More Castor Oil, Please

By Robert C. Balling Jr., Ph.D.
Arizona State University

As children, generations of us associated foul-tasting castor oil with certain death. We weren't entirely wrong: Castor bean plants and seeds contain ricin, which is extremely poisonous to people, animals, and insects. Used throughout human history in suicides and assassinations, it can kill a grown man (or woman) in doses as small as 1 mg. This is one houseplant for the Addams Family.

Extract the plant oil, however, and you do indeed have an arguably therapeutic elixir. Process the beans to eliminate the ricin, and you have a livestock feed. And even the ricin itself is now being used as a "natural" pesticide to kill aphids, the European corn borer, and the Southern corn rootworm. Add a little carbon dioxide, and you'll have even more plant material to work with.

This Jekyll-and-Hyde plant is the focus of recent research by German plant physiologists Grimmer and Komor. They begin their resulting article in the international journal Planta with a sentence straight out of Greening Up: "Plants grown under elevated atmospheric CO₂ concentrations show a higher relative growth rate, larger biomass and faster development."

The team grew castor bean plants for five to seven weeks in climate-controlled chambers with atmospheric CO₂ levels held constant at the natural, or ambient, level of 350 parts per million (ppm) and at 700 ppm. They found that the rate of net carbon assimilation (i.e., photosynthetic CO₂ intake minus CO₂ "exhalation") increased by 39 percent when CO₂ levels were doubled. This carbon assimilation basically translates into more plant material; indeed, the scientists found a 76 percent to 142 percent increase in dry matter in leaves. At all ages, the elevated CO₂ increased the net synthesis of carbohydrates (i.e., sucrose and starch). No measurements are available for the beans themselves, but given the increases in other plant components (along with a great many other experiments on beans), it is likely that the beans, too, will benefit from more CO₂.

We like to think of the humble bean as Greening Up's poster child. In part because beans are easy to grow, they have become a staple not only of the world's diet, but also of agricultural studies examining CO₂'s effects. As I was writing this column, I discovered yet another new article showing that beans benefit enormously from elevated CO₂.

CO₂ is the key to a greener planet. Nonetheless, though Mother Earth may be smiling about the CO₂-enhanced benefits to castor bean plants, children everywhere may find the news tough to swallow.

References:

Grimmer, C., and Komor, E., 1999, Assimilate export by leaves of Ricinus communis L. growing under normal and elevated carbon dioxide concentrations: The same rate during the day, a different rate at night. Planta, 209, 275–281.