CUCUMBER CROP RESPONSES TO CO₂ ENRICHMENT

Threefold CO₂ enrichment is normally practised throughout the winter and early spring until ventilation is required repeatedly for cooling the greenhouse. At this stage, usually in late April, it becomes uneconomic to maintain a CO₂ level of 1000 vpm. During the summer months the rate of ventilation is often insufficient to prevent cucumber crops from depleting the greenhouse atmosphere of CO₂. On a cloudless day at the summer solstice the CO₂ level inside a cucumber house with ventilator's fully open can be 10% or more below the normal outside value of 335 vpm. With the ventilators closed it is possible in bright sunny weather for the CO₂ concentration to fall to a level at which there is no net photosynthesis (usually between 50 and 100 vpm depending on light and temperature).

Photosynthesis CO₂ response curves show that CO₂ depletion results in a slower rate of CO₂ uptake by crops. There is therefore scope for improving crop productivity throughout the summer months by supplementing with enough gaseous CO₂ (supplied from a bulk vessel of liquid CO₂) to maintain the atmosphere at or just above 335 vpm.

Results obtained from an experiment in a multifactorial glasshouse unit with January-sown cucumbers (cvs Farbio, Corona, Primio and Tirana) show that by avoiding CO₂ depletion for a 17-week period from late April to early August the output of marketable fruits can be increased by 11%. This improvement in yield results from increases in both the number and size of fruits developed on the crop.

Measurements of CO₂ usage have shown that the technique is cost effective. Reports from growers and other researchers suggest that similar increases in yield may be obtained commercially with both tomato and cucumber crops.

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