1/28/23:
Surviving the flood
Banner photo by Daniel Koglin
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1/28/23:
The initiation of this blog last summer coincided with the initiation of the Root Lab's 2022 waterlogging trial. The updates that followed have been a sporadic interplay between the history of important figures in science, introductions to a diverse range of forest friends (fungi, insects, and the occasional Root Lab volunteer), foraging campaigns, and an excessive amount of vocabulary terms. Throughout all of this enjoyable, yet unfocused, meandering, I've realized that I have failed to commit to a deep-dive on perhaps the most dive-able topic of all: waterlogging. This next installment will attempt to summarize plant-water relations, as well as whole-plant and root specific responses to waterlogging.
Plants and water
The fate of life on our blue planet lies in the wet hands of the hydrosphere. From the oldest microbial fossils of the stromatolites that comprised Earth's first known reef systems, to the green flagellate ancestor that gave rise to the phylum streptophyta, life's reliance on H₂O is as clear as an overpriced bottle of evian water. If you don't know what I'm talking about and somehow missed the brazen evian product placement that took place in every 90's-00's movie, I suggest you hold a viewing party with some of your most observant friends, watch "When Harry Met Sally," and take note of your evian bottle count by the end of the film. Alternatively, if you came here to learn about plants and science and all the other wonders of the world, check out this informative presentation on the evolution of roots and the earliest vascular plants. As plants made their slow crawl onto the terrestrial landscape, they had to make adaptations for water regulation, structural support against gravity, reproduction, photosynthesis, uptake of nutrients from the Earth's mineral surfaces, and transport of nutrients and water throughout their systems. Plant-water relations are involved in all of the above adaptations and have guided our green friends' evolution for over 500 million years.
Now that you've been inundated with a wave of claims regarding water's influence on plants, let's run through a quick and dirty summary of early land plant lineages and the vital biological modifications that allowed for a plant dominated biosphere. The earliest land plants are the bryophytes. Coined the amphibians of the plant world, bryophytes include the brave mosses and co. whose ancestors decided to pack up and take a chance for a better life on land. Independence isn't always quickly cultivated, and these plants still required moist environments to thrive and reproduce. At this point in the fossil record we don't see evidence of true leaves or roots just yet, but what scientists have found are fossils of rhizoids and cuticle tissue, the cuticle being an important adaptation that would be conserved by all land plants hereafter. Below you can see the flattened, lobe-like liverworts, the needle-like hornworts, and the dense mat formed by mosses. Image source.
Next up we have the vascular lycophytes. If you ignored my previous video recommendation above, then 1) how dare you, and 2) you probably need an introduction to these land dwelling lads. Lycophytes arrived on the scene around 420 million years ago (mya) along with game-changing evolutionary features such as vascular tissue, true roots, lignin infused stems, and leaves. Vascular tissue includes xylem and phloem and allowed for nutrient and water transportation between plant organs. Roots, compared to their ancestral rhizoid counterparts, allowed for mineral and water uptake in addition to the primitive function of plant anchorage. Lignin provided the structural support for upward growth against that ever-persistent gravity that John Mayer is always wailing on about. Leaves, which were at first needle shaped and later evolved to increase surface area and light capturing abilities, increased photosynthetic capacity and aided larger growth. On the topic of leaves, research has provided evidence for another land-mark adaptation, stomata, occuring sometime around 400 mya. Stomata are yet another plant modification that aid in water regulation and gas exchange. These microscopic pores aren't only ubiquitous on the leaves and stems of vascular plants, they are also found on some bryophytes (hornworts and mosses only, sorry liverworts). Fully equipped with these complex physiological and anatomical designs that only nature itself could construct, lycophytes grew upwards and onwards to populate Earth's first forests. Check out the anatomy of the leaf below to see if you can identify some of the modifications we just covered. Image source.
We have now entered the late Denovian, something like 375 mya, give or take a few. Surrounding us are giant Lepidodendron complemented with an understory of tree-like sphenopsids. It wouldn't be until millions of years later, in the Carboniferous period (360-300 mya), that we would begin to see progymnosperm and true gymnosperm species that resemble the extant seed bearing trees in our own backyards. As land plants diversified and began to colonize different biomes, the Earth itself experienced many changes in climate and shifting landmasses. Glacial periods led to extreme global warming and cooling while mountains formed from plates of lithosphere being shoved together like misaligned pieces of a puzzle. Pangea's intro coincided with the global Carboniferous forests's outro, as the drying climate led to mass seed plant extinction. Below is what we think these forests looked like before annihilation! Image source.
