Balancing Photosynthesis

Balancing Photosynthesis



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Balancing photosynthesis and transpiration rates  

Plants face a dilemma when it comes to water relations. For plants to perform photosynthesis, they must keep their stomata open so they can take in carbon dioxide. But leaves must also retain a certain amount of water internally, to maintain turgor pressure and to provide water for photosynthesis. The result is that, on a sunny day when the plant is photosynthesizing like crazy, the plant is losing lots and lots of water, as water vapor is released through the open stomata. The plant can close its stomata to conserve water—but then photosynthesis would stop for lack of carbon dioxide. (This is what happens when temperatures rise above about 85 degrees F for many plants. Some desert plants have special adaptations to continue photosynthesizing in hot, dry weather.)

Rates of transpiration and water loss vary depending on the temperature of the air, humidity, wind, and the amount of leaf surface area. On a hot, dry, sunny day with a warm breeze, plants with large leaves lose a tremendous amount of water. On a cool, cloudy, humid day, plants transpire far less. You can get a sense of your plants’ transpiration rates by noting how you feel outdoors. On hot, sunny days you know you need to drink lots of fluids. Add to those conditions very low humidity and a steady wind (which increases surface evaporation) and you’d better drink even more!

Now you know why plants need so much water, as well as some of the environmental conditions that affect transpiration rates and therefore water requirements. So don’t get too irritated with your horticultural advisors, who tell you to keep your plant’s soil "evenly moist," rather than giving you specific amounts of water to apply. Who can predict your plant’s rate of transpiration!


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Hand lotion...for shrubs?
It’s pretty easy to imagine our plants "perspiring" on a hot, sunny day—we can relate! But some plants suffer on cold, windy days too. Rhododendrons and arborvitae are two examples. Evergreens don’t go fully dormant in the winter—they need a degree of metabolic activity to maintain their leaves or needles. When the ground is frozen, however, these plants can’t take up the water they need, and "winter burn" shows up as browned and withered foliage. Be sure plants are kept well watered in the fall, so that when the soil surface freezes, they still have a reserve of water deeper down upon which to draw. Some people have found commercially available antitranspirants helpful—these are sprayed onto foliage to partially seal it, reducing transpiration and therefore water loss. Best of all, know your plants' needs, and choose a site sheltered from the wind for susceptible plants.

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Leave it to leaves   Compare the leaf surface area in plants from different environments. Rainforest plants often have huge leaf areas. Though the air is hot, high humidity and lots of shade (in the lower canopy at least) limit the amount of water lost through evaporation. Grasses in the open plains, exposed to bright sunshine and drying winds, conserve water with narrow, strap-like leaves. Desert conditions are so severe that plants there must use other techniques to conserve water—such as heavy, waxy coatings and thick, fleshy leaves. Observe your garden plants, and think about their adaptations. You might even make some guesses about your ornamental and vegetable plants’ native habitats.


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