Biomass production of C3- and C4-plant species in pure and mixed culture with different water supply.

Pure and mixed cultures of the dicotyledons Atriplex hortensis L. (C3 plant) and Amaranthus retroflexus L (C4) on the one hand and of the grasses Avena sativa L (C3) and Panicum miliaceum L. (C4) on the other hand were maintained in a standard soil with different ground water tables. After 12 weeks the length, dry weight and nitrogen-content of the aboveground and belowground parts of the plants, and in addition the carbon-and ash-content and the δ13C value of the aboveground parts were determined. It turned out that the length and the dry weight of the shoots of the C3 species showed on increasing tendency with increased water supply, while the values of the C4 species were drastically diminished at the highest water level only. The roots showed in most cases an increased length and dry weight at drier conditions, more pronounced in the C4 than in the C3 species. The nitrogen content of the shoots was mostly higher in the shoots of the C3 plants and in the roots of the C4 plants; it changed in a non-regular manner with variations in water supply. Since the carbon content did not change markedly, the C/N ratio was variable. There was a slight tendency for a higher carbon content and mostly also for a higher C/N-ratio in the shoots of C4 plants. The δ13C values of both C3 as C4 plants were in general not at all influenced by the water supply; they were fixed genetically. The ash content of the analyzed species did not show a clear relationship to the type of photosynthetic CO2-fixation or to the water regime.The influence of light intensity was studied with mixed cultures of all four plant species, again with different water supply. There was a strong effect of light intensity on the competitive behaviour of the C3 and C4 plants under modified water conditions. The wild C3 plant Atriplex hortensis was most successful under conditions of relatively low light intensity and high water availability, while the cultivated 'artificial' species Avena sativa showed much less differences between full-light grown and shadow plants. The C4 plant Amaranthus retroflexus is most successful under competitive conditions at high water stress in full light. The C4 grass Panicum miliaceum showed maximum shoot growth in light, but was successful under competitive conditions especially also with good water supply. The light intensity had no effect on the δ13C values. - There was no indication that the soil-type as such has a distinct influence on the success of C3 or C4 plants in mixed cultures.