Root-zone CO₂ enrichment increases biomass accumulation in lettuce and pepper grown hydroponically and aeroponically

Enhancing CO₂ levels in commercial glasshouses is a widely used technique to increase productivity, but has high-energy costs and detrimental environmental impacts due to frequent ventilation of the glasshouse (to prevent plant diseases) releasing CO₂ into the atmosphere.
Previous studies suggest that root-zone (RZ) CO₂ enrichment may be a more economic and sustainable alternative to aerial CO₂ enrichment.
These experiments aimed to compare the effects of RZ CO₂ enrichment by adding either bicarbonate or gaseous CO₂ into hydroponic and aeroponic systems respectively, and to determine the physiological mechanisms by which plants respond to RZ CO₂. Root-zone CO₂ enrichment (1500 ppm) of aeroponically-grown lettuce increased shoot dry weight by around 20% compared to those grown with 400 ppm RZ CO₂. Supplying hydroponically grown plants with different HCO₃^– concentrations, that increased the levels of dissolved inorganic carbon (DIC), increased biomass accumulation of lettuce (10% increase at 1 and 5 mM HCO₃^–) and pepper (10% increase at 1 mM HCO₃^–). Plants exposed to 1 mM NaH¹³CO₃ showed a significant increase of foliar δ13C values over time, therefore confirming the uptake of DIC by the roots.
The δ13C values of roots increased significantly over time, however, higher values at the beginning of H¹³CO₃^– exposure suggested root-to-shoot transport of DIC. Nutrient solution pH did not affect root carbon uptake, but shoot δ13C values were lower in those plants exposed to lower pH levels (5.8) compared to those exposed to fluctuating pH (between 6.3 and 6.7), suggesting differences in root-to-shoot transport of DIC. Thus, root carbon uptake was independent of the form in which CO₂ was provided (gaseous CO₂ at pH 5.8; HCO₃^– at higher pHs). How this additional carbon promotes plant growth is still unclear.
Potential mechanisms of action such as increased rates of photosynthesis, altered amino acid concentrations and changes in phytohormone concentrations will be investigated in future studies.

XXX International Horticultural Congress IHC2018: II International Symposium on Soilless Culture and VIII International Symposium on Seed, Transplant and Stand Establishment of Horticultural Crops

E. Leibar-Porcel, M.R. McAinsh, I.C. Dodd

bicarbonate, root-zone CO₂, hydroponics, aeroponics, plant growth, lettuce, pepper

https://doi.org/10.17660/ActaHortic.2020.1273.13