34.20:
Epiphytes, Parasites, and Carnivores
Unlike most animals, plants cannot move from one place to another in search of food. To survive, plants must gather light and nutrients by growing and moving in place. Plants obtain food through two different means: autotrophy and heterotrophy.
Autotrophic plants synthesize their own food through photosynthesis. Light energy—captured by chloroplasts—drives the creation of sugars that nourish the plant.
Heterotrophs, however, rely on another organism for nourishment. Heterotrophic plants, such as dodder vine, often lack chloroplasts and cannot make their own food.
Dodder vine is a leafless, climbing, plant parasite. It absorbs nutrients from its plant host, causing the host harm or sometimes death.
Modified root projections called haustoria invade the host plant’s vascular tissues to divert water and nutrients for the parasite’s growth. Dodder can also spread to neighboring plant hosts, creating a nutritional supply system for itself.
The staghorn fern has chloroplasts and is, hence, an autotroph. Staghorn fern illustrates a different kind of nutritional adaptation—it is an epiphyte or “air plant,” that grows on other plants, such as trees, for physical support.
Epiphytes benefit from this relationship because the host plant often secures them a higher position in the forest canopy that provides more light for photosynthesis, while the host plant does not suffer damage.
Epiphytes have specialized roots that may anchor them to the host. They also take up water and nutrients from the air or organic debris that accumulates near their anchoring point. Additionally, the leaves of epiphytes absorb moisture and nutrients from the air and rain.
Pitcher plants are mixotrophic—both autotrophic and heterotrophic—carnivores that live in sunny, acidic, nutrient-poor bogs. Since bog soil is nitrogen-deficient, pitcher plants rely on carnivory to supplement their nutrition. The pitcher plant’s specialized leaves form funnels, which trap and drown prey, such as insects and other small animals, in its digestive fluid.
Despite being stationary, plants evolved remarkable adaptations to reduce the effects of environmental stressors, such as low light or poor nutrient availability. Plants like the staghorn fern, pitcher plant, and dodder vine access valuable resources by utilizing other organisms in their environment.
34.20:
Epiphytes, Parasites, and Carnivores
Plants often form mutualistic relationships with soil-dwelling fungi or bacteria to enhance their roots’ nutrient uptake ability. Root-colonizing fungi (e.g., mycorrhizae) increase a plant’s root surface area, which promotes nutrient absorption. While root-colonizing, nitrogen-fixing bacteria (e.g., rhizobia) convert atmospheric nitrogen (N2) into ammonia (NH3), making nitrogen available to plants for various biological functions. For example, nitrogen is essential for the biosynthesis of the chlorophyll molecules that capture light energy during photosynthesis. Bacteria and fungi, in return, gain access to the sugars and amino acids secreted by the plant’s roots. A variety of plant species evolved root-bacteria and root-fungi nutritional adaptation to thrive.
Other plant species, such as epiphytes, parasites, and carnivores, evolved nutritional adaptations that allowed them to use different organisms for survival. Rather than compete for bioavailable soil nutrients and light, epiphytes grow on other living plants (especially trees) for better nutritional opportunities. Epiphyte-plant relationships are commensal, as only the epiphyte benefits (i.e., better nutrient and light access for photosynthesis) while its host remains unaffected. Epiphytes absorb nearby nutrients through either leaf structures called trichomes (e.g., bromeliads) or aerial roots (e.g., orchids).
Unlike epiphytes, parasitic plants absorb nutrients from their living hosts. Non-photosynthetic dodder, for example, is a holoparasite (i.e., total parasite) that completely depends on its host. Hemiparasites (i.e., partial parasites), such as mistletoe, use their host for water and minerals but are otherwise fully photosynthetic. While both dodder and mistletoe employ haustoria to divert hosts’ nutrients, other parasitic species tap into mycorrhizae associated with other plants to absorb nutrients (e.g., Indian pipe). Indian pipe is non-photosynthetic and relies on this interaction for survival. In parasite-plant relationships, parasites derive nutrients at hosts’ expense.
Carnivorous plants are photosynthetic but live in habitats that lack essential nutrients, such as nitrogen and phosphorus. These plants supplement their nutrient-poor diet by trapping and consuming insects and other small animals. Carnivorous plants developed modified leaves that assist in capturing prey through funnel (e.g., pitcher plant), sticky tentacle (e.g., sundew), or jaw-like (e.g., Venus flytrap) mechanisms. Carnivorous plant-small animal relationships are fundamentally predator-prey relationships. Understanding these plant nutritional adaptations reveals important ecological information, such as which nutrients are essential for plant growth as well as the nutrient status of a given habitat.
Suggested Reading
Adibah, MS Ruzana, and A. N. Ainuddin. "Epiphytic plants responses to light and water stress." Asian Journal of Plant Sciences 10, no. 2 (2011): 97. [Source]
Hedrich, Rainer. 2015. “Carnivorous Plants.” Current Biology 25 (3): R99–100. [Source]
Twyford, Alex D. 2018. “Parasitic Plants.” Current Biology 28 (16): R857–59. [Source]