ADAPTATION TO DROUGHT IN EUROPEAN CHESTNUT. EVIDENCES FROM A HYBRID ZONE AND FROM CONTROLLED CROSSES BETWEEN DROUGHT AND WET ADAPTED POPULATIONS

Species colonising wide and differentiated areas must exhibit a relevant variability of adaptivness to environmental constraints. The natural variability of European chestnut was deeply studied during the past decade concerning both population genetic and stress physiology. Joint genetic-physiological studies recently addressed this variability to the adaptivness of chestnut to environments with markedly different water availability. Particularly, a substantial variability was found among populations spread along a Turkish transect from the Black Sea coast to the Mediterranean coast. Along this transect the climate gradually changes from sub-humid to sub-arid. Eastern populations were compared to Mediterranean ones in a common site in Italy, also including artificial hybrids obtained from controlled crosses among individuals from two extreme populations as well as natural hybrids belonging to a hybrid zone in Turkey. To elucidate the physiological basis of a differential adaptivness to water availability this pool of plant material was analysed for carbon isotope discrimination (Δ; it gives information about plant water-use efficiency), photosynthesis, above ground biometry and phenology. The Mediterranean drought adapted chestnut type showed higher Δ (lower water-use efficiency), higher photosynthesis, lower juvenile growth, higher long term growth and higher earliness of bud burst with respect to the Eastern chestnut type. Intermediate and variable functional responses were generally observed on both artificial and natural hybrids. Particular emphasis is devoted to highlight possible adaptive and evolutionary significance of the hybrid zone.