Stomatal sensitivity in Corylus avellana (L.): first analysis from a global dataset

The cultivated area planted with hazelnut is rapidly increasing. The species is now being planted in areas where precipitation is scarce and irrigation is needed for ensuring profitable orchards. Moreover hazelnut is known to be sensitive to water shortage as it responds quickly to the increase of vapor pressure deficit by reducing stomatal aperture and consequently carbon assimilation. However modeling stomatal responses in different climatic conditions is still a challenging task. The general aim of the research was to predict stomatal responses by collecting data in different climatic conditions and soil water availability. We installed five measurement sites with different cultivars (‘Tonda Gentile delle Langhe’, ‘Tonda di Giffoni’, ‘Ennis’) in different countries (France, Georgia, Australia, Chile) and under different irrigation regimes. In each site we measured the sap flow in six trees, the soil water content, the stem diameter variations and the meteorological parameters. In two sites (Australia and France) we measured the stomatal conductance and the carbon assimilation by using a portable gas analyzer. Results showed that in all sites and cultivars, hazelnut trees strongly reduce the stomatal aperture when the vapor pressure deficit exceeds 10 hPa, which leads to a decrease in the assimilation rate. The daily course of stomatal aperture showed that the maximum is reached very early in the morning (about 9 AM) and then it drops abruptly. Furthermore, the species responses at leaf level were coherent with those at the tree canopy level, establishing a crucial basis for the future modelling of the water use at the orchard level. We showed that the carbon assimilation is mainly controlled by stomatal sensitivity to vapor pressure deficit and that, even in conditions of high soil water content, the photosynthetic capacity of the species is clearly reduced.