The effects of substrate-heating on gas exchange and leaf water potential of gerbera {Gerbera jamesonii H. Bolus ex Hook.) cv. Maria grown in rockwool as substráte were studied under a plastic greenhouse. Measurements of net photosynthetic rate (Pn), stomatal conductance to water vapour {g^, leaf to air vapour pressure difference (VPD), and leaf water potential ('P) were made along the day, at different dates, during two winter seasons. and were higher in plants grown with substrate-heating than in control plants. Control plants without substrate-heating showed higher VPD values. T' was generally more negative for the control plants. These differences may be ascribed to differences in root activity and/or root development, which are related to the temperature of the substráte.