The weak sinks of the fruit in cucumber (Cucumis sativus) are assumed to result in low sink–source ratios for carbohydrates, which may inhibit photosynthesis. Kläring et al. measure CO2 exchange in roots and shoots and find that removing all fruit from the plant results in a doubling of the root-zone respiration within a few hours. Respiration of the shoot, however, remains unaffected and photosynthesis is only marginally reduced, if at all. They deduce that this increased level of root zone respiration is due primarily to the exudation of organic compounds by the roots and their decomposition by micro-organisms.