Further understanding of the range of environmental influence on source-sink relationships in quinoa is important to streamlining future crop improvement and efforts concerning geographic expansion of cultivation areas. In the present study a photoperiod sensitive quinoa cv. 'Achachino' and photoperiod neutral cv. 'Titicaca' were studied under short (10h) and long (17.5h) days, with respect to C and N distribution as well as partitioning of newly assimilated C to plant organs. An extended photoperiod resulted in ¹⁴C decreasingly being allocated to stem growth and lower leaves in 'Titicaca', but increasingly in 'Achachino'. Both cultivars increased biomass accumulation under extended photoperiod, but in the short day cultivar 'Achachino' the extension mostly favoured stem and lower leaf growth and resulted in deteriorated seed development. In contrast, 'Titicaca' responded to extended photoperiod with an immediate increase in carbon allocation to upper leaves, and over time to the reproductive structures, resulting in a more than 50% increase in final yield. Collectively the results indicate that even though the photoperiod sensitive cultivar flowered under long photoperiod it did not develop seeds, whereas the photoperiod neutral cultivar in comparison has a wider range in photoperiod plasticity and ability to specifically utilize additional light towards reproductive growth, resulting in an increased yield potential in regions outside of the tropical zone.