Ecological significance of C4 photosynthesis: A comparison of C3 and C4 subspecies of Alloteropsis semialata and other NADP-ME Panicoid grasses

The photosynthetic efficiency of C3 grasses declines with increasing temperature and C4 grasses are predicted to be advantaged at temperatures above 15 °C. We demonstrated an above-ground productivity difference in a commongarden experiment with Alloteropsis semialata and show that it is related more to differences in life-history than to photosynthesis. Frost caused leaf mortality in the C4 but not C3 subspecies, which maintained photosynthetic leaf area throughout the winter. As a consequence the C3 plants were detrimentally affected by fire while the C4's were not. C4's retained a greater proportion of biomass belowground and through reallocation were able to re-grow leaf area faster than the C3's. Differences in growth and allocation may be related to the greater photosynthetic productivity and nitrogen use efficiency of C4 plants. Drought experiments on Alloteropsis and other Panicoid grasses showed differential effects on C3 and C4 productivity, removing the C4 photosynthetic advantage due to greater metabolic limitations of photosynthesis and prolonged recovery on re-watering. This drought response may explain the paradox of declining NADP-ME species numbers with decreased rainfall, despite the apparent water use efficiency advantage of C4 photosynthesis.