Why can some plants survive for almost six months of the year in watery darkness, totally submerged by the floods of the rivers in the Amazon Region, while others from the same species cannot live for even two weeks under water? A group of Brazilian botanists and ecologists compared the seeds and seedlings of the sucuuba (Himatanthus sucuuba), a scented tree from Brazil’s North Region, which they had gathered on the flooded plains and in the dry uplands around the city of Manaus. The comparison enabled the team to identify a number of clues that help one understand this evolutionary enigma. Although the plants from the flooded lowlands are identical on the outside to their siblings from the forest’s dry uplands, the floodplain plants have developed a unique way of storing and using energy, which ensures their survival in an environment that is hostile for most plants: darkness with very little oxygen. This unique system probably results from a long adaptation process that led to the sucuuba’s proliferation on the banks of rivers. The succubas on riverbanks store roughly 30% more carbohydrates in their roots and consume the sugars that keep them alive more economically than the plants on the dry uplands.Although seeds from the floodplain and the upland sucuuba trees have the same chemical reserves, the concentrations of these compounds are different, which also helps one explain why the plants from the flooded lowlands do not perish within water. The seeds from the flooded regions are harder (they have more of the polysaccharides that form cell walls) and, essentially, waterproof. “From the morphological point of view, the trees from the two different environments are equal and belong to the same species. Physiologically, however, they behave as if they were two different species,” says botanist Cristiane Ferreira, from the University of Brasília (UnB), one of the authors of the first scientific article on this plant, informally called Amazonian jasmine because its flowers and scent are similar to those of the jasmine plant. The article was published in the November Annals of Botany. “We observed the sucuuba during its speciation process,” says ecologist Maria Teresa Fernandez Piedade, from Inpa, the National Institute for Research on the Amazon Region, at Manaus, the paper’s other author. “And most likely that the same thing is happening with other native species of the region that have also adapted to different environments.” Preliminary genetic analyses have shown that the DNA of the two varieties is tending to grow apart.The sucuuba tree lives up to 70 years and grows as tall as 25 meters in its adult stage. Its latex has anti-inflammatory, analgesic and vermicide properties. It is not the only plant that grows both on the floodplains and dryer uplands of the Amazon Region. Experts estimate that some 30% of the species found in the floodplains also grow in the non-flooded uplands. However, this tree is an extreme example of adaptation and is perfect for a study on understanding the mechanisms that enable plants to survive on floodplains. “We prospected for a symbolic example of this adaptation,” says Maria Teresa. The tree’s habitat is the most complex in terms of contact with the wet environment of the floodplains along the riverbanks. This is the first area to be flooded and the last to dry up. Here, the waters inevitably overflow the river banks, even when there is less rainfall, unlike what happens in the uplands, where there is not always flooding and when there is, it occurs on a lesser scale and for a shorter time.
The sucuuba’s resistance in water is impressive. By the time it reaches the age of 10, when it has probably become tall enough (about six meters) for the upper portion of its trunk and its crown to be beyond the reach of the waters, the tree will have spent nearly one half of its life totally under water. Like other plants growing on flooded plains, where the light only reaches down to three meters below the water surface at most, the trunks of adult sucuuba trees exhibit the rings caused by the latest floods. When submerged in the underwater darkness, covered for five months by water as much as six meters deep, the floodplain succubas enter a phase vaguely reminiscent of animals’ winter hibernation: they use up their energy reserves at a slower pace and do not invest in vertical growth. They save as much sugar as they can so that they can resort to it at a more suitable time, i.e., in the dry season, when the priority is to grow tall enough to escape the next seasonal flood or minimize the next flood’s effects. When the waters flow back into the riverbeds, the trees only have three months in which to germinate and stretch their trunks as far up as possible before they find themselves once again submerged in floodwaters. This survival strategy is impressive, given that plants do not produce photosynthesis (or do so only marginally) in a dark environment and therefore tend to burn up their entire stock of carbohydrates to stay alive. This is what happens to a sucuuba from the non-flooded uplands when transferred to a floodplain – it germinates too fast and “starves to death,” unlike the floodplain sucuuba. An experiment performed by Brazilian scientists with one-month old, 10-centimeter high sucuuba seedlings (plantlets in botanical jargon) illustrates the different adaptive skills of each tree population. The floodplain trees survived when they were placed and maintained in the dark for over a month in water-filled recipients, an artificial environment that simulated natural floodplain conditions. They also flourished in vases that were not water-drenched. On the other hand, the sucuuba from the non-flooded uplands were unable to adapt to excess water and darkness and died in the flooded environment.
According to botanist Marcos Buckeridge, from the University of São Paulo (USP), who coordinated the studies on the carbohydrates, the two sucuuba populations show how plants from the same species adapted to different environments can use their energy stocks at completely different times and for entirely different purposes. The sucuubas from the floodplains stock up more sugars in their roots than those from the uplands; however, this is not what makes the metabolism of each example of the species function differently. “The total amount of carbohydrates in the two sucuuba populations is not what distinguishes one from the other. The difference is how these compounds are arranged in the plant and how the plant uses each kind of energy reserve,” says Buckeridge. The seeds of the sucuubas from the floodplains, for example, have five times less soluble sugar than those from the non-flooded uplands. This carbohydrate distribution makes a lot of sense from the evolutionary point of view. When it is submerged, the floodplain tree has to draw on its energy in a controlled manner to ensure its survival for a longer time. Externally, the floodplain plants and those from the non-flooded uplands are identical. However, like twins biologically identical twins at birth who, however, behave differently, the two tree populations adapt, so to speak, to specific life styles because of the environment in which they live. It is possible that the two examples from the two forest regions are also growing apart from the anatomical point of view (scientists have already observed changes in the internal structure of their roots). However, this issue will only be clarified in the future. For the time being, despite their different metabolisms, the succubas from the floodplains and those from the non-flooded uplands still produce seeds at the same time (between June and July), after the flood season. The seeds are carried by the wind or by water in the case of the flooded environment. Besides being important for understanding the mechanisms that enable a tree to adapt to the Amazon Region floodplains, the study of the two sucuuba populations is also important for a practical reason. The plant has given rise to significant commercial interest thanks to its medicinal properties and its light wood, which is suitable for flooring. “If someone were to plant the sucuuba in a floodplain, he would have to make sure that the given plant adapted to flooded areas,” Cristiane points out. Otherwise, growing the species on riverbanks would be impossible.
FERREIRA, C. S. et al. The role of carbohydrates in seed germination and seedling establishment of Himatanthus sucuuba, an Amazonian tree with populations adapted to flooded and non-flooded conditions. Annals of Botany. V. 104 (6), p.1.111-19. Nov. 2009.