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Amazing strategies plants use to keep cool in the heat

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This lettuce is beating the heat with the help of protective shade cloth. (Photo courtesy of Carlyle Stout.)
Daylily foliage turns brown at the tips because the plant is transpiring more water than it’s absorbing. (Photo by Rhonda Nowak)
Adaptation: Globe thistle (Echinops ritro) has adapted to hot temperatures with deeply dissected leaves and modified flower petals. (Photo by Rhonda Nowak)
Balloon flower (Platycodon grandiflorus) angles its vertically aligned leaves away from the sun’s hot rays. (Photo by Rhonda Nowak)

I found myself dreaming of vegetables so shiny and beautiful they could only be Hollywood vegetables, professional vegetables with agents. I stood in my picture-perfect garden with a reed basket over my arm, my hair in long braids with red ribbons on the ends, picking runner beans. One of the kids (like mine, but clean) was standing by, gazing up at me with admiration, marveling at my mastery of Mother Nature.” – Abbi Waxman, “The Garden of Small Beginnings,” 2017

Lilian Girvan, the protagonist in Waxman’s novel, wears herself out by pondering whether to plant a garden in her backyard. She falls asleep, dreaming about an idyllic landscape, while her “reality was a mostly concrete backyard, a tiny lawn huddled at one end, with several muddy Polly Pockets half buried in the flower beds … ”

It isn’t only the planted dolls that has unnerved Lili about making a garden; it’s also the “sea of weeds” and the “menacing squirrel” that Lili imagines “regarded me balefully from the one small tree we had.”

For Lili, the squirrel conjures up Tennyson’s famous line, “Nature, red in tooth and claw” (“In Memoriam A.H.H.,” 1850), in which the poet describes nature as violent and callous in regard to humans. At this point in the story, Lili apparently envisions gardening as a contest of wills between her and the natural world (plus a bunch of Polly Pockets).

During the summer in Southern Oregon, gardening in the heat, and now wildfire smoke, can certainly feel like a battle with nature. The hot temperatures, drought conditions and unhealthy air quality combine to make working outdoors feel not just a little overwhelming. It doesn’t help to know that humans initiated this battle by our actions (burning fossil fuels) and inactions (allowing fire fuels to accumulate in woodlands).

While humans are struggling to confront climate change, the natural world has a much longer history of adaptation and resilience. Last week, I wrote about how pollinators modify their behavior during hot weather, so this week I’m focusing on how plants protect themselves from extreme heat.

Plants produce food for the whole world by photosynthesizing — converting energy from sunlight into chemical energy — within their leaves, and leaf temperature affects the rate of photosynthesis. Leaf temperature can exceed ambient temperature, in the same way that a sidewalk or metal roof becomes hotter than the air temperature by capturing and accumulating heat energy. Optimal leaf temperatures range from about 60 to 100 degrees F.; photosynthesis decreases when leaf temperatures are outside of this range.

Heat-stressed plants respond by closing their stomata (leaf pores), which reduces or stops photosynthesis. Studies have shown that when plants do not photosynthesize for extended periods of time, they are less able to recover their optimal photosynthesizing capabilities. So plants have evolved different mechanisms that keep their leaf temperature stable to protect their ability to continue photosynthesis.

Some plants keep cool by angling their leaves away from the sun or rolling their leaves inward along the midrib. Over time, plants have evolved structural adaptations to cope with the heat, such as more vertical orientation of the leaves, fine hairs on the leaves, or a waxy coating. Leaves become smaller and/or deeply dissected to maximize heat loss.

Plants also sweat, or transpire, during which water is released through the stomata and cools down the leaves by evaporating. During high temperatures, plants wilt when they transpire more water than they absorb. Too much water loss from plant cells reduces turgidity and causes plants to flop over.

To minimize moisture loss from cell membranes, some plants have developed a higher proportion of fatty acids. Other plants produce “heat-shock” proteins that protect essential enzymes and nucleic acids from deteriorating in excessive heat.

All of these responses to extreme heat requires plants to focus their energy away from photosynthesizing and producing flowers and fruit. It’s common for plants to drop their flowers and immature fruit when they are heat stressed. Also, less than optimal photosynthesis results in less than optimal taste and nutrients in food that is produced.

Besides dropping flowers and fruit, there are other ways plants show outward signs of heat stress. Leaf scorch (brown edges), stunted growth and premature bolting are all frequent indications that garden plants have become too hot. Beet roots will develop rings in a bull’s eye pattern, and the outer layers of onion bulbs may turn gray.

Gardeners can protect their plants from heat stress by providing adequate water and protective covering. Shade cloth is particularly important during late afternoons when the heat and wind combine to dry out plants. Keep in mind, though, that exposure to some heat stress encourages plants to rev up their adaptive mechanisms — like the saying goes, “What doesn’t kill you will make you stronger.”

Despite protection, prolonged heat stress is likely to kill off some of our plants. In that case, I suppose we can find solace in another of Tennyson’s most famous lines from the same poem: ‘‘Tis better to have loved and lost, than never to have loved at all.’’

Rhonda Nowak is a Rogue Valley gardener, teacher and writer. Email her at Rnowak39@gmail.com. For more about gardening, check out her podcasts at https://mailtribune.com/podcasts/the-literary-gardener.

My gardening to-do list this week

• Protect vegetables from late afternoon heat/wind with cover cloth.

• Check for signs of heat stress among garden plants; take notes.

• Decrease water to potatoes when tops start dying back; harvest to use fresh or wait until vines are dead from frost to store.

• Look for garlic and onion tops beginning to fall over; withhold water for a week or two before harvesting.

• Spray blossoming pepper plants with a mixture of 4 Tbsp. Epsom salts per one gallon water (spray in the morning) to make them crisper and sweeter.