© 2021, Institute of Molecular Biology, Slovak Academy of Sciences.The metabolism of starch is sensitive to changes in light and plants respond to different light regimes by adjusting their carbon metabolism and regulating enzymes that participate in starch biosynthesis. Although there are several studies showing the influence of the circadian clock mechanism on starch biosynthesis on model plants, there is still limited information on how the circadian regulation of carbon assimilation and utilization works on crop plants and long-day plants. In our previous study, we examined lentil (Lens culinaris Medik.), a long-day crop plant, and determined the influence of circadian control on starch metabolism by investigating the transcriptional regulation of large (LS) and small (SS) subunits of ADP glucose pyrophosphorylase (AGPase). However, the regulation mechanism of the enzymes responsible for the formation of the starch granule remains unclear. In this study, the transcriptional regulation of soluble starch synthase isoforms I and III (SSSI and SSSIII) and starch branching enzyme isoforms I and II (SBEI and SBEII) were examined in sink and source tissues under different photoperiods in lentils by quantitative real time PCR (qPCR). The results showed that the temporal distribution of gene expression was altered when isoforms for both enzymes from the stem and leaf tissues were compared for different photoperiod regimes, exhibiting a rhythmic period of 4–6 h with maximal expression times and levels altered due to the shifting photoperiod. These results were in agreement with our previous observations on lentil AGPase supporting the circadian control of carbohydrate metabolism.