Circadian control of carbohydrate availability for growth in <i>Arabidopsis</i> plants at night

<jats:p> Plant growth is driven by photosynthetic carbon fixation during the day. Some photosynthate is accumulated, often as starch, to support nocturnal metabolism and growth at night. The rate of starch degradation in <jats:italic>Arabidopsis</jats:italic> leaves at night is essentially linear, and is such that almost all of the starch is used by dawn. We have investigated the timer that matches starch utilization to the duration of the night. The rate of degradation adjusted immediately and appropriately to an unexpected early onset of night. Starch was still degraded in an appropriate manner when the preceding light period was interrupted by a period of darkness. However, when <jats:italic>Arabidopsis</jats:italic> was grown in abnormal day lengths (28 h or 17 h) starch was exhausted ∼24 h after the last dawn, irrespective of the actual dawn. A mutant lacking the LHY and CCA1 clock components exhausted its starch at the dawn anticipated by its fast-running circadian clock, rather than the actual dawn. Reduced growth of wild-type plants in 28-h days and <jats:italic>lhy/cca1</jats:italic> mutants in 24-h days was attributable to the inappropriate rate of starch degradation and the consequent carbon starvation at the end of night. Thus, starch degradation is under circadian control to ensure that carbohydrate availability is maintained until the next anticipated dawn, and this control is necessary for maintaining plant productivity. </jats:p>