Slowly but surely over the next 300 million years, cycads, ginkos, and then conifers, dominated Earth's flora. In the Cretaceous period (145-65 mya), angiosperms flipped the plant evolutionary script with their showy flowers and protected seeds, quickly radiating into the estimated ~300,000 species known today. Right about now you may be asking, or demanding, depending on how much I have tested your patience with this detour down the terrestrial plant family tree: what does this have to do with water? As all this commotion was going on (evolution, tectonic plate shifts, global climate fluctuations, extinctions, radiations, volcanic eruptions, giant sloths slothing around, etc...), Pangea was breaking up into continents. Life on our planet was experiencing elevated rates of speciation as plants and animals were subjected to a diverse array of new biomes, isolated from other land masses, and supplied with limited resources. Plants were now growing in high elevations, low elevations, cold climates, hot climates, dry climates, humid climates, nutrient poor environments, nutrient rich environments, floodplains, temperate forests, tropical forests, swamps, and so on. As life migrated from the ocean to the land, water availability became one of the main factors determining species distribution. Take a look at how the continents have also migrated over the years. Image source.
We are left without an ending as we enter modern day, when both plants and our planet are strikingly different from the setting of our starting-off point. The initial adaptations that made the water to land transition possible have undergone millions of years of innovation. Some lineages of plants have developed even more complex anatomical and physiological mechanisms to aid in water regulation, and this is especially important for species who find themselves in environments with extreme high or low water availability and precipitation patterns.
Lexikon Blogion
Streptophyta: plant phylum including both unicellular (algae) and multicellular (not algae) organisms. Grouped within streptophyta are embryophytes, a.k.a. land plants. Take that taxonomic classification with a grain of salt. I spent about 90 minutes reading online arguments between online guys debating the correct ancestral lineage and taxonomic grouping of land plants. Yet, I am still no closer to the truth.
Bryophyte: informal group name for any nonvascular seed plant. This includes hornworts (division Anthocerotophyta), mosses (division Bryophyta), and liverworts (division Marchantiophyta).
Rhizoid: uni- or multicellular filamentous outgrowth from the underside of the thallus (body of nonvascular plants) that aids in anchorage and water transport. Rhizoids can also be found on algae in addition to bryophytes.
Cuticle: a protective surface covering the epidermis (skin) layer of aboveground plant parts. Its main functions include prevention of water loss and protection against pathogens and UV radiation. The cuticle is composed of two waxy polymers called cutin and cutan. Very cute.
Lycophytes: first vascular land plants, preceded ferns. While lycophytes were lucky enough to receive the evolutionary update of vascular tissues, they still lag behind on the leafy side of things, having single veined 'microphylls.' All vascular plants that followed have more complex 'megaphylls,' which sport bigger leaves with branched veins.
Xylem: one of two types of transport tissues that make up the vascular system in plants, responsible for water and mineral uptake and transport. Fun fact: Swiss botanist Carl Nägeli introduced the terms 'xylem' and 'phloem' in 1858... According to wikipedia, he's also infamous for being the dude who convinced Gregor Mendel to forego his research in genetics. Look here if you want to check out some of their letters. I think I will use Mendel's funny salutation "HIGHLY ESTEEMED SIR" for all of my future correspondences.
Phloem: one of two types of transport tissues in the vascular system of plants, responsible for transporting the products of photosynthesis (photosynthates) throughout the plant. The movement of these soluble sugars is called translocation.
Lignin: a complex structural polymer found in the cell walls of plants, the second most abundant natural material following cellulose. Not only is lignin important for structural purposes, but it also aids in water transport and stress resistance. Particularly important in wood and bark cells.
Stomata: microscopic pores present on most stems and leaves of terrestrial plants. Their main functions include uptake of CO₂, controlling plant water loss, and regulation of internal temperature. Stomata open and close with the help of special turgor-controlled guard cells that line the pore walls.
Lepidodendron: the cool, giant, trees you see in recreations of the Carboniferous forest. This genus is now extinct, but maybe we can invest in research for a more chill version of Jurassic Park where we bring back ancient plants rather than giant carnivorous lizards.
Sphenopsids: early vascular plants, also known as 'horsetails.' Today, Equisetum is the only living genus of this group and around 15 species can be found in North America.
