| The Magazine of the CALIFORNIA ACADEMY OF SCIENCES |
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Feature The Chocolate Tree
Of all the foods the Americas gave the world, few have garnered as much notoriety as chocolate. Prized since ancient times, chocolate has a reputation for inspiring passion in those lucky enough to consume it. Love of this flavorful treat led humans to domesticate the chocolate-producing cacao plant to meet our culinary desires. But our meddling may be responsible for cacao's tragic ecological fate. The cacao plant, or Theobroma cacao, evolved in the tropical rainforests of the Americas, but its origins remain something of a mystery. The species has been domesticated for so long that, like corn, it is now quite different from its wild relatives. Twenty-two wild species of Theobroma still flourish between central Mexico and the southern edge of the Amazon basin. Of those, four have been used as a food source by native peoples in Central and South America. But only Theobroma cacao, known largely as a cultivated crop, is used to make the delicacy we know as chocolate. Scientists are now trying to piece together cacao's genetic heritage by comparing the distribution and form of cacao's various strains. One hypothesis holds that cacao was once naturally widespread throughout Central Amazonia and the Guianas to southern Mexico. Over time, two subspecies evolved, one on either side of the isthmus of Panama. According to this theory, each subspecies has given rise to the two major strains, or cultivars, of T. cacao grown today. The second hypothesis, bolstered by recent genetics research, argues that T. cacao originated in the Amazon basin and upper Orinoco river region. The widest variations in domesticated cacao pod color, shape, and surface texture are found in this region. The late ethnobotanist Richard E. Schultes, who spent much of his career working in the Amazon basin, believed that cacao was spread throughout the area by people. Schultes surmised that as the region's early human residents worked their way northward and westward through the lowland forests bordering the Atlantic, they brought this favored food plant to Panama and eventually Mexico. Though Amazonian tribal people used the copious pulp surrounding cacao seeds to brew a favorite beverage, there is no evidence to suggest they exploited the plant for its seeds. The peoples of Mesoamerica were the first to selectively breed cacao as far back as 2,500 years ago. Proponents of this theory believe that the two major cacao cultivars originated from subspecies in the Amazonian basin. A third hypothesis suggests cacao originated as a natural hybrid between distinct forms in Central and South America. Ancient peoples may have accelerated the process of speciation by selecting hybrids with traits that suited their preferences. Genetic studies will, no doubt, eventually help clarify the plant's murky evolutionary history. The two main cultivars are known as Criollo and Forastero. Criollo have fruits (actually specialized berries commonly called pods) that are red or yellow-red and shaped roughly like a deflated toy football. They have deeply furrowed and warty skin; pod husks that are thin and easily cut; and seeds (often called beans) that are white or yellowish-white inside and round in cross-section. This cultivar has been propagated since prehistoric times in southern Mexico and Central America. It produces the most flavorful cacao, but the plants are very susceptible to several diseases. Forastero cultivars look very different. Their green, oval pods are smooth-skinned, rounded at both ends, and shallowly furrowed. A thick husk that is difficult to cut protects the seeds inside, which have purple interiors and are usually flattened like almonds in cross-section. This form originated in the Amazon and Orinoco valleys of South America. It produces cacao with a less intense chocolate flavor, but the plants are hardy, vigorous growers. Today, Forastero is the most widespread cultivar, and is grown from Brazil to West Africa. But as would be expected for a crop that has been domesticated for centuries, variants of both cultivars are common. One of the better known is a Criollo-Forastero hybrid called Trinitario, from the mouth of the Orinoco River. Cacao is a finicky plant. It has exacting moisture, temperature, and soil requirements, and falls victim to many insect pests and fungal diseases. For this reason, the plant was an unlikely candidate to coax out of the rain forest and into a plantation setting. It grows best in places where rain falls year round and the air is warm and humid. Accustomed to the shelter of the rainforest, cacao trees do not tolerate wind and need well-drained but moist, deep soils. Those soils harbor symbiotic fungi, or mycorrhizae, that live with the feeder roots of cacao. These mycorrhizae permit cacao trees to absorb critical nutrients like