored during the day inside chloroplasts and broken down at night to provide sugars to sinks, allowing metabolism and growth regardless of light or dark. Previous studies have reported that starch is accumulated and is degraded almost linearly, rather than exponentially, drawing a pyramid shape in the day-night cycle. More interestingly, the shape of pyramid adaptively changes in response to the various length of light period. Here we propose a model of adaptive starch metabolism in response to changing photoperiod. We assume key metabolic rates, carbon partitioning, starch degradation, and sucrose export, to be circadian regulated in leaves and their phases of oscillation are shifted in response to sucrose starvation. We performed gradient descents for various photoperiod conditions to find the corresponding optimal sets of phase shifts that minimize starvation best. Results at convergence were all consistent with experimental data: i) diurnal starch profile showed linear increase during the day and linear decrease at night; ii) reducing light period tended to increase starch synthesis speed during the shorter day while decreasing its degradation speed during the longer night; iii) sudden early dusk showed slower starch degradation during the longer night. This theoretical study would establish a framework for future research on starch and plant productivity.