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Summary 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.

<p>Springtime heatwaves are common phenomena in the Mediterranean region, named ‘Sharav’ or ‘Hamsin’. During these heatwaves, air temperatures (Ta) and vapor pressure demand (VPD) increase rapidly over 3-5 days, followed by a dramatic drop of at least 5℃ in Ta and 1 kPa in VPD back to the pre-event values.</p><p>Here, we used our mobile lab in an irrigated lemon orchard in Rehovot, Israel to carry out eddy covariance (EC) flux measurements of net ecosystem exchange of CO<sub>2</sub> (NEE), water vapor, and carbonyl sulfide (COS), as well as canopy Sun-induced fluorescence (SIF) together with other spectral indices (NDVI, PRI, NIRv). This was supplemented with leaf-scale measurements of Pulse Amplitude modulated (PAM). Five heatwave events were detected during a two-months measurement campaign. Two other events were defined as intermediate days, with VPD values higher than normal but lower than in the full-scale heatwaves.</p><p>During both the heatwave and intermediate days, the COS fluxes (Fcos), far-red SIF, and electron transport rate (ETR), decreased during midday to the same level, compared to the control days. In contrast, NEE responded differentially between the heatwave and intermediate days, with midday values of -5.9±0.9, -3.7±0.7 and -0.69±0.62 µmol m<sup>-2</sup>s<sup>-1</sup> CO<sub>2</sub>, in the control, intermediate and heatwave days, respectively. No differences were detected in both NDVI and NIRv values. The PRI index, related to energy transfer through the non-photochemical quenching (NPQ) pathway, showed a similar pattern to that of NEE. The recovery of the ecosystem from the heatwave events was rapid and occurred within a day after the end of the events.</p><p>The results indicate a link between the far-red SIF and the ETR in the response to the heatwaves. Moreover, the reduction in far-red SIF was negatively associated with the increase in NPQ, which was reflected in both the spectral (PRI) and the PAM (NPQ value) measurements. The observed decrease in Fcos is expected to reflect a decrease in stomatal conductance to a similar extent in the heatwave and intermediate days. However, the lower rate of CO<sub>2</sub> assimilation in the full-scale heat wave days suggests that additional factors further decreased its rates beyond that limited by conductance. This can be related to the increased effect of the heat stress on other energy-demanding pathways (e.g. photorespiratory, isoprene production) that can suppress net assimilation in these days.</p><p>This work demonstrated that the relation between carbon assimilation and far-red SIF can be complex, and that combining SIF and COS measurements can help partition the effects of heat stress on conductance and other physiological effects.</p>

<p>       The production of electricity from solar radiation should replace power production by burning fossil fuel and help reduce atmospheric concentrations of CO<sub>2</sub>. However, large photovoltaic (PV) fields can also influence the climate in more direct ways. The albedo of solar panels is low to allow efficient light absorption, but actual conversion efficiency is below  20%. The remaining 80% of the energy is reflected, re-emitted as thermal radiation or dissipated as sensible heat (H). These effects can heat the surface, influence local air circulations, and lead to the formation of “heat-islands”. Such effects are particularly significant in desert areas with high radiation load and high background albedo. The ultimate objective of this study will be to estimate the cost (in number of years) of CO<sub>2</sub> emission suppression of a PV power generation (a “cooling effect”) associated with the albedo radiative forcing and the surface "warming effects" and the partitioning to its components.</p><p>       We used a state-of-the-art field laboratory to carry out eddy covariance flux measurements of sensible and latent heat, and the radiative balance of incoming and outgoing short- and long-wave radiations. A research drone equipped with a thermal and a multi-spectral camera was used to estimate the spatial average reflected and emitted radiation from the solar panels field. Measurements were carried out on campaign basis during 2018-2019, both inside and outside a PV field in the Arava desert in southern Israel.</p><p>       The preliminary results indicated that summer noon incoming solar radiation (S) is ~1000 Wm<sup>-2</sup> and the desert surface albedo is on average 0.40. The mean solar panel field albedo is 0.23 (with panels projected area about 1/3<sup>rd</sup> of the PV field area), which is translated to ~170 Wm<sup>-2</sup> higher S absorption by the PV field. A large fraction of the energy is converted to sensible heat flux with mid-day H values of 450 Wm<sup>-2</sup>, compared with 250 Wm<sup>-2 </sup>in the desert, or about 200 Wm<sup>-2</sup> of extra heating above the PV field. A first approximation of the summer daily carbon suppression (assuming 12h daily average sunlight of ~500 Wm<sup>-2</sup>, PV efficiency of 0.2, and conventional power efficiency of ~200 gC/KWh) indicated ~0.08 Kg C per day per m<sup>-2</sup> PV area. These preliminary results are being extended to include thermal emission effects and the annual scale perspective to assess the “PV forest” radiative forcing effect. But it is evident that the land use change examined here has a large impact on the surface energy budget and its surrounding environments.</p>

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