Elevated Atmosphere Partial Pressure of CO₂ and Plant Growth. III. Interactions Between Triticum aestivum (C₃) and Echinochloa frumentacea (C₄) During Growth in Mixed Culture Under Different CO₂, N Nutrition and Irradiance Treatments, With Emphasis on Below-Ground Responses Estimated Using the δ¹³C Value of Root Biomass

Abstract

Wheat (Triticum aestivum L.), a C₃ species, and Japanese millet (Echinochloa frumentacea Link), a C₄ species, were grown in pots in monoculture and mixed culture (2 C₃ : 1 C₄ and 1 C₃:2 C₄) at two ambient partial pressures of CO₂ (320 and 640 μbar), two photosynthetic photon flux densities (PPFDs) (daily maximum 2000 and 500 µmol m^-2 s^-1) and two levels of nitrogen nutrition (12 mM and 2 mM NO₃^-). Growth of shoots of both components in mixed culture was measured by physical separation, and the proportions of root biomass due to each component were calculated from δ¹³C value of total root biomass. In air (320 μbar CO₂) at high PPFD and with high root zone-N, the shoot biomass of C₃ and C₄ components at the first harvest (28 days) was in proportion to the sowing ratio. However, by the second harvest (36 days) the C₄ component predominated in both mixtures. Under the same conditions, but with low PPFD, C₃ plants predominated at the first harvest but C₄ plants had over- taken them by the time of the second harvest.

Elevated atmospheric CO₂ (640 μbar) stimulated shoot growth of Triticum in 15 of 16 treatment combinations and the stimulation was greatest in plants provided with low NO₃^-. Root growth of the C₃ plants was generally stimulated by elevated CO₂, but was only occasionally sensitive to the presence of C₄ plants in mixed culture. However, growth of the C₄ plants was often sensitive to the presence of C₃ plants in mixed culture. In mixed cultures, elevated CO₂ plants stimulated growth of C₄ plants at high PPFD, high-N and in all low-N treatments but this was insufficient to offset a marked decline in shoot growth with increasing proportion of C₃ plants in mixed cultures. The unexpected stimulation of growth of C₄ plants by elevated CO₂ was correlated with more negative δ¹³C values of C₄ root biomass, suggesting a partial failure of the CO₂ concentrating mechanism of C₄ photosynthesis in Echinochloa under low-N. These experiments show that for these species nitrogen was more important than light or elevated pCO₂ in determining the extent of competitive interactions in mixed culture.