Rising CO2 promotes weedy rice

There has been a lot of research, recently, showing how global change — especially warming — can alter the habitat and preferred range of marine and terrestrial species. But rising levels of greenhouse gases can also, directly, do a number on agricultural ecosystems, a new study shows. At least for U.S.-grown rice, rising carbon dioxide levels give a preferential reproductive advantage to the weedy natural form — known colloquially as red rice (for the color of its seed coat).

Agriculture Department scientists raised rice in controlled lab environments. They grew some in CO₂ levels reflecting atmospheric concentrations from a century ago — through to what’s expected to exist in coming decades. And with each stepwise rise in CO₂, the weedy rice increasingly hybridized with the crop plants, reinserting wild genes that breeders had spent great effort to remove or modify.

The result was a diminishing of the value and quality of the cultivated rice — essentially transforming it into a weed, explains Lewis Ziska of the USDA Agricultural Research Service in Beltsville, Md. “That’s sort of the science fiction aspect of this,” he says. Likening it to Invasion of the Body Snatchers, “Whatever [seed] the good plant produces is now going to be bad seed.”

In theory, this flow of genetic traits could go in the opposite direction as well — moving genes from crop plants to the weedy rice. But in the new trials that didn’t seem to happen. It was a one-way gene transfer from the weeds to the crop plants, Ziska and his colleagues report May 23 in PLoS ONE.

And because some half of cultivated rice grown in the United States has been bred to resist the effects of the most widely used weed killer, any hybrid offspring — the bad seed — can now carry this trait as well.

Ziska’s group grew conventional rice crops in: 300 parts per million CO₂, a value representative of about 1900; 400 ppm CO₂, a value close to what exists in the lower atmosphere today; and 600 ppm CO₂ — a concentration that could develop within the current century. In each test plot, the researchers included one feral rice plant for every seven crop plants. This ratio typifies what’s found in southern U.S. rice fields today. And in the two higher CO₂ environments, the weedy rice outperformed the crop rice.

For instance, the weedy rice began flowering earlier. Now its pollen production was in sync with crop plants (where previously, most feral rice had flowered too late to pollinate cultivated rice). In addition, the red rice grew taller stems and more flowers — each conferring additional reproductive advantages, since pollen production increased and its release from unusually tall stems allowed it to travel beyond the plant that produced it (most rice tends to be self-pollinating).

The ability of the feral rice to successfully cross with the crop plants tripled between the lowest and highest CO₂ environments, the plant physiologists showed. The new seeds tended to be more fragile (with the hulls cracking easily — creating a grain that commands less money in the marketplace) and to have a diminished nutrient content. These impacts have likely been developing in rice fields over the past half-century, unbeknownst to farmers, Ziska says, as CO₂ concentrations have been climbing.

In addition, hybrids in the new tests retained the crop plant’s genetic immunity to a weed killer. Indeed, Ziska says, the latter feature may partially explain the diminishing value of herbicide treatments on rice in recent years.

Last year, Carol Mallory-Smith and Elena Sanchez Olguin of Oregon State University in Corvallis reported that resistance to the widely used weedy-rice-killing herbicide imidazolinone was being witnessed in commercial U.S. fields. Moreover, the Oregon State team noted, the lifetime of dormant red-rice seed in soil far exceeds that of cultivated rice, allowing the resistant hybrids to easily survive to infest the next season’s crop.

“But we’re going to try to make lemonade out of lemons,” Ziska vows. If the weedy red rice outperforms currently cultivated rice in high-CO₂ environments, then maybe it’s time to find out why, and to breed those traits into cultivated rice lines. He’s currently engaged in a project that aims to do just that. “And just maybe, 10 years down the road, we can offer farmers this new rice line.”