Radiation: evolutionary process in which species diversify at high rates over a short period of time. This can happen as a result of new resource availability and niches in the environment.
Nonstomatal vs. stomatal photosynthesis probz: Okay, so this isn't technically a term, but I thought it would be better explained here than wherever I typed it in this post. Stomatal derived photosynthesis issues involve the decreased or failed functioning of the guard cells that open and close these microscopic pores. This is perceived to be one of the main problems impacting the photosynthesis rates in species intolerant to waterlogging. Nonstomatal photosynthesis limitations include degradation of leaf pigments, accumulation of salts, RuBisCO performance, and so on.
Waterlogging
Now that you've been primed with an exciting and triumphant story about the resilience of life and the wonders of change, let's move on to extreme weather events and asphyxiation! Don't worry, I'll pepper in some ill-fitting humor, which will be sure to make you a) feel a tinge of secondhand embarrassment for me as you read, or b) laugh hysterically, brighten up your day, and tell random strangers about the comedic and educative brilliance of this blog. Jokes aside, I want to take a moment to define waterlogging, how it happens, and the impacts it has on both the environment and plants.
Waterlogging occurs when water fully saturates soil pores and restricts the flow of oxygen (O₂) and other gases below the soil surface. Flooding is a similar term that is used somewhat interchangeably in common conversation, the technical difference being that flooding involves partial or complete submergence of the plant above the soil surface. For the sake of specificity, I will only be referring to the technical definitions of waterlogging and flooding from this point forward. The distinction between the two becomes important when explaining how waterlogging occurs.
There are multiple causal factors of waterlogging, most of which involve flooding events and site conditions. In undisturbed environments like swamps and forested wetlands, waterlogging can be a natural occurrence because of their slow draining mineral soils. Elevation and nearby water sources play a role here too, as lowlands and riparian ecosystems experience frequent flooding and waterlogging events. In developed urban areas, compacted soil and poor soil drainage due to improper site design can cause waterlogged soils. This is exacerbated by the higher frequency and intensity of flooding events occurring in regions of North America and Europe currently. Extremes in precipitation patterns are projected to increase as a result of climate change, making our understanding of plant responses to waterlogging, and how to design our cities in the most functional and sustainable manner, an increasingly important and time sensitive issue.
Floods leading to waterlogged neighborhoods in Chicago 2020. Photos from the Chicago Tribune and The Washington Post.
Let's dig in to what's happening belowground when soils are waterlogged. The most detrimental impact is the slowed movement and availability of O₂ to plant roots and other aerobic soil organisms. Gases move 10,000x slower in water than in air, and for most aerobic organisms that leads to an insufficient supply of O₂, regardless of how many Wim Hof breathing tutorials they watch on youtube. As a result of these anoxic conditions, microbial communities and processes are impaired. This leaves the fungi, bacteria, protists, and nematodes that populate these belowground metropolises to meet an undesirable fate. Some microbes are able to survive if they can adapt and/or withstand the stress. Others are left without a life raft. Soil saturation also leads to pH changes and this further disrupts chemical reactions and the composition of soils. All of these ecosystem changes indirectly impact plants and adds to the already stressful conditions induced by O₂ deprivation.
So what do we know about plant responses to waterlogging? To give a straightforward answer: quite a lot and nothing at all. From the first naturalists and explorers who sailed the seas in their puffy collared shirts and high socks, substantial attention was paid to plant species distributions. Prior to rich people boating and journaling around the globe (no disrespect to Charles Darwin, I would have gladly joined him on his travels if women were allowed on the crew), human societies everywhere used lineal knowledge from living on and cultivating the land to better understand and take advantage of plant-water relations. These observations have guided research on flora that are adapted to soil saturation including plants found in swamp, wetland, riparian, and coastal habitats. Because of this work, we know that plant responses and tolerance to waterlogging are highly species dependent. Scientists have also proven that factors such as waterlogging duration, time of season, and individual plant characteristics (age, sex, and developmental stage) influence tolerance and survival.
Focusing on the literature, most of what's known about plant responses to waterlogging involves aboveground impacts such as plant growth, die-back, and leaf necrosis. To understand plant performance in waterlogged conditions, scientists have used tools to measure plant gas exchange. This remains the most researched area of interest because impaired gas exchange leading to subsequent reductions in photosynthetic capacity, seems to be the ultimate cause of death for plants unadapted to waterlogging. Tolerant species generally show better maintenance of gas exchange rates, whereas sensitive species' rates can drop dramatically, or entirely, and this leads to an internal energy crisis. There are two main mechanisms in which this decline in photosynthesis occurs: nonstomatal and stomatal. All of the above findings on aboveground responses are undoubtably vital in understanding whole plant responses to waterlogging, but these photosynthesis probs aren't the ROOT of the problem.
