i've seen crop yields increased by large factors in greenhouses. according to the plant breeder i pestered, plant growth is limited by (roughly in order): water, nitrogen, phosphorous, micronutrients, and suitable temperature. too much or too little sunlight is also a problem in some climates. predation by insects, competition by weeds, and infection by viruses and fungi also knock down yields.

under sterile, controlled conditions, well, the plants are much happier. i imagine if we were to breed or genengineer plants that didn't even bother to try to fight disease or worry about weeds, we might be able to eek out further yield increases.