phosphorous from the organic-rich mulch in the upper soil layers. In exchange, the trees provide carbohydrates and sugars to the fungi. Smart cacao farmers ensure that a portion of the original forest remains intact within all plantations to encourage the spread of mycorrhizae. Today, cacao is often grown in plantations that can extend across more than 200 acres. Such single-crop fields are a bonanza for cacao pests and pathogens. Beetles, aphids, ants, and moths love nibbling on the trees, while several hundred species of fungi destroy up to a third of the world's cacao harvest each year. Modern breeding techniques have created cultivars tolerant or nearly resistant to some of these diseases, but many pests can be controlled only with costly chemical applications. Cacao trees also help feed much larger rainforest creatures. Monkeys, squirrels, bats, parrots, and woodpeckers chew on the pods to get at the sugary sweet pulp surrounding the seeds. A mature cacao tree can grow nine to twelve feet tall, which makes it part of the understory in tropical forests. It has a leafy crown of large oblong leaves that tend to hang downward. The plants are easy to recognize; new leaves are a vivid salmon pink. One of the most curious features of cacao is its habit of sprouting flowers on trunks and leafless branches. Called cauliflory, this phenomenon is thought to make flowers easier for pollinators to reach, and makes the fruit more obvious to large foraging mammals that can disperse the seeds. The flowers of the cacao plant are complex in shape and striking in color. Each of the flower's five petals consists of a translucent basal pouch that tapers into a strap. This in turn widens into a yellowish flag that hangs around the perimeter of the flower. The flag may attract the attention of pollinators and direct them to a suitable landing site. Each of the petal pouches conceals a pollen-bearing stamen. In addition, a whorl of five erect, wine-red filaments (stamens lacking pollen) forms a fencelike barrier around the ovary. Following pollination it takes five to six months for a pod containing 30 to 40 seeds to ripen. Cacao flowers appear adapted for insect pollination. Once, they probably attracted the same fast-moving trapline bees that pollinate many flowering plants in the neotropics. But there are no such bees to be found in the vast cacao plantations in Africa or Brazil. The pollination of modern cacao cultivars certainly isn't very efficient. A persistent mystery about modern cacao production everywhere has been the low number of pods on each tree. According to one estimate, of the 10,000 flowers produced by each tree in a year, just 20 to 30 develop into mature pods carrying the seeds we make into chocolate. Entomologist A. M. Young of the Milwaukee Public Museum spent many years studying the reproductive biology of cacao. He learned early in his field work that hand-pollinated flowers yielded far greater numbers of mature pods than those pollinated naturally. Young hypothesized that the fragrant, calorie-rich oils produced by wild Theobroma are what attract trapline bees, which have high energy requirements. But the hairlike structures that might have produced the oils are either vestigial or completely absent in most cacao cultivars. While breeding the plant to produce better chocolate, humans may have inadvertently destroyed the plant's primary means of attracting pollinators. By studying trees grown in commercial plantations, Young found that small flies known as midges visit cacao flowers. He contends that midge females are probably attracted to the protein-rich pollen of cacao flowers for the same reason female mosquitoes seek out a blood meal—to fortify their eggs. Yet flies are typically generalist pollinators, and cacao flowers aren't popular feeding sites even among midges. After expanding his studies to include plants in abandoned plantations as well as those next to mature forest, Young concluded that pollinator availability was limited by suitable breeding sites. In well-tended cacao plantations, the rotting pods, leaf litter, and other debris where midges like to lay their eggs are cleared to curtail the spread of disease-causing fungi. As a result, fewer midges are available to do the job of pollination. We still know precious little about the pollination biology of cacao in a natural setting, yet this kind of information is precisely what's needed to improve cacao harvests. Young describes the pollinator attraction system of cacao cultivars as dysfunctional. The products of artificial selection and asexual cloning over thousands of years, cultivated cacao plants, unable to produce floral oils, have become dependent on the fickle tastes of opportunistic midges. It is upon this group of minute and seemingly insignificant insects that our future chocolates depend. Frank Almeda is Senior Curator in Botany at the California Academy of Sciences. |