Root responses to waterlogging
As we wade into this last chapter covering the eternal dance between water and plants, let's look below the surface, into the soil horizons at our main plant organ of interest: the root system. Roots have many morpho-anatomical and physiological adaptations to tolerate and recover from waterlogging. These can be grouped in many ways, but through reviewing the research I have found it most helpful to think of these adaptations in three categories: anoxia-avoiding strategies, accommodation strategies, and hormonal strategies. This is not my own personal categorization, as many authors have published similar groupings. Also, some of the adaptations we'll cover do not distinctly fit into one category, and there is overlap between groups (e.g., anoxia avoiding strategies, such as adventitious root growth, are initiated by hormonal responses). Before diving into these adaptations, let's look at what belowground issues arise when plants are waterlogged.
Just like the waterlogged induced photosynthetic decline experienced by their aboveground counterparts, there are a couple of root functions known to be disrupted by fully saturated soils. First and foremost, the depletion of O₂ in the soil environment inhibits root respiration. Just like us, nematodes, giant sloths, and the 74-manned crew of the HMS Beagle, roots need to breath. This is an important point to emphasize because many people don't think of plants as respiring. Don't be blinded by all of the hype surrounding photosynthesis. You, reader, can do your duty as a citizen scientist by randomly shouting the following sentence at strangers: Roots, shoots, and leaves take in O₂, and respire CO₂, too! Let me know how it goes.
When respiration is disrupted, this causes a cascade of catastrophic calamities. All energy requiring processes are impaired, root to shoot hydraulic conductivity shuts down, nitrogen uptake and metabolism are thrown out the window, and transport of metabolites and toxic compounds is disrupted. If these roots were a house, there would be no electricity, plumbing issues, a lack of protein rich snacks in the pantry, and all of the hallways would be filled with trash. However, if you are a plant with a few adaptive tricks up your sleeve, you may be able to prevent, or clean up, such a domestic disaster.
Anoxia avoiding strategies
Many plants employ strategies to avoid low O₂ by both macro-scale anatomical adaptations like adventitious roots, and smaller changes such as root suberization. What all of the following adaptations have in common is their function of promoting gas exchange within the root system. Before explaining the mechanisms by which different types of adventitious roots aid in gas exchange and diffusion to the roots, it would be helpful to review the basics of plant root systems.
Honing in on trees, all seedlings begin with a tap root that (momentarily or permanently, depending on the species) persists from their embryonic stage, and any other root that grows thereafter is a lateral root or an adventitious root. Lateral roots grow from other root tissue, whereas adventitious roots can arise from aerial stem and leaf tissue. This doesn't mean that adventitious roots are always aboveground, as specialized stems can be found at or below the soil surface in some tree species. There is a diverse variety of adventitious roots that serve different structural and functional purposes, but that's a topic rich enough for another post (if you can't wait to learn more, check out this article for a review on hormonal drivers of adventitious root formation). Here we will be focusing on a type of adventitious root called a stilt root, and a modified lateral root called a pneumatophore.
If you look really closely you can see stilt roots on a mangrove tree. In addition to assisting with gas exchange, this tangled mess structurally supports the tree. Anchorage is more important than beauty, folks. Image source.
An oil painting by botanist and artist Marianne North. This piece, from her travels to Indonesia in 1876, depicts a grove of old banyan trees and their prop roots. Head on over to the Marginalian to learn more about Marianne's interesting life.
Stilt roots and pneumatomophores increase tree gas exchange and are both aided by lenticels and aerenchymous tissue. Stilt roots are common adaptions of mangrove trees who live in coastal intertidal zones. Pneumatophores can also be found in many mangrove species, as well as bald cypress (Taxodium distichum) and cotton gum/water tupelo (Nyssa aquatica). With the help of lenticels and aerenchyma, stilt roots and pneumatophores are able to diffuse atmospheric O₂ through their tissues and downwards to the stem and root system. Lenticels, a modified pore found on stems and roots, act as a gateway of entry for O₂. Aerenchyma, a type of soft tissue containing porous spaces, finishes the job by functioning as an air channel and moving the O₂ downward, eventually reaching distal portions of the root system. Aerenchyma, like lenticels and adventitious roots, can be constitutive or inducible, and is found in both lateral and adventitious roots. Research has shown higher abundance in modified and adventitious roots, like this study from Purnobasuki and Suzuki looking at aerenchyma development in pneumatophores of a common mangrove species.
