Trace gases generated in closed plant cultivation systems and their effects on plant growth

Interactions between plants and trace gases, especially ethylene, were investigated from two different viewpoints; ethylene is toxic for plant growth, whereas the ethylene release rate of plants can be utilized as a plant growth indicator. When lettuce plants and shiitake mushroom mycelium were cultivated in closed chambers, ethylene concentration increased with time. Ethylene was released both from lettuce plant and from shiitake mushroom mycelium. Dioctyl phthalate (DOP) and Dibutyl phthalate (DBP) were detected, and these concentrations reached 3.7 ngL-1 for DOP and 2.4 ngL-1 for DBP 4 days after closing. Organic solvents such as xylene and toluene and organic siloxane were detected with GCMS. Visible injury was observed in lettuce plants cultivated in the chambers and it seemed to result from trace contaminants such as DOP, DBP, organic solvents, dimethylsiloxane polymer, and ethylene. In order to obtain basic data of ethylene evolution from plants, ethylene concentration in a closed chamber in which the plants were cultivated under a controlled environment (25 degrees C air temperature, 60-70% relative humidity, 250-300 micromoles m-2 s-1 photosynthetic photon flux density (PPFD)) was measured. Lettuce (Lactuca sativa L. cv. Okayama) released ethylene more than Brassica rapa var. pervidis, Brassica campestris var. communis, and Brassica campestris var. narinosa. Ethylene release rate of intact lettuce plant was highly correlated with plant growth parameters such as dry weight, leaf area and photosynthetic rate. Ethylene release rates of intact lettuce plant were affected by cultivation conditions such as ambient CO2 concentration, light intensity and light/dark period. Increase in ambient ethylene level influenced lettuce growth even at the concentration of 0.1 microliter L-1. The level of ethylene inhibited leaf expansion and slightly accelerated chlorophyll degradation. It did not affect photosynthesis and transpiration, and also little affected dry matter accumulation. Thus, ethylene release characteristics were clarified and an effect of ethylene on lettuce growth was revealed. These findings are useful for determination of a threshold level of ethylene and a capacity of ethylene removal system in CELSS. On the other hand, a possibility of plant growth diagnosis by measuring ethylene concentrations was evaluated. As a result, it became clear that the measurement of ethylene concentration in CELSS is one of the useful non-destructive measurement methods for plant growth diagnosis. Further research is needed to investigate the applicability of the method to environmental stresses other than Ni and Co in nutrient solution.