J. R. Porter and B. Wollenweber claim in their Correspondence (Nature 463, 876; 2010) that improving the kinetics of the enzyme Rubisco and diminishing photorespiration are unlikely to increase crop productivity. But breeding as well as agronomy is necessary to enhance photosynthesis and meet the urgent global need for improved crop yields.

Enhancing photosynthesis by increasing Rubisco's carboxylation rate boosts biomass and yields, according to evidence from mechanisms such as C4 photosynthesis, which concentrates carbon dioxide, and studies of crop growth in a CO2-enriched atmosphere (E. A. Ainsworth and S. P. Long New Phytol. 165, 351–372; 2005). Improving Rubisco and engineering C4 photosynthesis into C3 crops such as rice are two approaches that we cannot afford to dismiss as gains from current agronomic strategies diminish.

Neither should we overlook the very real yield benefits of the 'Green Revolution'. Introducing dwarfing genes into cereals has enabled farmers to increase yields by using more nitrogen fertilizer without the risk of plants falling over. In wheat, dwarfing genes do not affect potential biomass production but increase allocation to grain (R. B. Austin et al. J. Agric. Sci. 112, 295–301; 1989).