Comparison of different photosynthetic pathways as tools for agriculture improvement in aquaponics.

Plants use photosynthesis to build biomass and different mechanisms of photosynthesis have evolved. The common C3 pathway has several drawbacks, which are a limitation for global agriculture. The C2 pathway has evolved to harness photorespiration to increase CO2 for the Calvin cycle, while allowing stomata to close more often to limit water loss. C2 photosynthesis is a bioengineering target for improving agricultural, but is largely untested due to species rarity. We tested leaf gas exchange, stomatal anatomy and biomass production between arugula species with different photosynthetic pathways (C2 - Diplotaxis tenuifolia and C3 -Eruca sativa) in a sustainable aquaponics system. Aquaponics provide plants with nutrients and water from fish efflux, reducing water and nutrient stress typical of traditional agriculture. We also grew individuals of each species in soil for comparison with aquaponics. We hypothesized that species in aquaponics would increase stomatal conductance, resulting in higher rates of photosynthesis and growth. We also hypothesized that C2 plants would outperform C3 plants in aquaponics compared to soil because of the potential capacity to better optimize stomatal behavior. For both species, increases in stomatal conductance were detected in aquaponics plants. Additionally, Diplotaxis tenuifolia (C2) exhibited increased stomatal density in aquaponics compared to soil, while Eruca sativa (C3) did not. Surprisingly, this did not lead to increased photosynthesis nor biomass improvement in aquaponics plants for either species. More work is needed, including additional leaf anatomical and stoichiometric data, to evaluate the efficacy of using the C2 pathway as a tool to improve agriculture.