Surviving the flood
Banner photo by Daniel Koglin
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
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.
Root responses to waterlogging
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.
Images of aerenchymous tissue from prop, anchor, and pneumatophore roots courtesy of the study linked above.
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.
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.