Rising carbon-dioxide levels are slightly helping crops compete against weeds.
Two rival designs of plant biochemistry compete to dominate the globe. One, called C3 after the number of carbon atoms in the initial sugars it makes, is old, but still dominant. Rice is a C3 plant. The other, called C4, is newer in evolutionary history, and now has about 21% of the photosynthesis “market.” Corn is a C4 plant. In hot weather, the C3 mechanism becomes inefficient at grabbing carbon dioxide from the air, but in cool weather C4 stops working altogether. So at first glance it seems as if global warming should benefit C4.
Certainly, in bright sunlight and warm temperatures, C4 plants grow faster than C3 ones and need less nitrogen fertilizer. Under these conditions, a C4 crop like corn or sugar cane can achieve higher yields and tolerate drought better than a C3 crop like wheat or rice. Of the 86 plant species that supply most of the world’s food, only a handful are C4, but they dominate tropical agriculture: The chief ones are corn, sugar cane, millet and sorghum.
But it is not quite that simple. Surprisingly, the C4 strategy first became common in the repeated ice ages that began about four million years ago. This was because the ice ages were a very dry time in the tropics and carbon-dioxide levels were very low—about half today’s levels. C4 plants are better at scavenging carbon dioxide (the source of carbon for sugars) from the air and waste much less water doing so. In each glacial cold spell, forests gave way to seasonal grasslands on a huge scale. Only about 4% of plant species use C4, but nearly half of all grasses do, and grasses are among the newest kids on the ecological block.
So whereas rising temperatures benefit C4, rising carbon-dioxide levels do not. In fact, C3 plants get a greater boost from high carbon dioxide levels than C4. Nearly 500 separate experiments confirm that if carbon-dioxide levels roughly double from preindustrial levels, rice and wheat yields will be on average 36% and 33% higher, while corn yields will increase by only 24%.
Another complication is that C4 has a larger share of the market in weeds. Of the 18 most pestilential weeds that trouble farmers, 14 are C4. So, all else being equal, and especially in temperate regions where C3 crops dominate, the battle against weeds should get easier as carbon dioxide levels rise—because C3 crops can accelerate their growth more than C4 weeds can.
Last year, Qing Zeng of the Institute of Soil Science in Nanjing and his colleagues published the first test of this prediction on a real farm. By emitting carbon dioxide over plots of rice, they enriched the air to almost twice the ambient level of CO2. They then measured the growth rate of both rice and its worst weed, barnyard grass (a C4 plant), in the experimental plots, compared with control plots nearby.
The ear weight of the rice was enhanced by 37.6% while the growth of the barnyard grass was actually reduced by 47.9%, because the vigorous rice shaded out the weeds. So the good news is that rising carbon-dioxide levels are, on balance, slightly helping crops (mostly C3) compete against weeds (mostly C4) rather than vice versa.
Still, that enormous yield advantage of C4 plants in hot weather suggests an obvious next goal for plant breeders.
Given that most rice grows in hot countries, fiddling with its genes to make it into a C4 plant could boost its yield by 50% and cut its nitrogen needs, transforming world food supply. This is the goal of the C4 Rice Project at the International Rice Research Institute in the Philippines. It takes heart from the fact that C4 “technology” has emerged naturally in many different lines of plants, so why not put it in rice, too?