Images of aerenchymous tissue from prop, anchor, and pneumatophore roots courtesy of the study linked above.
Fancy diamond-shaped lenticels on a white poplar (Populus alba). Image source.
Other adaptations to avoid anoxic conditions include root suberization, aquaporin maintenance and expression levels, and plasticity in rooting depth. After just now scrolling up through this post and realizing how much I've overindulged myself, I will quickly summarize these last three. A compound called suberin is present in the root endo- and epidermis to prevent water loss and act as a barrier of entry to pathogens. This outer layer of suberin is known to thicken with root age, but also in response to environmental stressors. Research has shown that suberization of lateral roots limits radial oxygen loss and allows distal portions of the root system to receive adequate O₂ supply. Aquaporins, protein channels found in all biological cells, are typically inhibited under anoxic conditions. Scientists have found genetically conserved markers for specific aquaporins in species tolerant to waterlogging that aren't present in sensitive species within the same genus. Take a look at this study on sorghum to learn more. Plasticity in rooting depth is perhaps the most practical approach roots take to avoid drowning, and there have been many interesting experiments showing some species' abilities to morph the structure of their root systems. Fujita and co. showed the ability of black pine (Pinus thunbergii) to maintain the same amount of root biomass as non-waterlogged trees by increasing growth in shallow soil layers.
The big blue guy above is an aquaporin. These membrane proteins are sandwiched between the phospholipid bilayers that make up cell membranes. Image source.
Accommodation strategies
Another strategy waterlogged plants may utilize involves metabolic adaptions, which can be in addition to, or excluding, the avoidance strategies I reviewed above. These biochemical changes include the ability to transition to anaerobic respiration, efficient use and allocation of energy reserves, synthesis of proteins and other macromolecules, and protection from post-anoxic injury. Let's jump in with an oversimplified explanation of aerobic and anaerobic respiration.
In the presence of O₂, most biological cells undergo aerobic respiration. This begins with an extracellular process called glycolysis, which forms pyruvate molecules, and THEN proceeds to go through some intracellular cycles, eventually resulting in 36 of these energy containing molecules called adenosine triphosphate. You have my consent to forget all of that and just remember: O₂ -> 36 energy thingies. For our drowning rooty friends who are without a supply of O₂, glycolysis is still carried out and pyruvate is produced. From there, three pathways and end products are a possibility: alanine, lactate, and ethanol. The takeaway here is: no O₂ -> 2 energy thingies + possibly toxic, possibly useful, fermentative end products.
What we know about plants that are tolerant to waterlogging is their ability to speed up the glycolytic cycle and to use and/or dispose of fermentative end products. In tolerant species, genes coding for enzymes that catalyze these fermentative pathways are up-regulated. In addition, the lactic-acid pathway tends to be inhibited due to the cytoplasmic acidosis and eventual cell death caused by the end product lactate.
Above you can see the process of cellular respiration under normal O₂ conditions. Below you can see the different pathways pyruvate can take in O₂ deprived plants. Instead of pyruvic acid entering the Krebs cycle, it can be used in different fermentative pathways, or to biosynthesize the amino acid alanine. Images from here and here.
How waterlogged plants use energy reserves, maintain synthesis of macromolecules, and protect themselves from post-anoxic injury, are all mysterious subjects that warrant intense investigation. Preferably from people in trench coats and fedoras, just to emphasize the obscure inner-workings of plant physiology. What we know is that tolerant species are able to catabolize those sugars and starches stored as nonstructural carbohydrates (NSCs) that I've rambled on and on about. Some species, like the magnolias in our waterlogging study, seem to have the ability to allocate NSCs to belowground pools and provide energy to produce new roots. The mechanisms underlying these adaptations are not yet clear, though they undoubtably involve a recovery in photosynthate and water conductance from shoots to roots post-waterlogging, synthesis of enzymes to break down sugars and starches, and root hormones that stimulate new growth.
