4.11: The battle against world hunger can use plants adapted to water scarcity.

Sudan. Ethiopia. India. Somalia. Many of the world’s regions with the highest rates of starvation are also places with the hottest, driest climates. This is not a coincidence. These climate conditions present difficult challenges for sustaining agriculture (FIGURE 4-23), and in the absence of stable crop yields, food production is unpredictable and the risk of starvation high. But evolutionary adaptations in some plants enable them to thrive in hot, dry conditions. Recent technological advances in agriculture use these innovative evolutionary solutions to battle the problem of world hunger. In this section, we discuss some adaptations that allow plants to thrive when water is scarce. We also look at how humans use these adaptations to grow food in the dry, inhospitable climates where starvation rates are highest.

When it gets too hot and dry, animals can seek coolness in the shade. Plants, however, are anchored in place and do not have this option. Consequently, plants in hot, dry climates can lose significant amounts of water through evaporation. Evaporation is a problem for plants because water is essential to photosynthesis, growth, and the transport of nutrients. Without water, plants cannot live long.

One method of combating water loss through evaporation is for plants to close their stomata (sing. stoma), small pores usually on the underside of leaves (FIGURE 4-24). These openings are the primary sites for gas exchange in plants: carbon dioxide for photosynthesis enters through these openings, and oxygen generated as a by-product in photosynthesis exits through them. When open, the stomata also allow water to evaporate from the plant. Closing their stomata solves one problem for plants (too much water evaporation) but creates another: with the stomata shut, oxygen from the “photo” reactions of photosynthesis cannot be released from the chloroplasts, and carbon dioxide cannot enter. If there are no carbon dioxide molecules for sugar production, the Calvin cycle tries to fix carbon but instead finds only oxygen. Plant growth comes to a standstill and crops fail.

In some plants, including corn and sugarcane, a process has evolved that minimizes water loss but still enables the plants to make sugar when the weather is hot and dry. In the process called C4 photosynthesis, these plants add an extra set of steps to the usual process of photosynthesis, which is usually called C3 photosynthesis (FIGURE 4-25). In these steps, the plants produce an enzyme that functions like the ultimate “CO₂-sticky tape.” This enzyme has a tremendously strong attraction for carbon dioxide; it can find and bind carbon even when CO₂ concentration is very low. As a consequence, the plant’s stomata can be opened just a tiny bit, and let in just a little CO₂. Reducing the amount of stomata opening reduces evaporation and conserves water for the plant. (In contrast, rubisco, the usual enzyme that plants use to pluck carbon from the atmosphere, functions poorly when CO₂ is scarce, necessitating greater stomata opening.)

This seems like such a good solution that we would expect all plants to use it. There is a catch, though. The extra steps in C4 photosynthesis require the plant to expend additional energy. Specifically, every time the plant generates a molecule of the “CO₂-sticky tape” enzyme, it uses one molecule of ATP. It is acceptable to pay this energy cost only when the climate is so hot and dry that the plant would otherwise have to close its stomata and completely shut down all sugar production. If the climate is mild, however, plants conducting the more energetically expensive C4 photosynthesis would be out-competed by the more efficient plants conducting standard C3 photosynthesis. Not surprisingly, we see few C4 plants in the temperate regions of the world and little C4 photosynthesis among photosynthetic organisms living in the oceans. In hot, dry regions, however, C4 plants are dominant and displace the C3 plants wherever both occur (FIGURE 4-26). With global warming, many scientists expect to see a gradual expansion of the ranges over which C4 plants grow, and believe that non-C4 plants will be pushed farther and farther away from the equator.

A third and similar method of carbon fixation, called CAM (for “crassulacean acid metabolism”), is also found in hot, dry areas. In this method, used by many cacti, pineapples, and other fleshy, juicy plants, the plants close their stomata during hot, dry days. At night, they open the stomata and let CO₂ into the leaves, where it binds temporarily to a holding molecule. During the day, when a carbon source is needed to make sugars in the Calvin cycle, the CO₂ is gradually released from the holding molecule, enabling photosynthesis to proceed while keeping the stomata closed to reduce water loss (see Figure 4-25). A disadvantage of CAM photosynthesis is that by completely closing their stomata during the day, CAM plants significantly reduce the total amount of CO₂ they can take in. As a consequence, they have much slower growth rates and cannot compete well with non-CAM plants under any conditions other than extreme dryness.

C4 and CAM photosynthesis originally evolved because they made it possible for plants to grow better in the world’s hot and dry regions. Researchers are now experimenting with these adaptations as a way to fight world hunger. They have introduced into rice plants several genes from corn that code for the C4 photosynthesis enzymes. Once in the rice, these genes increase the rice plant’s ability to photosynthesize, leading to higher growth rates and food yields. Whether the addition of C4 photosynthesis enzymes will make it possible to grow new crops on a large scale in previously inhospitable environments is not certain. Early results suggest, however, that this is a promising approach.