August heat tough on plants

Pity plants this time of year! In hot weather, we humans can jump into some cool water, sit in front of a fan, or duck into the shade. Our plants, though, are tethered in place no matter what the weather.

And don't think that plants enjoy searing sunlight. High temperatures can damage plant cells, dry out plants, and have them using up energy faster than they're making it.

Plants cool themselves, to some degree, by transpiring water. Transpiration, which is the evaporation of water from leaves, can cool a plant by about five degrees Fahrenheit. More than 90 percent of the water taken up by plants runs right through them, up into the air, exiting through little holes in the leaves called stomates. Carbon dioxide and oxygen, the gases plants need to carry on photosynthesis, also pass in and out through the stomates.

All this is fine provided there is enough water in the soil to keep fueling that transpiration. If not, stomates close, transpiration and photosynthesis stop, and the plant warms.

Even in moist soil, though, those stomates might close this time of year around midday, when the leaves lose water faster than the roots can imbibe it. Fortunately, there are ways around this dilemma of having to keep stomates open for photosynthesis and cooling, but closed so plants don't dry out.

Cacti and other succulent plants open their stomates at night instead of during the day, capturing the carbon dioxide needed for photosynthesis in special molecules. During the day, these special molecules release the carbon dioxide within the plant. The mechanism doesn't do much good as far as keeping a plant cool, but at least these plants can make food.

Another group of plants, called C4 plants, function efficiently at temperatures that have most other plants gasping for air and water. C4 plants capture carbon dioxide in special four-carbon molecules, rather than the three-carbon molecules of other plants. These four-carbon molecules cling so tenaciously to carbon dioxide that C4 plants do not have to keep their stomates open as much as do C3 plants....