Abstract: Factors controlling plant photosynthesis and primary production include parameters dictating photosynthetic activity together with export and allocation properties. In fact, phloem loading activity as well as phloem sap redistribution and velocity are considered as important players in plant carbon acquisition. But surprisingly, there is almost no information about how phloem sap properties such as sugar concentration and velocity, are controlled by photosynthetic rate. Here, we carried out gas exchange experiments coupled to isotopic labelling on sunflower leaves to monitor photosynthate export rate when photosynthesis was varied with CO₂ mole fraction. We took advantage of a compartmental isotope model to obtain an experimental estimate of phloem sugar pool size and thus sap concentration and velocity. Phloem sugar concentration was found to increase relatively slowly (from very low values to about 1 mol L^-1) with photosynthesis and export rate, while sap velocity remained in the same order of magnitude (0.2-0.6 mm s^-1). This phenomenon could be explained using a simple one-dimensional model of phloem sap generation and movement suggesting that at fixed photosynthesis, phloem sap was in a steady-state reflecting the balance between xylem-to-phloem and sieve tube hydraulic conductance, turgor pressure and osmotic pressure due to loading.