In general, cellular maintenance and growth require a constant synthesis of the macromolecules used to construct proteins, membrane lipids, and RNA. Tolerant species are able to stabilize these cellular components by slight molecular modifications that prevent decay in addition to synthesizing stress proteins that scavenge harmful reactive molecules, the later being a strategy we expected the silver maples in our study to use. The ability of some plants to protect themselves from the post-anoxic injury induced by the reoxygenation of their tissues and cells also depends on those stress proteins. Specific proteins and antioxidants of interest include ascorbic acid, glutathione, catalase, peroxidase, and superoxide dismutase. I implore all those who find themselves interested to grab their fedora, magnifying glass, and insatiable curiosity, to research any of the above topics. Mostly because I am a compassionate human who cares deeply about understanding the experience of drowning trees, and slightly because I prefer to cite sources that readers can follow up on and make their own conclusions rather than trusting me, an unreliable narrator ... >:)
Hormonal strategies
At this point, I know I'm "on thin ice" and you've "had it up to here" and all of the other expressions indicating exhaustion that my parents lovingly offered to my brother and I as we fought over rights to his Nintendo DS in the backseat of the family vehicle. Fortunately for you (if you're still here), this last section is extremely short and can basically be summarized in one sentence. Waterlogging induces increased ethylene production and build up, and in some plants, that ethylene triggers root extension, aerenchyma and adventitious root formation, and shoot elongation. Now, because I am an unreliable narrator (I warned you), I will proceed to go beyond my one sentence claim. If you are upset by this, then I will offer you another turn of phrase commonly used by parents and old people to instill peace, "life's not fair!"
Actually, I decided that the one sentence was enough for now. Bye!
Lexikon Blogion
Adventitious root: plant roots that form from any non-root tissue. Depending on the plant species, adventitious roots can be modified to perform many functions, including: energy storage, structural support, assimilatory purposes, haustorial or parasitic structures for nutrient absorption, epiphytic/hygroscopic/aerial structures for nutrient and water absorption from the air, and reproductive purposes.
Suberization/suberin: the conversion of plant cell walls into cork tissue via the deposition of suberin, a complex polyester made out of long-chain fatty acids and glycerol. Suberin forms a barrier called the 'Casparian strip' to reduce water loss and protect the inner tissue of plants from pathogen invasion. Suberization also occurs at a smaller degree in the root exodermis of some species, and also as a wound response.
Stilt root: a type of adventitious root that grows downward from the stem of the plant and provides structural support, also called 'buttress,' 'plank,' and 'ballast' roots in trees. This is not to be confused with a 'prop' root, which are adventitious roots growing downwards from branches, like those found on the banyan trees (Ficus benghalensis) in Marianne North's painting.
Pneumatophore: root snorkels. Just kidding, but really, you have to google this one for yourself because whatever picture my description paints in your head, it won't be the right one. These modified lateral roots extend upwards past the soil surface and above the water table to supply the belowground roots with O₂.
Lenticel: raised pore in the stem of woody plants that aids in gas exchange. The tissue underneath these pores is spacious and filled with air, similar to that found in aerenchyma. Depending on the species, lenticels can look like longitudinal slashes, or circular bumps, on the bark of trees.
Aerenchyma: modified parenchyma tissue that aids in gas exchange and diffusion between roots and shoots of plants. Contingent on how the tissue is formed, aerenchyma can be classified into two types: schizogenous and lysigenous. Schizogenous aerenchyma results from differential cell growth and cell separation, whereas the lysigenous variety follows the programmed cell death of root cortical cells (which just so happens to be triggered by our old friend ethylene).
Constitutive: regarding lenticels, adventitious roots, and aerenchymous tissue, constitutive refers to the formation of these structures/tissues in some plant species regardless of environmental conditions. In other words, plants that have a long evolutionary history of hanging out in wet or frequently flooded environments will grow these tissues even if they never experience a flood in their lifetime. Side note: if you're up for the challenge, take a voice recording of yourself saying 'constitutive' and email it to me. The only rule is you must not, under any circumstance, google the pronunciation beforehand. There has been some heated debate in the Root Lab surrounding the correct pronunciation and I want to settle this once and for all!
Inducible: lenticels, adventitious roots, and aerenchyma that arise from modified tissue as a result of environmental stressors. Plants that have the constitutive tissues described above can also have inducible tissues, but species that are purely inducible aren't constitutive.
Aquaporin: family of membrane proteins found in animals, plants, and microorganisms ... The unity of life! We're all the same, people! Anyway, these water channels facilitate water and glycerol transport between cells.
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