Carbon assimilation and leaf-to-fruit sugar transport are, along with plant water status, the driving mechanisms for fruit growth. De Swaef et al. develop a mechanistic water-flow and storage model to relate variations in stem diameter to phloem sugar-loading and sugar concentration dynamics in tomato, Solanum lycopersicum. The model allows instantaneous dynamics in plant water relations and gradual variations in plant carbon status to be distinguished. When combined with stem diameter measurements, the model also enables prediction of dynamic variables that are difficult to measure in a continuous and non-destructive way, such as xylem water potential and phloem hydrostatic potential. Phloem sugar-loading and sugar concentration dynamics can also be simulated from stem diameter variations.