Differential responses in a Mediterranean citrus orchard to two heatwave intensities is identified by combining measurements of fluorescence, carbonyl sulfide (COS), and CO 2 uptake

The impact of extreme climate episodes, such as heatwaves, on physiological functioning and survival may depend on the event intensity, which requires quantification. We unraveled the distinct impacts of intense (HW) and intermediate (INT) heatwave days on the carbon uptake, and the underlying changes in the photosynthetic system, in a Mediterranean citrus orchard using leaf active (Pulse amplitude modulation; PAM) and canopy level passive (sun-induced; SIF) fluorescence measurements, together with CO2 , water vapor, and carbonyl sulfide (COS) exchange measurements. Compared to normal (N) days, gross CO2 uptake fluxes (GPP) were significantly reduced during HW days, but only slightly decreased during INT days. In contrast, COS uptake flux and SIFA (at 760 nm) decreased during both HW and INT days, which was reflected in leaf internal CO2 concentrations and in the non-photochemical quenching, respectively. HW conditions also resulted in a substantial decrease in electron transport rates, measured using leaf-scale fluorescence, and an increase in the fractional energy consumption in photorespiration. Using the combined proxy approach, we demonstrate a differential ecosystem response to different heatwave intensities, that allow the trees to preserve carbon assimilation during INT days but not during HW days.