Roots and Soil: an underground relationship

Plants, like us, need "supplements".  Micronutrients such as iron, manganese, and copper are critical components of enzymes that drive chemical reactions in living organisms. Without them, necessary metabolites cannot be produced causing deficiencies within the organism.  When we are faced with a nutrient deficiency, we can simply change our diet.  But since plants cannot, they must rely on the soil as a ready source of micronutrients.  This is the tenet of our research: if we can understand the genetic effects controlling micronutrient uptake in plants, we can make bigger strides in improving plant performance under marginal soil conditions.

We can think of soil as a very complex solution of chemicals.  There are inorganic and organic elements present in varying quantities, depending on how local climate and geology have affected soil development over time.  No two soils are therefore exactly alike: all are products of their environments.  Adding to the complexity is that most inorganic elements, such as micronutrients plants need, occur in different oxidation states within soils. That is micronutrients in the soil may be more likely to possess a certain electron configuration, which affects their bonding with other compounds.  For plants, micronutrients are only usable in certain oxidation states because of bonding properties present from specific electron configurations.  Thus we use the broad term micronutrient deficiency to describe a plant that is negatively affected by the lack of a certain micronutrient in the soil, or the inability of the plant to obtain the element from the soil.

Nutrient availability chart. https://www.pioneer.com

The availability of a micronutrient in a given oxidation state depends both on soil pH and composition.  As we can see, not every soil pH allows micronutrients to be available to plants in the same amounts.  Especially at high pH, very few micronutrients are readily available as they become insoluble in the soil and therefore unavailable to plants.  Take for example the rust stains in your shower or bathtub: this high pH "hard water" is often cloudy and leaves precipitates as elements are not soluble, notably rust.  At high pH, this rust is due to iron oxide that is not soluble and deposited on bathroom surfaces.  Plant roots can only transport iron which is reduced and soluble in both water and soil and not in the oxidized form, like you would find in hard water.
Luckily plants can modify surrounding soil pH from the roots to make certain micronutrients such as iron more available. We can see this below using a chemical to stain for reduced iron that is available to plants. The Rhododendron seedling roots on top slightly lowered the media pH, while the one on the bottom lowered media pH more drastically.  Note the greater saturation of the purple color on the top indicating a greater amount of reduced iron in the media.  Photo credit: Elsa Eshenaur.

Hopefully this puts the mechanisms of the project in perspective. It is our goal to see how different soils have driven root adaptation to enhance iron acquisition in our research species. All in an attempt to illuminate and improve the reactions that occur beneath our feet.

Collecting in Arklahoma

My recent surveys on the Arkansas/Oklahoma border located a large, extensive Rhododendron viscosum population.  Located at the base of a large ridge, Rich Mountain, the population is comprised of many large individuals upwards of 10 feet in height.  The population occurs along a stream on the Arkansas side of the state line, is not divided into subpopulations, and  occurs in isolation with no other populations nearby.  Many individuals had pink flowers, something which I have not seen before.

R.viscosum from the population on the Arkansas/Oklahoma state line, showing characteristic pink flowers.

Summit of Rich Mountain.  The population was located at the base of this ridge.  Ridgetop flora were also unique, comprised of species that had been severely stunted by wind and ice.

While the species is documented further west in Oklahoma, I had no luck in locating populations.  The terrain here is much lower, hotter and drier.  Many of the forest lands in that part are heavily logged, and it seems that recent clearcutting events took altered a lot of prime habitat.  There was no shortage of prime habitat for snakes, and I could see and hear many scurry along through the rocky sand as I walked by.  I couldn't identify any; they were too fast.

McCurtain County, OK

I am currently in Texas, and will be collecting here this week.

Plants ho!

Fourche La Fave River, Ouachita NF near Y City, AR

I identified the first wild populations of R. viscosum on the banks of the Fourche La Fave River in west-central Arkansas.  After scouting probable habitat further north in the Ozarks without any luck, we made the decision to look further southwest.  Sure enough, some leads got us to the right place.

An R. viscosum population, naturally subdivided into 3 distinct subpopulations.  Red dots indicate occurrence of plants near flowering.

Parts of each subpopulation were situated in the floodplain of the river and, judging by the littered debris, appear that they are submerged when the river floods.  Others occurred on rocky bluffs adjacent to the river's edge. Below is a video showing the general habitat of subpopulation C, entirely situated in the floodplain.

None of the subpopulations occurred far from water.  A few individuals grew on embankments at the base of a hill, but after surveying the higher ground I concluded that this population occurred solely on the Fourche La Fave.  The soils in the floodplain varied differently in appearance than those on the high ground, and so I subdivided subpopulation B into a grid formation up the slope to sample the soils where R. viscosum was most abundant, somewhat present, and absent. This type of survey will be repeated in other populations to see if any soil properties are associated with increased or decreased occurrence.  

Subpopulation B schematic.  Plants were most abundant in the northeast end of this grid.

Photos of R. viscosum flower and spreading habit.  Scaled to my machete

     

The red individuals were the ones actually sampled for genetic analysis.  These individuals had flower buds that were either open or near opening, and will be used as "parents" to help us understand how certain traits behave.  I'll elaborate soon, but for now it is nice to have one population surveyed.  On to Oklahoma!

Why go to the trouble?

Why do we take the time to study native plant populations? It would require less logistical effort to conduct research on plants at hand. When plants are not readily available or the species is Uncharacterized, which is partly true in my case, it becomes necessary. That being said, there must be some compelling reason that I've undertaken this expedition

Practically speaking, I am working on a plant breeding project.  My research helps make improved varieties of the trees and shrubs in your yard. For the landscape plant industry as a whole, demand is driven by novelty. Consumers want plants with the newest color, form, and uniqueness. Unfortunately these demands expedite the transfer of exotic species around the globe to places where they often have negative ecological consequences.

Another consequence of this quest for novel plants is that plant performance can suffer. A plant that is "ideal" in one place at a given time is no indication that it will remain "ideal" when subjected to the stresses of different environments. An understanding of the genetic and environmental contributions that make a plant "ideal" must first be understood if the breeder wants to make improvements. Take for example yield increases in corn, which have seen dramatic increases over the last century. Corn yields did not increase markedly due to the selection of novel high yielding individuals. Rather, strategic approaches were taken to improve plant performance. For creating adapted, sustainable ornamental plants, it  will be breeders' obligations to implement strategic plant improvement.  There is a lot of beauty and novelty in our own backyards: let's build upon that and make it even better.

And so I find myself (currently in the Ozarks) trying to improve one US native plant, Rhododendron viscosum, for landscape use. If only I could find it..

Piney creek  bottoms, Ozark NF

Identified or likely populations of R. viscosum in eastern North America (yellow dots). Each will be visited, sampled, and studied

In 10 days, I will embark on a plant collection trip across 7 states: Arkansas, Oklahoma, Texas, Louisiana, Mississippi, Florida, and Pennsylvania.  I am seeking a wild azalea, Rhododendron viscosum, to study the broader issue of plant adaptation to diverse soils.   The wide geographic range and biology of the above species make it ideal for this kind of research, the details of which I will elaborate on in future posts.  My collaborators include U.S. Forest Service biologists, Department of Agriculture scientists and technicians (USDA-ARS), as well as public gardens and private landowners who have volunteered land and collections throughout the country.  I hope that this blog will serve as a narrative of my field notes to document this portion of my thesis work.  I also hope this will aid in communicating both current and classical genetic analysis as it applies to plant adaptation and improvement.  But ultimately I hope to showcase that the plants around us aren't just food for our sustenance, but food for our thought. Stay tuned!