• The effect is reported here of photoperiod on fructan accumulation in the tuberous roots of Dahlia sp. cv. Sunny Rose seedlings.
• Growth parameters were measured of shoots and roots on glasshouse-grown dahlia seedlings subjected to either short day (SD) or long day (LD; 4 h night photoperiod interruption) light regimes. The carbohydrate concentrations of tuberous roots was analysed by high performance anion exchange chromatography.
• Total plant dry weight was unaffected by photoperiod. The LD treatment inhibited tuberous root development but increased shoot dry weight. Tuberous root tissue of SD seedlings showed a 156% increase in total fructan (inulin) concentration compared with LD tuberous root tissue, which had higher reducing sugar concentrations than SD tuberous roots. A wide range of oligomers increased during the SD treatment.
• Sucrose appears to be the regulating factor in fructan metabolism in dahlia. Photoperiod is a valuable tool for studying fuctan metabolism in vivo, as it provides a nondestructive means of regulating sucrose partitioning.