Catalyst: Plants and increased levels of carbon dioxide

DR GRAHAM PHILLIPS:

The next big food issue could be how rising levels of carbon dioxide affect our fruit and vegies. Now, we know that plants love CO2, so rising levels of it will affect their metabolisms. And it seems almost certain that for many foods the levels of nutrition will go down and, for some, toxin levels will go up — both serious issues when trying to feed a world with increasing population.

DR ROS GLEADOW:

We're tracking worst-case scenario with carbon dioxide at the moment. And we need to predict what sort of things are going to happen in the future.

DR GRAHAM PHILLIPS:

Carbon dioxide is crucial for plant growth. Plants breathe in CO2 and convert the gas into usable energy.

DR ROS GLEADOW:

Plants use carbon dioxide. It's a carbon catcher and storage system, so you think if there's more carbon dioxide they would just capture and store more. But, in fact, that doesn't happen. Plants do grow faster at elevated carbon dioxide, usually, but not as fast as you'd expect.

DR GRAHAM PHILLIPS:

Ros Gleadow has been studying the effects of raised C02 on plant growth.

DR ROS GLEADOW:

Leaves of plants grown at elevated carbon dioxide have a lot less protein in them. Wheat, barley, rice, all of those in probably only 50 to 60 years' time will have 15% to 20% less protein in them than they do now.

DR GRAHAM PHILLIPS:

So why would a plant have less protein in a raised-carbon dioxide environment? In the process of photosynthesis, plants carbon dioxide into sugars, using a protein called Rubisco to capture the CO2 from the atmosphere. If carbon dioxide levels increase, plants will need to produce less Rubisco to capture the same amount of carbon dioxide. Less of this protein means a less nutritious meal. And the plant's extra resources can then be diverted into protecting the plant. Plants protect themselves using spines and thorns or make themselves less appetising by producing toxic chemicals. Eucalypts produce phenols.

DR ROS GLEADOW:

In about 50 years' time or even 100 years' time eucalyptus leaves will have trouble supporting arboreal herbivores, like koalas, because the phenolic concentration will be too high and the protein level too low.

DR GRAHAM PHILLIPS:

Phenols are one way that plants protect themselves from animal attack, but there is another way — cyanide.

PROFESSOR HOWARD BRADBURY:

If an insect comes and eats the leaf, then it immediately gets a nasty taste of bitter hydrogen cyanide, so it goes somewhere else. So this is a really great mechanism for protecting the plant. And there's about 2,000 plants use this mechanism, including apples, apricots, peaches.