Carbon gain is coordinated with enhanced stomatal conductance and hydraulic architecture in coffee plants acclimated to elevated [CO2]: The interplay with irradiance supply.

We recently demonstrated that, under elevated [CO2] (eCa), coffee (Coffea arabica L.) plants grown at high light (HL), but not at low light (LL), display higher stomatal conductance (gs) than at ambient [CO2] (aCa). We then hypothesized that the enhanced gs at eCa/HL, if sustained at the long-term, would lead to adjustments in hydraulic architecture. To test this hypothesis, potted plants of coffee were grown in open-top chambers for 12 months under HL or LL (ca. 9 or 1 mol photons m-2 day-1, respectively); these light treatments were combined with two [CO2] levels (ca. 437 or 705 µmol mol-1, respectively). Under eCa/HL, increased gs was closely accompanied by increases in branch and leaf hydraulic conductances, suggesting a coordinated response between liquid- and vapor-phase water flows throughout the plant. Still under HL, eCa also resulted in increased Huber value (sapwood area-to-total leaf area), sapwood area-to-stem diameter, and root mass-to-total leaf area, thus further improving the water supply to the leaves. Our results demonstrate that Ca is a central player in coffee physiology increasing carbon gain through a close association between stomatal function and an improved hydraulic architecture under HL conditions.

Growth and Leaf Gas Exchange Upregulation by Elevated [CO2] Is Light Dependent in Coffee Plants.

Coffee (Coffea arabica L.) plants have been assorted as highly suitable to growth at elevated [CO2] (eCa), although such suitability is hypothesized to decrease under severe shade. We herein examined how the combination of eCa and contrasting irradiance affects growth and photosynthetic performance. Coffee plants were grown in open-top chambers under relatively high light (HL) or low light (LL) (9 or 1 mol photons m-2 day-1, respectively), and aCa or eCa (437 or 705 µmol mol-1, respectively). Most traits were affected by light and CO2, and by their interaction. Relative to aCa, our main findings were (i) a greater stomatal conductance (gs) (only at HL) with decreased diffusive limitations to photosynthesis, (ii) greater gs during HL-to-LL transitions, whereas gs was unresponsive to the LL-to-HL transitions irrespective of [CO2], (iii) greater leaf nitrogen pools (only at HL) and higher photosynthetic nitrogen-use efficiency irrespective of light, (iv) lack of photosynthetic acclimation, and (v) greater biomass partitioning to roots and earlier branching. In summary, eCa improved plant growth and photosynthetic performance. Our novel and timely findings suggest that coffee plants are highly suited for a changing climate characterized by a progressive elevation of [CO2], especially if the light is nonlimiting.

Elevated [CO2] benefits coffee growth and photosynthetic performance regardless of light availability.

Despite being evolved in shaded environments, most coffee (Coffea arabica L.) is cultivated worldwide under sparse shade or at full sunlight. Coffee is ranked as greatly responsive to climate change (CC), and shading has been considered an important management strategy for mitigating the harmful CC outcomes on the crop. However, there is no information on the effects of enhanced [CO2] (eCa) on coffee performance in response to light availability. Here, we examined how carbon assimilation and use are affected by eCa in combination with contrasting light levels. For that, greenhouse-grown plants were submitted to varying light levels (16 or 7.5 mol photons m-2 day-1) and [CO2] (ca. 380 or 740 µmol mol-1 air) over six months. We demonstrated that both high light and eCa improved growth and photosynthetic performance, independently. Despite marginal alterations in biomass partitioning, some allometric changes, such as higher root biomass-to-total leaf area and lower leaf area ratio under the combination of eCa and high light were found. Stimulation of photosynthetic rates by eCa occurred with no direct effect on stomatal and mesophyll conductances, and no signs of photosynthetic down-regulation were found irrespective of treatments. Particularly at high light, eCa led to decreases in both photorespiration rates and oxidative pressure. Overall, our novel findings suggest that eCa could tandemly act with shading to mitigate the harmful CC effects on coffee sustainability.