Non-natural day length does not negatively affect lettuce growth as a result of acclimated carbohydrate rhythms

• Plant growth and morphology are unaffected by non-natural day lengths (20, 22, 26 and 28 h). • Lack of circadian resonance does not necessarily result in poor growth. • Extending the photoperiod (by reducing the dark period) does not affect plant growth. The plant endogenous clock determines optimal physiological and growth responses to daily environmental changes. Aligning the day length with the endogenous clock period (different from 24 h) is known as circadian resonance and enhances the growth of Arabidopsis thaliana via precise timing of carbohydrate metabolism and a range of other physiological processes. Furthermore, long photoperiods within a 24 hour day enhance plant growth compared to short photoperiods. This study aimed to elucidate the interaction between day length and photoperiod on growth, and morphology, and how daily rhythms of non-structural carbohydrates explained these effects. We grew two lettuce cultivars in a climate chamber under day lengths of 20, 22, 24, 26 and 28 h combined with dark periods of 2, 4 and 6 h and under continuous light. Growth and morphology were unaffected by the interaction between day length and photoperiod, as well as by day length alone. Extending the photoperiod by reducing the dark period from 6 to 2 h did not affect growth in one cultivar, however, it improved growth in the other cultivar. This positive effect on growth was associated with higher leaf area and projected leaf area. Non-structural carbohydrates showed rhythms in agreement with the day length applied, which implies acclimation to non-natural days likely via endogenous clock regulation. Our findings show that a lack of circadian resonance does not necessarily result in poor growth and remark the benefit of extended photoperiods to achieve similar or enhanced growth compared to shorter photoperiods at equal DLI.