Drought tolerance is becoming an increasingly important criterion for the selection of tree species, especially in urban areas characterized by low water availability. Apart from drought tolerance, the introduction of non-native species should be considered for new planting programs under such conditions to enhance the resilience of urban forests. The present study is aimed at evaluating the in situ physiological responses of Magnolia grandiflora and Magnolia × soulangeana to severe drought that frequently occurs in urban environments in the Southeastern Europe. Transpiration rate, stomatal conductance, intercellular CO2 concentration, water-use efficiency and intrinsic water-use efficiency showed notable differences both between species and between the measured periods (wet and dry). Among the chlorophyll a fluorescence parameters, effective photochemical quantum yield of PS II, quantum yield of light-induced non-photochemical fluorescence quenching, quantum yield of non-regulated heat dissipation, fluorescence emission and index of susceptibility of leaves to light stress revealed significant differences both between the two species and the periods of measurements. The reduction of net photosynthesis in both magnolia species occurs as the result of non-stomatal limitation obtained by the reduction of electron transport rate coupled with simultaneous increase in intercellular CO2 concentration. Moreover, M. grandiflora was the species less vulnerable to water shortage conditions, while M. soulangeana exhibited a photosynthetic capacity sensitive to drought-induced stress. M. grandiflora can therefore be considered as a promising alternative to M. soulangeana for urban sites under the predicted climate change scenarios. Keywords: Chlorophyll a Fluorescence, Drought, Leaf Gas Exchange, Photosynthesis, Urban Tree Selection Schlüsselwörter: Chlorophyll a Fluoreszenz, Trockenheit, Blattgasaustausch, Photosynthese, Stadtbaumauswahl
