RHIZOSPHERE MICROFLORA OF WINTER-WHEAT PLANTS CULTIVATED UNDER ELEVATED CO2

We studied an effect of elevated atmospheric CO2 on rhizosphere microo rganisms in a hydroponics system where young wheat plants provided the only source of C for microorganisms. Plants were cultivated in minera l solution in sterile silica sand and exposed to control (ambient) and elevated (double) CO2 concentrations for periods of 13, 20, 25 and 34 days. Microbial biomass C (C content in fraction of size 0.3-2.7 mu m ) was not affected by the elevated CO2 concentration during the first 25 days of plant growth and was increased after 34 days of plant growt h. A content of poly-beta-hydroxybutyrate (PHB) reserve compounds (mea sured as derivatized product of 3-hydroxy-butyric acid and -tert-butyl dimethylsilyl-N-methyltrifluoracetamide using GC-MS) was lowered signi ficantly (p<0.001) in the elevated CO2 after 25 and 34 days. It was ac companied with a shift of bacterial distribution towards the nutrition al groups utilising more complex organic material (number of CFUs on m edia with different sources of C and N). A coincidence of several even ts connected with plant and microbial carbon economy (decrease of an a ssimilation rate and relative growth rate of plants, small increase of microbial biomass, PHB decrease and suppression within the bacterial nutritional group requiring the most readily available source of C and energy) was observed in the system under elevated CO2 on the 25th day . A modification of the CC-MS method for the detection of low levels o f PHB compounds in natural samples was developed. We excluded the lipi ds fractionation step and we used EI MS/MS detection of the main fragm ent ions of the derivatized compound. This guarantees that the ion pro files have high signal-to-noise ratio at correct retention time. The d etection limit is then about 30 pg g(-1) of sand or soil. The rhizosph ere microflora responded very sensitively to the short-term changes in C partitioning in plants caused by the elevated CO2.