Effect of variable CO2 enrichment on greenhouse production in mild winter climates

Variable CO2 enrichment was evaluated under the autumn-winter climatic conditions prevailing in a coastal zone of southern Spain. Two adjacent identical greenhouses were used: control (without CO2 enrichment) and enriched. Pure CO2 was supplied to a greenhouse cucumber crop, maintaining in the greenhouse the air CO2 concentration at close to 700 μmol mol−1 when the greenhouse was closed and 350 μmol mol−1 when the vents were open. CO2 dynamics, efficiency of radiation and CO2 use, and crop responses were determined over the growing season and compared to those of a similar cucumber crop grown in a non-enriched greenhouse. While the average diurnal concentration remained above 400 μmol mol−1 in the enriched greenhouse, significant CO2 depletion was observed in the unenriched compartment, where the CO2 concentration fell below 300 μmol mol−1 during 60% of the daytime when the crop was fully developed (leaf area index (LAI) ≈ 3). In the unenriched greenhouse, the average diurnal CO2 depletion as well as the average mean concentration were highly correlated to the radiation intercepted by the crop, i.e. to the potential for biomass production. The average efficiency of CO2 enrichment, i.e. the ratio of the amount of CO2 sequestrated in the crop to the amount of artificial CO2 delivered, was about 2% with respect to vegetative biomass, and about 6% with respect to fruit biomass. CO2 enrichment affected mainly fruit biomass and had only a slight effect on leaf area index and vegetative biomass. The average increase in fruit production for both dry and fresh matter due to CO2 enrichment was 19%, which agrees well with previous results on the agricultural response of cucumber to moderate CO2 enrichment.