Parental Leaves

by David Sugarman

Parents guide the development of their children, helping them deal with conditions they might encounter. But does anything like that happen in the plant world?

Scientists in England found a nifty way to see if mature leaves influence the development of young ones. They decided to use the number of "breathing holes" on the leaves as an indicator. These microscopic holes, called stomata, are found on both their upper and lower sides. Stomata can open and close, depending on the availability of moisture and carbon dioxide. The number of stomata influences the rate of photosynthesis and how efficiently a plant uses water.

The scientists isolated mature leaves of thale cress, Arabidopsis thaliana (a cousin of cabbage), to expose them to higher concentrations of carbon dioxide than younger leaves. They found that the number of stomata on young leaves was reduced. When carbon dioxide levels were lowered for mature leaves, young leaves produced more stomata. Light had an effect too. When light exposure for mature leaves was limited, young leaves developed fewer stomata.

It makes good sense for mature leaves to influence the number of stomata young leaves develop. When carbon dioxide is plentiful, plants don't want too many holes that let out precious water. But when carbon dioxide is scarce, plants need more stomata in order to grow. And with less light, plants make food at a slower rate, so they need fewer stomata.

This may have a big effect on plant growth and atmospheric carbon dioxide levels, since 40 percent of the carbon dioxide in the air passes through stomata yearly.