Water is the resource that most often limits a plant’s ability to grow and function. We see this easily when we forget to water our houseplants or garden. And when you drive across a continent, you see that wet places look very different from dry ones. Water has such a significant effect on the growth and functioning of plants because plants use large amounts of water. Why do plants require so much water?

The answer is not water’s role as an electron donor in photosynthesis (Chapter 8): That process accounts for less than 1% of the water required by vascular plants. Instead, most of a plant’s need for water arises as a consequence of CO₂ uptake from the air.

As we saw in Chapter 25, CO₂ is a minor constituent of air, at present about 400 parts per million. This low concentration limits the rate at which CO₂ can diffuse into the leaf. Therefore, how fast a plant can take up CO₂ depends in large part on the degree to which leaves can expose their photosynthetic cells to the surrounding air. If we look inside a leaf, we see that each mesophyll cell is surrounded by air spaces. The mesophyll cells obtain the CO₂ that they need for photosynthesis directly from these air spaces. If the air spaces within the leaf were completely sealed off, photosynthesis would quickly run out of CO₂ and come to a halt. But they’re not sealed off—the leaf’s air spaces are connected to the air surrounding the leaf by pores in the epidermis. As photosynthesis lowers the concentration of CO₂ molecules within the leaf’s air spaces relative to the concentration of CO₂ in the outside air, the difference in concentration between the inside and the outside of the leaf causes CO₂ to diffuse into the leaf, replenishing the supply of CO₂ for photosynthesis.

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The ability of leaves to draw in CO₂ comes at a price: If CO₂ can diffuse into the leaf, water vapor can diffuse out (Fig. 29.4). Furthermore, water vapor diffuses out of a leaf at a much faster rate than CO₂ diffuses inward. Molecules diffuse from regions of higher concentration to regions of lower concentration, and the rate of diffusion is proportional to the difference in concentration. On a sunny summer day, the difference in water vapor concentration between the air spaces within a leaf and the air outside can be more than 100 times greater than the difference in concentration of CO₂. Add the fact that water is lighter than CO₂, and so diffuses 1.6 times faster for the same concentration gradient, and it becomes clear why, on a sunny summer day, several hundred water molecules are lost for every molecule of CO₂ acquired for photosynthesis.

The loss of water vapor from leaves is referred to as transpiration, and the rates at which plants transpire are often quite high. A sunflower leaf, for example, can transpire an amount equal to its total water content in as little as 20 minutes. To put this rate of water loss in perspective, you would have to drink about 2 liters per minute to survive a similar rate of loss. Vascular plants can sustain such high rates of water loss because they can access the only consistently available source of water on land: the soil. Water moves from the soil, through the bodies of vascular plants, and then, as water vapor, into the atmosphere. Therefore, the challenge of keeping photosynthetic cells hydrated is met partly by the continual supply of water from the soil. As we discuss next, it is also met by limiting the rate at which water already in leaf tissues is lost to the atmosphere.

Quick Check 1 Why do plants transpire?