Impact of a DANA Event on the Thermal Response of Nectarine Trees.

This field experiment focuses on the effects of a heavy rainfall event (DANA, that occurred on 12-14 September 2019 (DOY, Day of the year, 255-257), in southern Spain on plant water status and the thermal response of nectarine trees. Two irrigation treatments were applied during the summer-autumn postharvest period (DOY 158-329): full-irrigated (CTL) and non-irrigated (DRY). Volumetric soil water content (θ), air temperature (Ta) and canopy temperature (Tc) were monitored in real-time and the crop water stress index (CWSI) was calculated. The difference in Tc between the DRY and CTL treatments (Tc' - Tc) is proposed as a new thermal indicator. Stem water potential (Ψ) and leaf gas exchange measurements were recorded on representative days. During the DANA event, only the Tc measured by the infrared radiometer sensors could be monitored. Therefore, the effects of the DANA forced the soil water content sensors to be switched off, which prevented Ψ and leaf gas exchange determinations from DOY 255 to 275. Before the DANA event, withholding irrigation caused a gradual decrease in the soil and plant water status in the DRY treatment. Significant differences appeared between treatments in the studied thermal indexes. Moreover, Tc' - Tc was more sensitive than Tc - Ta in assessing nectarine water stress. The effects of the DANA reduced these differences, suggesting different baselines for the calculation of CWSI. In this respect, the relationship Tc - Ta vs. VPD improved the coefficient of determination after the DANA event in full-irrigated trees. Similar values of Ψ and leaf gas exchange were found in both treatments after the DANA event, even though thermal indexes showed some significant differences. In addition, the strong relationship found between Tc - Ta and CWSI vs. Ψ worsened after DANA occurred, revealing a lower sensitivity of Ψ compared to canopy temperature to accurately assess nectarine water status in these saturated soil conditions. This research underlined the robustness of infrared thermography to continuously monitor plant water status under these extreme weather conditions.