Elevated air [CO₂] improves photosynthetic performance and alters biomass accumulation and partitioning in drought-stressed coffee plants

Coffee (Coffea arabica L.) is an important global commodity grown in tropical areas where increased drought severity and frequency are believed to become progressively important due to climate changes. Nonetheless, elevated air [CO₂] is thought to be able to mitigate heat and drought stresses. In this study, we tested how carbon assimilation and use are affected by elevated [CO₂] in combination with a progressive drought, and how this could impact shifts on biomass accumulation and partitioning. For that, we cultivated coffee plants in open top chambers under greenhouse conditions. Plants grown in 12-L pots were then submitted to ambient (386 ± 20 ppm) or elevated (723 ± 83 ppm) [CO₂] during approximately seven months, as well as to varying soil water availabilities (100, 50, 37.5, 25 or 20 % of soil field capacity). Our results demonstrate that elevated [CO₂] improved carbon assimilation rates (>60 %) with unaltered stomatal conductance and no signs of photosynthetic downregulation. This was accompanied by increases in water-use efficiency, respiration rates and biomass accumulation regardless of watering, and decreased photorespiration rates and oxidative pressure under drought. Improved growth under elevated [CO₂] was more evident under drought than under full irrigation, and was unlikely to have been associated with global changes on hormonal pools, but rather with shifts on carbon fluxes. Finally, elevated [CO₂] promoted key allometric adjustments linked to drought tolerance, e.g., more biomass partitioning towards roots with greater root length. Collectively, our results offer novel and timely information on the mitigating ability of elevated [CO₂] on the photosynthetic performance and growth of coffee plants under drought conditions.