Day‐length effects on carbon stores for respiration of perennial ryegrass

• The mechanism controlling the use of stored carbon in respiration is poorly understood. Here, we explore if the reliance on stores as respiratory substrate depends on day length. • Lolium perenne (perennial ryegrass) was grown in continuous light (275 μmol photons m⁻² s⁻¹) or in a 16 : 8 h day : night regime (425 μmol m⁻² s⁻¹ during the photoperiod), with the same daily photosynthetic photon flux density (PPFD). Plants in stands were labelled with ¹³CO₂ : ¹²CO₂ for various time intervals. The rates and isotopic signatures of shoot‐ and root‐respired CO₂ were measured after labelling, and water‐soluble carbohydrates were determined in biomass. The tracer kinetics in respired CO₂ was analysed with compartmental models to infer the sizes, half‐lives and contributions of respiratory substrate pools. • Stores were the main source for respiration in both treatments (c. 60% of all respired carbon). But, continuous light slowed the turnover (+270%) and increased the size (+160%) of the store relative to the 16 : 8 h day : night regime. This effect corresponded with a greatly elevated fructan content. Yet, day length had no effect on sizes and half‐lives of other pools serving respiration. • We suggest that the residence time of respiratory carbon was strongly influenced by partitioning of carbon to fructan stores.