Potato is an important staple food with increasing popularity worldwide. Elevated temperatures significantly impair tuber yield and quality. Breeding heat‐tolerant cultivars is therefore an urgent need to ensure sustainable potato production in the future. An integrated approach combining physiology, biochemistry, and molecular biology was undertaken to contribute to a better understanding of heat effects on source‐ (leaves) and sink‐organs (tubers) in a heat‐susceptible cultivar. An experimental set‐up was designed allowing tissue‐specific heat application. Elevated day and night (29°C/27°C) temperatures impaired photosynthesis and assimilate production. Biomass allocation shifted away from tubers towards leaves indicating reduced sink strength of developing tubers. Reduced sink strength of tubers was paralleled by decreased sucrose synthase activity and expression under elevated temperatures. Heat‐mediated inhibition of tuber growth coincided with a decreased expression of the phloem‐mobile tuberization signal SP6A in leaves. SP6A expression and photosynthesis were also affected, when only the belowground space was heated, and leaves were kept under control conditions. By contrast, the negative effects on tuber metabolism were attenuated, when only the shoot was subjected to elevated temperatures. This, together with transcriptional changes discussed, indicated a bidirectional communication between leaves and tubers to adjust the source capacity and/or sink strength to environmental conditions. Here, we performed a comprehensive study to analyse changes in potato plants under elevated temperatures. A special experimental set‐up allowed us to separately apply temperature stress to either above‐ or belowground tissues and thereby to distinguish between source and sink signals. Our results show that elevated temperatures impair photosynthetic assimilate production and partitioning towards tubers together with a decreased expression of the mobile tuberization signal. Our data led us to conclude that there is a bidirectional signalling between source leaves and sink organs (tubers), and candidate genes are discussed which may be involved in the metabolic readjustment.