The total citrus area irrigated in Brazil has increased over the decades. The main cause of this increasing is due to the use of rootstocks tolerant to Citrus Sudden Death however they are less tolerant to drought than Rangpur lime. This research aims to study the influence of rootstock and soil type on transpiration of young plants of Valencia orange. The experiment was conducted in a greenhouse at the Department of Biosystems Engineering, ESALQ/USR Orange seedlings were planted in boxes of 500 L. It was determined simultaneously sweating of 20 plants through thermal dissipation probes (sap flow). It was monitored solar radiation, relative humidity and air temperature sensors were installed to 2 m tall at the center of the greenhouse. The reference evapotranspiration (ETo PM) was calculated by the standard method suggested by FAO. According to these results it is concluded that plant transpiration of Valencia oranges are influenced not only by the type of rootstock but also by leaf area growth and phenological stage, and its relationship with ETo pM is not linear in the whole range of evaporative demand of the atmosphere.
