ROOT OXYGEN USE DURING PROPAGATION OF CUCUMBER ON ROCKWOOL CUBES

Cucumbers were propagated in rockwool cubes in a climate cell for four weeks. The complete root system of each cucumber was enclosed in an airtight box. Each box was connected to an air bag, which acted as an air reservoir. A peristaltic pump ensured air circulation in the system. Treatments included maintenance of oxygen levels at 21%, 7% and 2% in specific box-bag systems. The goal of this experiment was to measure the critical oxygen supply rate for normal production. An additional goal was to characterize plant and root growth affected by sub optimal oxygen avail¬ability. With a fiber optic oxygen meter it proved possible to measure and monitor the oxygen level in the box-bag system in time at various points. A critical oxygen maintenance level between 8 and 12% was found for this system. A maximum oxygen use of 5.8 mg.h^-1.plant^-1 was reached by plants with a growth equal to the reference plants. The above ground growth reaction to mild but prolonged sub-optimal oxygen supply rates included a 20-50% reduction in leaf area, fresh and dry above ground and below ground mass production and, less pronounced, a reduction in plant length. The dry mass root/shoot ratio remained constant despite widely different oxygen supply. The root oxygen use rate during the light period was 5-10 times higher than during the dark period. It is argued that the growth reduction is not so much related to a critical oxygen level in the air but to local oxygen depletion in the substrate. Local oxygen depletion might be the result of the interaction of oxygen supply rate and substrate diffusivity or the accumulation of gasses as carbon dioxide and ethylene to phytotoxic concentrations.