Rooted single leaves of sweet potato (Ipomoea batatas L.) produce and translocate photosynthates, thus representing an ideal model for characterising the source–sink relationships and responses to various environments. A hydroponics culture study was conducted with rooted single leaves of sweet potato to determine intraspecific variation in growth, biomass partitioning, and associated physiological changes in response to variable potassium (K) supply among genotypes Ji22 (low K-use efficiency), Nan88 (high K-uptake efficiency) and Xu28 (high K-use efficiency). Potassium deficiency suppressed biomass accumulation in blades, petioles and roots in all three genotypes. Root length of diameters <0.25 mm and 0.25–0.5 mm was significantly less for K-deficient than K-sufficient roots of all genotypes, but the difference was proportionally greater in the K-inefficient genotype Ji22 than the other two genotypes. Potassium deficiency also severely inhibited net photosynthesis of blades in Nan88 and Ji22, as well reducing photosynthate translocation, increasing starch, hexose and sucrose concentrations, and decreasing K concentration in blades. The genotypes varied in photosynthesis-related responses to the K deficiency. Xu28 had greater blade K concentration and net photosynthesis as well as stable maximum quantum yield of photosystem II (F_V/F_M, with F_V = F_M – F₀) under K deficiency, possibly because of a better source–sink balance and more efficient translocation of photosynthates to roots and K to blade compared with genotypes Ji22 and Nan88. Impaired phloem loading during K deficiency was associated with a decline in photosynthetic rate and decreased carbohydrate supply from blades, resulting in restricted root growth.