Soil Ca₂SiO₄ Supplying Increases Drought Tolerance of Young Arabica Coffee Plants

Silicon (Si) may benefit the growth and physiology of various cultivated species, especially under stress conditions. Here, we hypothesized that soil Si supplying as Ca₂SiO₄ would increase the drought tolerance and water use efficiency of young Coffea arabica L. (Arabica coffee) plants, by maintaining shoot water status and photosynthesis under low water availability. To test such a hypothesis, morphological and physiological (leaf water potential, leaf gas exchange, photochemical activity, chlorophyll content) traits of coffee plants were evaluated under varying soil Ca₂SiO₄ applications (0, 3000, 6000 kg ha^-1) and water availability. The chemical composition of plant tissues was evaluated under well-watered conditions after six months of Ca₂SiO₄ application, with fertilized plants showing higher concentrations of Ca (leaves and roots) and B (all plant organs) as compared to plants not supplied with Ca₂SiO₄ (control treatment). As there were no changes in Si concentration in plant organs under Ca₂SiO₄ application, our data indicate that the coffee species is a Si non-accumulator, or at least the cultivar 'Catuaí Vermelho' evaluated herein. Additionally, the photosynthetic capacity of coffee plants increased with 6000 kg Ca₂SiO₄ ha^-1 compared to the control under well-watered conditions, as given by increases in gross and net photosynthesis under light saturation, light saturation point, maximum RuBisCO carboxylation rate, maximum electron transport-dependent RuBP regeneration, and maximum rate of triose phosphate use. Such photosynthetic improvements underlined high leaf CO₂ assimilation, transpiration, carboxylation efficiency, and chlorophyll content in plants grown under Si supplying and well-watered conditions. The negative impact of water deficit on leaf gas exchange was alleviated by Ca₂SiO₄ application, but the instantaneous water use efficiency was maintained as similar in both water regimes, as expected for Si non-accumulator species. Morphologically, coffee stem diameter was increased under Ca₂SiO₄ application, regardless of water regime. In conclusion, our data revealed that high Ca₂SiO₄ doses benefit coffee performance and also suggest that the use of steel slag-an industrial byproduct rich in Ca₂SiO₄-can be considered as a sustainable practice for residue recycling in agriculture while improving C. arabica growth and physiology under varying water availability.