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

Summary

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.