The influence of root-zone bicarbonate and carbon dioxide enrichment on lettuce, pepper and tomato growth

Enhancing atmospheric 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 enrichment of the root zone (RZ) with CO2 (RZ CO2) may be a more economic and sustainable alternative to aerial CO2 enrichment. This thesis aimed to compare the effects of RZ dissolved inorganic carbon (DIC) enrichment by adding either bicarbonate (HCO3-) or gaseous CO2 to hydroponic and aeroponic systems, and to determine the physiological and molecular mechanisms by which plants respond to RZ DIC. Supplying hydroponically grown plants with high bicarbonate concentrations (20 mM) inhibited growth of lettuce, pepper and tomato. However, lower concentrations increased biomass accumulation in lettuce (10% increase at both 1 mM and 5 mM HCO3-) and pepper (10% increase at 1 mM HCO3-), but had no effect in tomato. Exposing plants to 1 mM NaH13CO3- significantly increased shoot δ13C values over time, therefore confirming the uptake of DIC by the roots. Root δ13C values also significantly increased over time, however higher values at the beginning of NaH13CO3- 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). Adding 1 mM HCO3- to hydroponically grown plants did not change foliar nutrient content, but K, P, N, Zn, Cu and Mn concentrations decreased at 20 mM HCO3-, suggesting nutrient deficiencies could limit growth. Applying 2000 ppm RZ CO2 to hydroponically grown lettuce, tomato and pepper did not affect biomass accumulation. Applying 1500 ppm CO2 to the RZ of aeroponically grown lettuce increased shoot biomass between 19-25% (in 4 independent experiments) compared to those grown with 400 ppm RZ CO2. However, leaf gas exchange measurements were inconsistent and therefore increased biomass could not be attributed to higher photosynthetic rates. In another 3 independent experiments, applying 1500 ppm CO2 to the RZ of aeroponically grown lettuce did not stimulate biomass accumulation, probably because the plants were exposed to higher night temperatures. Similarly, pepper and tomato did not show any biomass response to elevated RZ CO2, suggesting that the responses to RZ CO2 concentration are environment- and species-dependent. Nutrient analysis indicated that aeration with high RZ CO2 decreased lettuce foliar Mg and S concentrations, whereas root N concentrations were higher than control plants. Multi-hormone analysis of foliar and root tissues revealed that lettuce plants showed few differences in hormone status following RZ CO2 enrichment. High RZ CO2 increased foliar jasmonic acid concentration of lettuce, but the physiological significance of this change is not clear. Pepper plants showed significantly higher foliar 1-aminocyclopropane-1-carboxylic acid and lower trans-zeatin and salicylic acid concentrations, as well as lower root N6-(Δ2-isopentenyl) adenine and higher salicylic acid and gibberellic acid concentrations. These hormonal responses were associated with lower leaf area expansion of pepper plants exposed to elevated RZ CO2. Finally, transcriptome analysis of lettuce plants indicated that fatty acid biosynthesis, amino acid biosynthesis and carbon metabolism appeared to be the major pathways enriched in roots exposed to elevated RZ CO2. In addition, proteins related to the cell walls and membranes seemed enhanced under elevated RZ CO2. Although increased CO2 concentration around the roots caused major transcriptomic restructuring, the aerial parts of the plants showed limited transcriptomic changes. Taken together, this thesis is the first study of the responses of several horticultural species to elevated RZ CO2 within different growing systems in order to decipher the impact that elevated RZ CO2 has on crop productivity. Although bicarbonate enrichment of hydroponics and RZ CO2 enrichment of aeroponics stimulated biomass accumulation of lettuce in many experiments, further work is required to fully understand the physiological response mechanisms to RZ CO2. Whether the root transcriptomic changes in response to elevated RZ CO2 represent an adaptive response to their environment requires a better temporal understanding of changes in specific genes. Ultimately, whether these changes are functionally significant to shoot growth seems to be strongly environmentally regulated.