Consequences of C4 photosynthesis for the partitioning of growth:: a test using C3 and C4 subspecies of Alloteropsis semialata under nitrogen-limitation

C-4 plants dominate the world's subtropical grasslands, but investigations of their ecology typically focus on climatic variation, ignoring correlated changes in soil nutrient concentration. The hypothesis that higher photosynthetic nitrogen use efficiency (PNUE) in C-4 than in C-3 species allows greater flexibility in the partitioning of growth, especially under nutrient-deficient conditions, is tested here. Our experiment applied three levels of N supply to the subtropical grass Alloteropsis semialata, a unique model system with C-3 and C-4 subspecies. Photosynthesis was significantly higher for the same investment of leaf N in the C-4 than C-3 subspecies, and was unaffected by N treatments. The C-4 plants produced more biomass than the C-3 plants at high N levels, diverting a greater fraction of growth into inflorescences and corms, but less into roots and leaves. However, N-limitation of biomass production caused size-dependent shifts in the partitioning of growth. Root production was higher in small than large plants, and associated with decreasing leaf biomass in the C-3, and inflorescence production in the C-4 plants. Higher PNUE in the C-4 than C-3 subspecies was therefore linked with greater investment in sexual reproduction and storage, and the avoidance of N-limitations on leaf growth, suggesting advantages of the C-4 pathway in disturbed and infertile ecosystems.