Plant Nutritional Strategies
How do plants make their food?
Plants are famous for being green and, relatedly, for their ability to fix carbon. Unlike animals and fungi, when plants are exposed to carbon dioxide, water, and sunlight, they are able to generate food in the form of simple sugars through a process called photosynthesis. Because of this, they are known as autotrophs (literally “self feeding”) while those of us that rely on externally fixed carbon are called heterotrophs (“other feeding”).
Does that mean plants are self-sufficient?
Plants need more than just carbon to survive. In order to build cells and their components–including the proteins involved in photosynthesis–plants require other mineral nutrients, especially nitrogen (N) and phosphorus (P). When N and P are abundant in biologically available forms, plants can simply use their root system to retrieve these elements from the soil. However, in many parts of the world N and/or P is scarce, meaning that plant growth is limited by their ability to extract a small amount of element from a large amount of soil. Sort of like searching for a needle in a wide, deep haystack without being able to move your feet– very difficult! To further complicate the issue, without access to the nutrients they need, plants are unable to extend their root systems, but without large root systems, they are unable to gather the necessary nutrients. When soil nutrient levels are too low, plants simply cannot grow.
So if plants experience scarcity (or nutrient limitation) in most terrestrial ecosystems, how does the Earth remain so green? Well, most plants have help!
A variety of organisms live in the soil, many of which engage in complex interactions with the plants we see above ground. Two particularly influential categories of soil-dwellers are mycorrhizal fungi and nitrogen-fixing bacteria. Let’s begin with the former.
Mycorrhizal fungi participate in what are called mycorrhizae: symbiotic associations between themselves and plant roots. In practice, these associations involve fungal hyphae (or strand-like structures) growing between or inside of the cells of live root tissue. From there, the hyphae extend far into the soil and forage for N or P that would otherwise remain inaccessible to their host. In return, the plant transfers fixed carbon in the form of sugar into the heterotrophic fungus. Most mycorrhizal fungi could not persist without the symbiosis, and in many habits worldwide, the opposite is true of their hosts. In extreme cases, plants will transfer more than 30% of their fixed carbon to fungi in exchange for all the nitrogen they need to survive. Additionally, this relationship is ancient. The first plants to ever colonize land are thought to have done so with the assistance of fungal partners, and today more than 80% of vascular plants maintain a mycorrhizal symbiosis. Partnering with mycorrhizal fungi, it seems, is far and away the most popular plant nutritional strategy.
The relationship between nitrogen fixing bacteria (also known as diazotrophs) and plants is similar to the plant-fungal relationship in a number of ways. In this symbiosis, plants provide carbon in exchange for usable nitrogen. However, rather than extending into the soil and scavenging for buried treasure nutrients, the bacteria live completely encased within special root structures known as nodules. This housing allows them to perform the energy intensive reaction that turns N2 from the atmosphere (which is not biologically available) into ammonia (a nitrogen compound that plants can process into proteins and other structures). While not quite as widespread as mycorrhizal fungi, symbiotic nitrogen fixers are considered important drivers of ecosystem function because they have the unique ability to draw new nitrogen in from the atmosphere, rather than simply helping trees to access what was already there. Additionally, plants that associate with nitrogen fixers tend to be abundant where the climate is warmer, due to the fact that the nitrogen fixation reaction is generally more efficient at higher ambient temperatures.
What about plants that aren’t green?
In other words, how do plants that lack the pigments and structures necessary for photosynthesis feed themselves (see note 1)? Surprising though it may be, not all plants are autotrophic. Some, like the Ghost Pipe pictured above, leverage their established relationship with mycorrhizal fungi to gain carbon from nearby plants. To understand what I mean, you must first understand that mycorrhizal fungi can be in contact with the roots of more than one plant host. In some cases, one of those plants (known as a myco-heterotroph, myco- being the prefix that refers to fungi) takes fixed carbon away from the fungus, rather than providing it in exchange for nutrients. These complex interactions are sometimes called tripartite symbioses because they involve three distinct species– two plants and one fungus. However, they can also be thought of as a coupled mutualism (an interaction in which both participants benefit) and parasitism (an interaction in which one participant benefits at the expense of the other). In either case, it is generally agreed that most myco-heterotrophs were engaged in a more conventional mycorrhizal association at some point in their evolutionary history, but lost the ability to photosynthesize due to the fact that they live in the highly shaded forest understory. Call them cheaters or call them clever, myco-heterotrophic plants certainly provide an interesting answer to the question of where they’ll get their next meal.
