The variation of irrigation regime in hazelnut cv. TGL has a little impact on canopy conductance

The hazelnut (Corylus avellana L.), cultivar Tonda Gentile delle Langhe (TGL) remains one of the most appreciated nut trees by the food industry and it was exported to extra native sites to allow extensive plantations. Still TGL is renowned for being highly sensitive to the climate outside its native ecological niche (Piemonte, Italy), where its sensitivity is expressed as a rigid stomata behavior (stomata closure in the early morning, sharp closure at 10 hPa of VPD). We hypothesized that because of its sensitivity to VPD, TGL could have a limited benefit by increasing the irrigation regime because of the small impact in the total canopy conductance and, thus, on the carbon assimilation and yield. We monitored six TGL hazelnut trees during two growing seasons in San Sebastian, Chile. We measured all climatic parameters, sap flow and soil water content on two irrigation treatments: T100 and T200 respect to the local standard. Our result showed that the response of sap flow to VPD remained consistent in the two treatments. Although the sap flow density in T200 was significantly higher as compared to T100, the diurnal canopy conductance was >17% in T200 respect to T100 only in a restricted interval of VPD, i.e. 4-12 hPa around a peak of 7 hPa. Our results suggest that a doubled irrigation causes an increase in canopy conductance only in few climatic circumstances. Indeed, a higher benefit by increased irrigation in a VPD-sensitive species as it is hazelnut can be reached when the irrigation is applied at lower VPD. In such conditions, stomata are more prone to gas exchanges and carbon sequestration, a proxy for crop yield.