Whew, this has been one hot week, not only for us, but for our leafy friends. We can pick up a nice cold glass of water, sweat like crazy, take a dip in the pool -- but what about our plants?

Some plants are just naturally tough, with an innate ability to survive challenging environmental conditions. Heat is the biggest challenge of all. To see how they cope, let's start at the bottom.

Specialized roots

The roots' thickness and how deep they plunge into the soil affect a plant's heat resistance. Specialized or thick roots include rhizomes (found on German bearded iris), tubers (potatoes, dahlias), bulbs (tulips, daffodils), fleshy roots (peony, sedum, balloon flower). These types of roots have the ability to store water for later use.

Roots that go deep are called taproots (some refer to them as carrot roots). A taproot is a chunky, single main root that eventually has smaller side roots and root hairs protruding from it. Because this root grows deep, it is rarely affected by surface heat. Its ability to use rainwater that has percolated down into the subsoil layers makes it drought tolerant.

Plants with taproots include baby's breath, Russian sage, carrots, radishes, oak trees and the perennial favorite, dandelion. When plants with tap or specialized roots wilt, you know the ground is parched.

The leaf

The leaf provides heat resistance in two ways.

First, its surface is covered with a waxy protective layer called the cuticle (the waxy material itself is called cutin), which helps seal moisture inside the leaf. Some leaves have more cutin than others, and those with more tend to withstand heat and drought better.

Take the houseplant schefflera. If you own one, chances are you've shined its leaves. In doing so, you've simply buffed the cutin. Its leaf requires more cutin than a maple leaf because it is native to the South, where it needs more protection from the blazing heat. Fruits such as cucumber, tomato or apple are also naturally covered in cutin to protect their internal moisture.

Secondly, the thickness of the leaf affects moisture. Paper-thin leaves like those of a fern are not designed to withstand heat; hence, they grow in the coolness of shade. Thick, succulent leaves of sedum and cactus are able to store extra water, which gets the plant through heat and drought.

Stomata

Flip the leaf over and you will find the stomata, tiny microscopic holes. They are responsible for gas exchange (carbon dioxide in, oxygen out) and the release of water vapor into the air during respiration. These openings are shaded by the leaf. They are not covered with cutin because they must open and close to absorb and release. If they were on top of the leaf, they would be sealed in cutin or affected by the heat, thus remaining continually open. The leaf would face immediate desiccation.

Last weekend, as I was weeding around our pumpkins, some of the vines got moved a bit, flipping a few leaves upside down. With their bottoms facing the sun, it seemed like only a nanosecond before the whole leaf wilted.

Concern is growing over global warming and the fact that close to 20 percent of the world's food crop is lost annually because of heat-related problems.

University of Florida researchers may have stumbled onto one of nature's secrets: There may be a link between a gene that helps produce the amino acid beta-alanine (known to assist in metabolizing glucose, which provides energy) and increased heat resistance in plants.

Researchers at the university enhanced tobacco plants with this gene and then subjected the plants to intense, nonstop 95-degree heat for a week. The plants not only survived but thrived, compared with their non-enhanced cousins.

According to a March 2006 release, years of research still lie ahead. But the ability to grow a wider variety of crops in high heat zones and increase heat resistance of currently grown crops are goals.

Plants may not be able to sweat or hang out in air-conditioning, but they, too, have ways of coping with this heat.

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