Professor Peter Horton FRS called my attention to his blog post discussing some opportunities to improve crop photosynthesis and yield, in ways that were missed by past natural selection. He notes that:
"linking photosynthetic activity at the leaf level (the pre-occupation of the plant scientist) to crop yield per unit land area (the concern of the farmer) has proven very difficult."Sometimes, there are tradeoffs between plant traits that maximize photosynthesis per unit leaf area and those that maximize photosynthesis by the community of crop plants. Thicker leaves (with more nitrogen per square cm) have higher photosynthesis rates per square cm. But, if a young plant doesn't yet have enough resources to completely shade the ground with thick leaves, making thick leaves means making too few leaves to intercept all the available sunlight. Photons that reach the soil without hitting a leaf don't contribute to photosynthesis. This tradeoff is among those discussed in a paper by Condon et al., (2004), cited and discussed in my book.
Professor Horton also suggests that:
"Put simply, stability and survival (a low risk strategy) in the natural environment are driving forces of evolution, not necessarily high growth rate and photosynthetic rate (a high risk strategy) or high grain yield."
Our major grain crops are all annuals, which produce seed and then die. But "survival" could mean "producing at least some seed that survive and grow in subsequent years." I agree that a plant strategy that almost always ensures that there will be some surviving offspring is probably not the plant strategy that maximizes good-year grain production.
What strategies work best for farmers -- and for society as a whole? A subsistence farmer might accept lower average yields, rather than risk occasional crop failures. A farmer in a market economy, though, might bank enough money most years to survive the occasional crop failure. This could be true even if the overall result of many farmers making that decision turns out to be greater risk of regional, or even global, food shortages.
But back to plants. Professor Horton closes with the suggestion that increased understanding of how optimization by natural selection may conflict with our goals for agriculture:
"may also be necessary to offset the inherent conservatism of plants that could thwart current attempts to increase photosynthetic efficiency, and hence yield"