TECHNIQUES OF CO₂-EXPERIMENTATION

For the CO₂ working group we prepared a bibliography of nearly all CO₂ publications since 1960. It comprises about 170 fairly extensive reports on reasonably wall conducted trials with elevated CO₂ supplies. In 95% of these trials propane, kerosene and solid or liquid CO₂ were applied in compartmented or separate greenhouses of unknown uniformity. There are serious limitations to this kind of experimentation. Of course, these experiments are very valuable in demonstrating what simple CO₂ enrichment can do. But they do not show all the possibilities which additional CO₂ supply could have from the plant physiological point of view.

There is inadequate information on the quantitative CO₂ response of plants, to say nothing of the interactions between CO₂ effects and those of light, temperature, mineral nutrition etc. The CO₂ effect varies with the air temperature applied. Findings of improved CO₂ effects by elevating the temperature already have been taken as a proof for a higher optimum temperature when CO₂ enrichment is done. In truth corresponding temperature effects are found under atmospheric concentrations of CO₂. But by raising the CO₂ concentration the temperature sensitivity of the plants is increased.

Concerning most of the plant physiological aspects of CO₂ efficiency one is still thrown upon speculations based on photosynthesis measurements. The results apply to detached or single leaves. For entire plants or crops no measurements have yet been published. Now, it is obvious that a tomato plant 2 m tall will react differently from one of its leaves since each leaf has a particular physiological state and a particular micro-environment. Furthermore the vast majority of results on the effect of elevated CO₂ concentrations on the CO₂ uptake of plants only apply for rather short periods. From others one knows that the CO₂ effect may sharply decrease in course of the day. So in all we need photosynthesis measurements of the total daily CO₂ uptake and nocturnal release and the degree of saturation and/or fatigue after several days of CO₂ enrichment for plants with tissues already grown under the CO₂ concentrations to be tested. Perhaps the N.I.A.E. cuvette-greenhouse experimentation will provide the answer for ecological conditions.

Appropriate assimilation experiments could be extremely helpful but the best measurements of the gas exchange cannot explore into which physiological "sinks" the additional assimilates, resulting from an increased CO₂ supply, are transported. These productivity increases may be worthless if they promote tissue ageing, or induce developmental changes counteracting a further utilization of additional CO₂ supplies. These developmental aspects of CO₂ action - and particularly its range of action - have not been investigated systematically. Obviously there are 2 action components having different optima:

1. Variation in the production of plant matter.
2. Variation of the time required for each developmental stage and consequently changes of the proportion of plant parts.

These and other considerations led us to construct an experimental set-up to meet following requirements: