CO₂ ENRICHMENT IN GREENHOUSES: PROBLEMS OF CO₂ ACCLIMATION AND GASEOUS AIR POLLUTANTS - REFEREED PAPER -

CO₂ enrichment of the greenhouse atmosphere greatly improves the output, quality and value of vegetables, cut-flowers and ornamental plants. Raising the greenhouse CO₂ concentration enhances photosynthesis and growth by increasing the rate of CO₂ fixation concomitantly with a suppression of photo-respiration. Prolonged exposure of plants to elevated CO₂ concentrations can, however, greatly accelerate the decline in the photosynthetic capacity of individual leaves with age.

Commercially, CO₂ for enrichment is normally obtained in liquid form or produced directly in the greenhouse atmosphere by burning hydrocarbon fuels such as natural gas, LPG propane, and premium kerosene (paraffin) which all contain low and acceptable levels of sulphur. Plentiful supplies of natural gas in both Britain and The Netherlands have also encouraged growers in these countries to practise CO₂ enrichment by ducting flue-gases into their greenhouses from centralized, gas-fired boiler installations.

Generating CO₂ from hydrocarbon fuels can give rise to several gaseous air pollutants that are potentially damaging for crop production. The pollutants that cause most of the trouble in CO₂-enriched greenhouses are nitrogen oxides such as nitric oxide and nitrogen dioxide, and unburnt hydrocarbons such as ethylene and propylene. Additionally, improvements to the insulation of heated greenhouses restrict air exchange and increase the hazard of certain plasticisers such as the alkyl esters of phthalic acid which are used to give flexibility to PVC.

The risk of incurring losses in yield due to gaseous air pollutants can be minimized by using low-sulphur fuels, avoiding leaks of fuel gases, servicing burners regularly, limiting fuel consumption and improving ventilation in near-airtight structures. Longer-term measures to improve the productivity of crops grown in polluted greenhouse atmospheres include the design of pollution-free burners, and the development and use of cultivars that are tolerant of gaseous air pollutants.