Uncovering the wide protective responses in Coffea spp. leaves to single and superimposed exposure of warming and severe water deficit.

Climate changes boosted the frequency and severity of drought and heat events, with aggravated when these stresses occur simultaneously, turning crucial to unveil the plant response mechanisms to such harsh conditions. Therefore, plant responses/resilience to single and combined exposure to severe water deficit (SWD) and heat were assessed in two cultivars of the main coffee-producing species: Coffea arabica cv. Icatu and C. canephora cv. Conilon Clone 153 (CL153). Well-watered plants (WW) were exposed to SWD under an adequate temperature of 25/20°C (day/night), and thereafter submitted to a gradual increase up to 42/30°C, and a 14-d recovery period (Rec14). Greater protective response was found to single SWD than to single 37/28°C and/or 42/30°C (except for HSP70) in both cultivars, but CL153-SWD plants showed the larger variations of leaf thermal imaging crop water stress index (CWSI, 85% rise at 37/28°C) and stomatal conductance index (IG, 66% decline at 25/20°C). Both cultivars revealed great resilience to SWD and/or 37/28°C, but a tolerance limit was surpassed at 42/30°C. Under stress combination, Icatu usually displayed lower impacts on membrane permeability, and PSII function, likely associated with various responses, usually mostly driven by drought (but often kept or even strengthened under SWD and 42/30°C). These included the photoprotective zeaxanthin and lutein, antioxidant enzymes (superoxide dismutase, Cu,Zn-SOD; ascorbate peroxidase, APX), HSP70, arabinose and mannitol (involving de novo sugar synthesis), contributing to constrain lipoperoxidation. Also, only Icatu showed a strong reinforcement of glutathione reductase activity under stress combination. In general, the activities of antioxidative enzymes declined at 42/30°C (except Cu,Zn-SOD in Icatu and CAT in CL153), but HSP70 and raffinose were maintained higher in Icatu, whereas mannitol and arabinose markedly increased in CL153. Overall, a great leaf plasticity was found, especially in Icatu that revealed greater responsiveness of coordinated protection under all experimental conditions, justifying low PIChr and absence of lipoperoxidation increase at 42/30°C. Despite a clear recovery by Rec14, some aftereffects persisted especially in SWD plants (e.g., membranes), relevant in terms of repeated stress exposure and full plant recovery to stresses.

Intrinsic non-stomatal resilience to drought of the photosynthetic apparatus in Coffea spp. is strengthened by elevated air [CO2].

Growing water restrictions associated with climate changes constitute daunting challenges to crop performance. This study unveils the impacts of moderate (MWD) or severe (SWD) water deficit, and their interaction with air [CO2], on the photosynthetic apparatus of Coffea canephora Pierre ex A. Froehner cv. Conilon Clone 153 (CL153) and Coffea arabica L. cv. Icatu. Seven year-old potted plants grown under 380 (aCO2) or 700 µl l -1 (eCO2) [CO2] gradually reached predawn water potentials between -1.6 and -2.1 MPa (MWD), and below -3.5 MPa (SWD). Under drought, stomata closure was chiefly related to abscisic acid (ABA) rise. Increasing drought severity progressively affected gas exchange and fluorescence parameters in both genotypes, with non-stomatal limitations becoming gradually dominating, especially regarding the photochemical and biochemical components of CL153 SWD plants. In contrast, Icatu plants were highly tolerant to SWD, with minor, if any, negative impacts on the potential photosynthetic functioning and components (e.g., Amax, Fv/Fm, electron carriers, photosystems (PSs) and ribulose-1,5-bisphosphate carboxylase oxygenase (RuBisCO) activities). Besides, drought-stressed Icatu plants displayed increased abundance of a large set of proteins associated with the photosynthetic apparatus (PSs, light-harvesting complexes, cyclic electron flow, RuBisCO activase) regardless of [CO2]. Single eCO2 did not promote stomatal and photosynthetic down-regulation in both genotypes. Instead, eCO2 increased photosynthetic performance, moderately reinforced photochemical (PSs activity, electron carriers) and biochemical (RuBisCO, ribulose-5-phosphate kinase) components, whereas photoprotective mechanisms and protein abundance remained mostly unaffected. In both genotypes, under MWD, eCO2 superimposition delayed stress severity and promoted photosynthetic functioning with lower energy dissipation and PSII impacts, whereas stomatal closure was decoupled from increases in ABA. In SWD plants, most impacts on the photosynthetic performance were reduced by eCO2, especially in the moderately drought affected CL153 genotype, although maintaining RuBisCO as the most sensitive component, deserving special breeder's attention to improve coffee sustainability under future climate scenarios.

Resilient and Sensitive Key Points of the Photosynthetic Machinery of Coffea spp. to the Single and Superimposed Exposure to Severe Drought and Heat Stresses.

This study unveils the single and combined drought and heat impacts on the photosynthetic performance of Coffea arabica cv. Icatu and C. canephora cv. Conilon Clone 153 (CL153). Well-watered (WW) potted plants were gradually submitted to severe water deficit (SWD) along 20 days under adequate temperature (25/20°C, day/night), and thereafter exposed to a gradual temperature rise up to 42/30°C, followed by a 14-day water and temperature recovery. Single drought affected all gas exchanges (including Amax ) and most fluorescence parameters in both genotypes. However, Icatu maintained Fv/Fm and RuBisCO activity, and reinforced electron transport rates, carrier contents, and proton gradient regulation (PGR5) and chloroplast NADH dehydrogenase-like (NDH) complex proteins abundance. This suggested negligible non-stomatal limitations of photosynthesis that were accompanied by a triggering of protective cyclic electron transport (CEF) involving both photosystems (PSs). These findings contrasted with declines in RuBisCO and PSs activities, and cytochromes (b559 , f, b563 ) contents in CL153. Remarkable heat tolerance in potential photosynthetic functioning was detected in WW plants of both genotypes (up to 37/28°C or 39/30°C), likely associated with CEF in Icatu. Yet, at 42/30°C the tolerance limit was exceeded. Reduced Amax and increased Ci values reflected non-stomatal limitations of photosynthesis, agreeing with impairments in energy capture (F0 rise), PSII photochemical efficiency, and RuBisCO and Ru5PK activities. In contrast to PSs activities and electron carrier contents, enzyme activities were highly heat sensitive. Until 37/28°C, stresses interaction was largely absent, and drought played the major role in constraining photosynthesis functioning. Harsher conditions (SWD, 42/30°C) exacerbated impairments to PSs, enzymes, and electron carriers, but uncontrolled energy dissipation was mitigated by photoprotective mechanisms. Most parameters recovered fully between 4 and 14 days after stress relief in both genotypes, although some aftereffects persisted in SWD plants. Icatu was more drought tolerant, with WW and SWD plants usually showing a faster and/or greater recovery than CL153. Heat affected both genotypes mostly at 42/30°C, especially in SWD and Icatu plants. Overall, photochemical components were highly tolerant to heat and to stress interaction in contrast to enzymes that deserve special attention by breeding programs to increase coffee sustainability in climate change scenarios.