Differential responses to two heatwave intensities in a Mediterranean citrus orchard are identified by combining measurements of fluorescence and carbonyl sulfide (COS) and CO2 uptake
The impact of extreme climate episodes such as heatwaves on plants 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 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 (gross primary production, GPP)
were significantly reduced during HW days, but only slightly decreased during INT days. By
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 nonphotochemical quenching,
respectively. Intense (HW) heatwave 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, which allows the trees to preserve carbon assimilation during
INT days but not during HW days.