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

P>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 mu mol photons m-2 s-1) or in a 16 : 8 h day : night regime (425 mu mol m-2 s-1 during the photoperiod), with the same daily photosynthetic photon flux density (PPFD). Plants in stands were labelled with 13CO(2) : 12CO(2) for various time intervals. The rates and isotopic signatures of shoot- and root-respired CO(2) were measured after labelling, and water-soluble carbohydrates were determined in biomass. The tracer kinetics in respired CO(2) 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.