Pressure-probe measurements and single-cell sampling and analysis techniques were used to determine the effect of photosynthetic production and accumulation of sugars on osmotic and turgor pressures of individual cells of barley ( Hordeum vulgare L.) source leaves. In control plants, the changes in osmotic pressure in individual cells during the photoperiod were different for mesophyll (increase of 276 mOsmol/kg), parenchymatous bundle sheath (PBS; increase of 100 mOsmol/kg) and epidermis (remains constant). There was also an increase in osmotic pressure at the tissue level. Cooling of roots and the shoot apical meristem restricted the export of sugars from leaves, and the resulting changes in osmotic and turgor pressure were monitored. In contrast to the control leaves, mesophyll, PBS, and epidermal cells showed a similar increase in osmotic pressure (up to 500 mOsmol/kg). Cooling also increased the turgor pressure in epidermal and (to a greater extent) PBS cells. The difference in turgor pressure between epidermal and PBS cells is consistent with the presence of a water potential gradient within the leaf, from the vascular bundles towards the leaf surface.