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

Danielly Dubberstein,Paula Scotti-Campos,Isabel Marques,Ana P Rodrigues,Fábio L Partelli,Magda C Semedo,Eliemar Campostrini,José C Ramalho,José N Semedo,Duarte Gouveia,Weverton P Rodrigues,Ana I Ribeiro-Barros,Sónia Martins,Maria C Simões-Costa,Fernando C Lidon,Jean Armengaud,I Moura,Fábio M Damatta,Isabel P Pais

doi:10.3389/fpls.2020.01049

Abstract

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.

Highlights

The global CO2 emissions have been increased steadily from the industrial revolution onwards, from around 280 μL CO2 L-1 until a global atmosphere average of 407.4 in 2018 (NOAA, 2019)
The stomatal density (SD) was not significantly altered by drought in Clone 153 (CL153), but it gradually increased with the imposition of heat, significantly only at 42/30°C (Figure 2)
Stomatal density (SD) was reduced in Icatu with the single imposition of either severe drought or heat stress, reaching the lowest values upon stress superimposition (SWD plants at 42/30°C)

Summary

Introduction

The global CO2 emissions have been increased steadily from the industrial revolution onwards, from around 280 μL CO2 L-1 until a global atmosphere average of 407.4 in 2018 (NOAA, 2019). Heat and drought stresses, which are major environmental constraints to plant growth and crop productivity, have been associated with decreases in both stomatal conductance (gs) and net CO2 assimilation rates (Pn) (Long et al, 2006). These decreases predispose leaves to photoinhibitory damage due to decreased energy use through photosynthesis (Baker and Rosenqvist, 2004; Lambers et al, 2008; Haworth et al, 2018), which can be exacerbated when these stresses are superimposed. This might be even more relevant for perennial crops such as coffee, which can have a lifespan up to 30 years (Bunn et al, 2015) or even more

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