And what about plants that eat meat?
As stated above, about 80% of the vascular plants populating our ecosystems rely on some kind of mycorrhizal symbiosis to meet their nutritional needs. But what about the other 20%? This remaining portion can be split into two categories: the habitat specialists and the nutritional specialists. As the name suggests, plants in the former category are extremely well-suited to a particular habitat niche (especially niches that lack abundant fungi), allowing them to extract nutrients from their immediate environment without assistance. This includes epiphytes (which grow on the surfaces of other plants, rather than in the soils) and plants that take advantage of disturbances like fire or rock slides by quickly colonizing the newly barren ground. The latter category refers to plants that have overcome the challenges of extremely nutrient-poor soil via some non-mycorrhizal adaptation. Perhaps the most famous (and my favorite) example from this category is the carnivorous plants (see note 2).
These organisms use specialized structures to trap and digest animals, usually insects, in order to harvest the nitrogen, phosphorus, or other trace nutrients from their bodies. In North America, carnivorous species are often found in wet, boggy regions where highly acidic conditions prevent nitrogen from accumulating in the soil. In other regions, carnivores can double as epiphytes and use rewards like sweet nectar to attract larger kinds of prey. In Borneo, for example, Nepenthes lowii attracts an arboreal shrew in this way. However, rather than consuming the rodent itself, this pitcher plant digests the shrew’s nitrogen-rich feces, which is usually released into the bowl of the plant while the shrew rests upon it (see note 3). Isn’t nature so freaking cool??
Why should I care about plant nutrition?
So why study the nutritional strategies of plants, anyway? First, and perhaps most importantly, it’s fun and exciting! You never know when you might discover a novel poop-based mutualism.
But on a more serious note, heterotrophic life relies on plant nutrition. Plants form the base of the food web in nearly every terrestrial ecosystem by generating the organic carbon that animals, fungi, and even some bacteria require to survive. Just as there would be no forests without fungi, there would be no fungi without forests. And without either one, there would be no you or I. In addition, plants structure ecosystems by providing habit for the diversity of life. By investigating the myriad ways that plants meet their own nutritional needs, we achieve a deeper understanding of the factors that determine what the world looks like and what can live in it.
Moreover, as I established above, plant growth largely depends on the concentration and combination of nutrients available to them within their environment. If they’re nutritional needs cannot be met, they will not grow. This issue is particularly salient in two contexts. The first is agricultural settings, in which humans often attempt to cultivate selectively bred food crops in a monoculture– picture a corn field and you’ll see what I’m getting at. Historically, this style of farming has resulted in soils that are low in nutrients and contain few symbiotic organisms. In order to maintain the level of food production necessary for humans and our livestock, farmers have turned to fertilizers. This practice has had numerous negative consequences for the functioning of our ecosystems, while some nutrients have become increasingly difficult for humans to access (see note 4). Therefore, many farmers have begun to seek alternative agricultural practices that do not require extensive fertilizer application. Understanding the diverse tools that plants use to satisfy their own nutritional needs is vital to this effort. Second, the global climate is changing, and in many places becoming increasingly volatile. This can have numerous effects on plants, including changing the availability of nutrients. By studying the ways that plants have evolved to deal with nutrient limitation, we may be able to better predict how ecosystems will respond to future disturbance.
Finally, it’s worthwhile to study plant nutrition because there is simply so much that we do not know. For example, we have much to learn about how the species or genetic make-up of the host plant affects the kinds of mutualisms it can participate in, and about how many individuals can interact with the same fungal partner at the same time. Similarly, we are continually learning more about why some nutritional strategies are more common in some places than others, and why some seem to be found all over the globe. My own interests as a researcher lie in determining how different fungal partners provide different kinds of benefits to their plant hosts, and how these differences shape whole ecosystems. It’s a rich and wild world out there; by being clever and curious and very often lucky, we learn more about it every day.
Green-ness ≠ photosynthetic ability. Some plants have purple or red leaves year-round, but still photosynthesize. There are a variety of red- and orange-colored pigments that also contribute to photosynthesis.
Though they consume other organisms to meet their nutritional needs, carnivorous plants are not considered heterotrophs. This is because autotrophy and heterotrophy describe the way that an organism obtains its carbon, specifically. Since carnivorous plants photosynthesize, they retain the title of autotroph.
Scat-based mutualisms are way more common than you might expect. For another example, check out this pitcher that acts as an outhouse/hostel for bats!
This issue is too complex to be covered here, but the resource linked above is a good jumping-off point for further exploration.
