Abstract. Water status plays an important role for fruit quality and quantity in tomato (Solanum lycopersicum L.).
However, determination of the plant water status via measurements of sap flow (FH2O) or stem diameter (D) cannot be done
unambiguously since these variables are influenced by other effectors than the water status. We performed a semi-seasonal
and a diurnal analysis of the simultaneous response of FH2O and D to environmental conditions, which allowed us to
distinguish different influences on DD such as plant age, fruit load and water status and to reveal close diurnal relationships
between FH2O and DD. In addition, an analysis of the diurnal mechanistic link between both variables was done by applying
a slightly modified version of a water flow and storage model for trees. Tomato stems, in contrast with trees, seemed to
maintain growth while transpiring because a large difference between turgor pressure (Yp) and the yield threshold (G) was
maintained. Finally, the simultaneous response of D and FH2O on irrigation events showed a possibility to detect water
shortages.