Carbon Dioxide Enrichment Increases Yield of Field Grown Red Raspberry Under High Tunnels

Anthropogenic sources of carbon dioxide (CO₂) released into the atmosphere may increasingly affect the global climate system.  While CO₂ capture and sequestration is currently costly and underdeveloped, producers of high-value agricultural crops such as raspberry may become advocates for CO₂ capture and recycling if substantive benefits to plants can be demonstrated.  We designed a completely randomized study with three replications to optimize a CO₂ application system to field grown raspberry (proprietary variety) under high tunnels and to compare plant photosynthesis, growth, and fruit yield with and without CO₂.  Leaves at the top of the canopy were 8% less efficient in using available light energy according to leaf fluorescence measurements so we placed the irrigation drip tape to apply CO₂ in the middle of the plant canopy.  We found that leaf CO₂ assimilation and stomatal conductance were reduced 55% and 30% during the midday, and therefore chose to apply CO₂ from 7:00 to 11:00 am and from 2:00 to 6:00 pm.  In this way, we maintained a CO₂ concentration of 436 μL·L^-1 for four months in tunnels 91.4 m long and 5.5 m wide.  CO₂ concentration in the tunnel was 18% higher in the morning hours; wind speed often increased in the afternoon, which may have moved the CO₂ even in the protected tunnel.  In CO₂ tunnels, yield and berry size from plots 6 m long increased 12% and 5%, respectively, compared to untreated tunnels.  The number of fruit baskets harvested from the entire tunnel increased 36% after CO₂ application when compared to previous floricane yield in the same tunnels; the number of fruit baskets also increased 16% in untreated tunnels.  We found no corresponding differential response in leaf CO₂ assimilation, stomatal conductance, and fluorescence, raspberry cane height, pruned cane biomass, cane diameter, or carbohydrate content of the fruit after CO₂ application; documenting plant physiological changes must be the focus of future research.  As the economic feasibility this system depends on a source of captured or recycled CO₂, we hope this study prompts further research and development of CO₂ capture and recycling to benefit agricultural producers.