RESUMO
In order to evaluate their responses to drought, we determined the photosynthetic activity, water potential, stomatal conductance, transpiration, water use efficiency, photosynthetic photon flux density and leaf temperature of Paulownia imperialis, P. fortunei and P. elongata in three different soil moisture conditions in the field. Our results showed that P. imperialis had greater photosynthesis (8.86 µmol CO2 m-2 s-1) and instantaneous water use efficiency (0.79 µmol CO2mmol H2O-1) than either P. elongata (8.20 µmol CO2 m-2 s-1 and 0.71 µmol CO2 mmol H2O-1) or P. fortunei (3.26 µmol CO2m-2 s-1and 0.07 µmol CO2mmol H2O-1). The rapid growth of Paulownia did not appear to be correlated with photosynthetic rates. Paulownia fortunei showed more transpiration (48.78 mmol H2O m-2 s-1) and stomatal conductance (840 mmol m-2 s-1) than P. imperialis (20 mmol H2O m-2 s-1 and 540 mmol m-2 s-1) and P. elongata (20 mmol H2O m-2 s-1 and 410 mmol m-2 s-1), which allowed these two Paulownia species to increase their tolerance to low soil moisture, and maintain higher water use efficiency under these conditions. According to our physiological gas exchange field tests, Paulownia imperialis does appear to be capable of successful growth in semiarid zones.