Science has made huge progress in improving the productivity of plants. Through breeding to increase the edible parts of crops, planting in new arrangements, and by developing new fertilizers and pesticides, scientists have helped feed ever more people. The Green Revolution in the 1950s and ’60s doubled and quadrupled yields of wheat, rice, and other crops around the world, and we’ve seen productivity increases ever since.
The question now is how science continues to make improvements as the global population grows, and demand for food keeps rising. We will need 70% more calories globally by midcentury, according to U.N. estimates.
Stephen Long, a plant scientist at the University of Illinois, says techniques developed during the Green Revolution are hitting their limits. For example, you can’t keep increasing the edible portion of individual plants “because you’ve still got to have some stems and leaves and structure to hold the grain,” he says. “Sixty to sixty-five percent is about as far as you’re going to get with that strategy.” (Wheat looks dramatically different today, with bigger ears.)
Yield increases–the amount of a plant you can grow in one space–are now flattening out, too. A recent analysis from University of Nebraska–Lincoln found that 30% of major cereal crops, like corn, wheat, and rice, have reached their “maximum possible yields.” And we know that continued use of heavy “inputs” like nitrogen fertilizer (a key part of the Green Revolution) are harmful to soil quality and often injurious to the wider environment.
“If we look at current rates of yield improvement globally, we’re going to come nowhere near that [required 70%] percentage increase. So we really need new ideas and breakthroughs,” Long says.
Long and others are looking to the fundamental molecular machinery of plants, particularly the way plants convert sunlight and carbon dioxide during photosynthesis. Long thinks this process is the best opportunity left to improve plant sizes and thus raise crop productivity. If you can’t increase the relative size of what’s edible in a plant, he says, you need to work on its absolute size, and that means getting into the guts of how plants grow.
It may seem unfair to plants to criticize the way plants do photosynthesis, because it’s really pretty amazing to be able to convert sunlight into food at all. But, viewed dispassionately, you can see that plants aren’t particularly efficient. “If you really look at the process in terms of energy efficiency, in the same way you do with photovoltaic cells, there’s a lot of room for improvement,” says Bob Blankenship, a professor at Washington University in St. Louis. “One percent of solar energy is converted into biomass by plants, compared to 10% or 15% converted by photovoltaics.”
In a recent paper in the journal PNAS, photosynthesis researchers laid out some ways we might improve the efficiency of the process. Some of the ideas are summarized below.
Plants absorb a relatively small part of the light spectrum. They’re like humans: They mostly see the visible bit. “There’s a lot of energy in that near-infrared region, but plants nowadays get nothing out of that part of the solar spectrum,” says Blankenship. One idea he’s working on is to modify the chlorophylls that plants use to pick up sunlight, for example by swapping in chlorophyll d, which absorbs more of the spectrum, for chlorophyll a, which takes in less. In theory, plants could be engineered so that they have more chlorophyll d in their lower areas where they typically receive less light.
Plants are also inefficient in that they don’t absorb all the visible light available to them. “When the sun comes up, plants work fairly efficiently, and then when it gets to 10 o’clock or so in the morning, they saturate,” says Blankenship. “They’re not able to use all that energy that you have in the main part of the day. Most of that energy ends up getting wasted.”
Another line of research: to engineer plants so light energy is distributed more evenly within the canopy. Because the top part of plants isn’t able to absorb all the light available, the idea is to limit the capacity of “antennae” chlorophylls in those areas, so they absorb less light and more of it passes through. The scientists do that by disrupting proteins that bind pigments in certain places. That should make plants more efficient as overall systems, though Blankenship, who’s not involved in this research, is a little skeptical. “In principle, it should work, but I think it’s turned out more complicated than it was supposed to be. The results have been suggestive rather than definitive that this is going to work,” he says.
A further possibility is to optimize the architecture of plant canopies to take in maximum light. “Everything from the color of the leaves, the angles of the leaves, to the amount of leaf you have–that all affects photosynthesis,” says Long. The ideal plant has lighter leaves at the top, sloping downward to allow more light to pass below. Then, at the bottom, it has darker, more horizontal leaves.
You would imagine that millions of years of evolution would have given us just that. But Long says there are good evolutionary reasons why plants aren’t designed that way. “If you have horizontal leaves at the top, it means the leaves or plants below aren’t growing as well, but it also means they’re not taking away your water and nutrients,” he says. Evolution is a competitive process, not necessarily one to maximize photosynthetic activity and therefore grow more food.
Long is modeling the entire photosynthetic system using computers at the National Center for Supercomputing Applications on the Urbana-Champlain campus. He breaks down the process to hundreds of equations, and then, using an “evolutionary algorithm” that selects at random, he sees which molecular combinations might produce the best results. It’s a complicated mathematical task: There are more than 100 proteins at play in photosynthesis. “There are hundreds of permutations of how you could look at the efficiency. We simulate each of those steps in the computer. You can see the effect of moving resources in here and taking it away from everywhere else,” he says.
With a five-year, $25 million grant from the Gates Foundation, Long is now testing some of the same best candidates indicated by the computer. These include reengineered rice and cassava plants, which have already grown 20% larger in the lab. Transposing certain “chloroplasts” from blue-green algae has shown potential to increase photosynthesis by 60%. Long is also trying to improve a key enzyme, RuBisCO, which catalyzes carbon fixation. RuBisCO makes up a quarter of plant protein, but, unfortunately, is slow in catalyzing reactions in photosynthesis.
Meanwhile, other researchers are trying to develop wholly new metabolic pathways for fixing carbon into sugars, using the building blocks from various organisms. And there’s the multinational C4 Rice Project, which engineers more efficient C4 photosynthesis into a C3 photosynthetic plant (rice).
Of course, there are other ways we could get more food so we’re not starving in the future. There is a still a big “yield gap,” for example, between the world’s best-performing and worst-performing farms that technology transfer could help bridge. Closing just half the gap could feed 850 million people, one analysis showed. We can use fertilizers more judiciously: At the moment, half of all nitrogen and phosphorous use is effectively pointless, as it goes beyond the limits of plants to absorb and make use of. And, of course, we waste an enormous amount of food, particularly in the developing world supply chain. But, in the long term, the work of Stephen Long and other photosynthetic designers might one day provide the sort of jump in agricultural productivity we’ll need.