Groups of crops, like groups of humans, might accomplish more if each crop specialized in what it does best. C4 photosynthesis needs less leaf nitrogen than C3, but C4 maize still needs lots of nitrogen to make seed proteins. A low-seed-protein maize would need little nitrogen fertilizer — the 1% of seed that’s planted could get a nitrogen-rich coating — but where would we get our protein? Nitrogen-fixing legumes could supply all our protein, if they didn’t waste their limited (C3-pathway) photosynthate making oil or starch. So breed low-protein maize for high starch or oil yields without nitrogen fertilizer, while breeding low-oil soybeans that yield lots of protein using nitrogen from symbiosis.
(This is my first entry to the National Academy of Sciences agricultural “Science Breakthroughs 2030” challenge.)