The receptacle of the sacred lotus is the main source of heat during the thermogenic stage of floral development. Following anthesis, it enlarges, greens and becomes a fully functional photosynthetic organ. We investigated development of photosynthetic traits during this unusual functional transition. There were two distinct phases of pigment accumulation in receptacles. Lutein and photoprotective xanthophyll cycle pigments accumulated first with 64 and 95% of the maximum, respectively, present before anthesis. Lutein epoxide comprised 32% of total carotenoids in yellow receptacles, but declined with development. By contrast, more than 85% of maximum total chlorophyll, β-carotene and Rubisco were produced after anthesis, and were associated with significant increases in maximum electron transport rates (ETR) and photochemical efficiency (F_v/F_m). Leaves and mature receptacles had similar Rubisco content and ETRs (>200 μmol m^-2 s^-1), although total chlorophyll and total carotenoid contents of leaves were significantly higher than those of green receptacles. Receptacle δ¹³C before anthesis was similar to that of leaves; consistent with leaf photosynthesis being the source of C for these tissues. In contrast, mature receptacles had significantly lower δ¹³C than leaves, suggesting that 14–24% of C in mature receptacles is the result of refixation of respired CO₂.