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

Abstract

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 d13C 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 d13C 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. © 2020 International Society for Horticultural Science. All rights reserved.