Plant Homeostasis Flashcards
Artificial selection began with
the selection of plants that had beneficial traits for cultivation and began to recognize effects of nutrient deficiency and optimal growing conditions
Eventually, industrialized agricultural practices began to yield much more product but had high input demands
Yield plateaued but demand increased
Green revolution
Intensive plant breeding programs
Genetic modification, engineering and biotechnology are
The most recent tools for improving crop yield and productivity -> Targeted trait improvement for various requirements and demands
Homeostasis
Regulating the internal environment to maintain a relatively stable state, compensating/adjusting for changes in the internal and external environment -> dynamic process
Homeostasis
Regulating the internal environment to maintain a relatively stable state, compensating/adjusting for changes in the internal and external environment -> dynamic process
Animal vs. Plant Nutrition
- Photoautotrophic behaviour forms the basis of the food chain (primary producers)
- Plants concentrate and assimilate nutrients that are present in low concentrations (CO2, minerals, inorganic solutes)
- Plants are unable to rely on many other sources for their nutrition -> synthesize vitamins and amino acids from primarily inorganic sources
- Humans have 9 essential amino acids and 13 essential vitamins -> must be obtained from our diet
Uptake by Root Systems
- Extensive root systems are key adaptations to exploit limited mineral nutrients and scarce water
- Can make up 20-50% (or more) of the total plant mass
- Act as selective filters to bring in important nutrients and exclude toxins
- Roots continue to grow as long as the plant lives
- Incredibly large surface area for rapid and efficient absorption (via root hairs)
- Uptake of water and minerals is achieved through active and passive transport mechanisms (All of the cell membranes of the root surfaces are packed with ion channels and transport proteins)
Passive Transport
- Requires no metabolic energy
- Substances moves along/with a concentration or electrochemical gradient
- Simple diffusion occurs for H2O, O2, and CO2
- All charged mineral ions require transport proteins (facilitated diffusion) to enter cells(e.g. ion channels and carrier proteins)
- Plants will attempt to rapidly translocate mineral nutrients farther away from the root surface to maintain the concentration gradient (sometimes achieved through active transport)
Identifying Essential Plant Nutrients
- identify all the elements present in plants by analyzing their ashes = identify all the necessary elements for plant growth
Using hydroponic culture, we can grow plants with liquid nutrient media that has the full suite of the identified elements
Remove one nutrient at a time and observe growth and survival
Essential elements
- Necessary for growth/reproduction
- Cannot be substituted
- Typically play one or more roles in metabolism that is critical for survival
Visual Indications of Nutrient Deficiencies
- Chlorosis: Yellowing of plant tissues due to lack of chlorophyll -> significantly reduces plant productivity and will negatively impact plant health
- Localization of the chlorosis is often indicative of the particular deficiency
- Any change to the leaf surface has the potential to reduce photosynthetic capacity and will impact plant survival
There are 17 essential elements for plants that
cannot be obtained directly from the atmosphere
Macronutrients essential in large quantities
- C, H, O from air and water account for 96% of dry mass (polysaccharides, lipids, cytosol)
- N, P, K, S, Ca, Mg, and Fe are mineral nutrients, available to plants through the soil as dissolved ions in water
- Components of nucleic acids (N, P) and amino acids (N, S)
- Some ions aid in the regulation of osmotic potential (K+ ) and signalling (Ca2+)
- Some macronutrients perform critical roles as enzyme cofactors (Fe and Mg are critical for photosynthesis)
Micronutrients are essential in trace quantities
- Cl, Cu, Ni, B, Mn, Zn, Mo
- Important activities as catalysts and enzyme cofactors but only required in very minute amounts (e.g. Zn-binding transcription factors) and are often not used up after they have been used
Bacterial-Mediated Nitrogen Cycling
- Abundant element in air (~78% is N2) but is among the most limiting to plants
- N2 has a triple bond that requires a specific enzyme and a lot of energy to break, plants can only accept NH4+ or NO3-
- Uptake of NO3- is preferable but plants will convert NO3- back to NH4+ to assimilate N into organic compounds
Nitrogen fixation:
- incorporates atmospheric N2 into ammonia (NH3) and ammonium (NH4+)
- Nitrogen-fixing bacteria (Diazotrophs): cyanobacteria, Green sulfur bacteria, Rhizobia, and others
Bacterial Ammonification breaks decaying organic N compounds (i.e. amino acids, nucleic acids) into NH4+
- Ammonification (Ammonifying) bacteria: Bacillus, Pseudomonas, Streptomyces, and others
- Also completed by some fungi
Bacterial Nitrification oxidizes NH4+ to NO2- and NO3-
Nitrification (Nitrifying) bacteria: Nitrosomonas, Nitrobacter
Bacterial-Mediated Nitrogen Cycling: Legume Root Nodules
- Legume root nodules form symbiotic associations with nitrogen-fixing bacteria - Rhizobia
- Atmospheric N2 is converted into NH3 which is directly accepted by the plant host in exchange for photosynthates
- Bypasses the need for ammonifying and nitrifying bacteria = much more efficient uptake of nitrogen
- Allows for greater accumulation of nitrogen-rich compounds = High Protein
Nutrient Limitations
- Plant growth and subsequent harvesting of crops remove all the mobile and readily available nutrients from the soil (stored in the harvested plant)
- Early agriculture typically involved shifting of lands used for plant growth to allow for the soil to replenish its lost nutrients (particularly Nitrogen) but land ownership, increasing populations and food demand required continuous repeated use of the same areas
By the 1930s, worldwide industrial farming had reached its maximum output but
put significant strains on the soil, severely depleting nutrients without allowing enough time to replenish
- Coupled with excessive dry conditions in North America (Dirty Thirties), worldwide agricultural production greatly slowed down and caused massive food shortages = Millions of people suffered malnutrition and starvation
A substantial contributor to the lack of production was:
Nutrient limitation (Nitrogen, Phosphorous, and Potassium)
How to improve nutrient limitations?
- Exhumed human skeletons to grind the bones into soil supplements (high in N, P, K), continued use of manure and organic fertilizers (compost, various excrement, etc.)
- Rotating of crops was often used to improve soil Nitrogen content but is a large commitment with inconsistent yields
- Farmers will switch between crops year by year and will include a legume crop
Nutrient Supplementation
- Post-WWII, the Haber-Bosch process was employed to produce ammonia fertilizer (instead of bombs), Phosphate rock and Potash mining continued -> Big Three Fertilizers: N-P-K
- Fertilizer application drastically improved plant growth and survival allowing for increased food production
Plants tended to grow much taller and had increased fruit + seed set ->
- Created humungous top-heavy plants = Lodging (fallen over and can’t get back up)
Green Revolution: Acceptance of Supplementation
- Selective breeding of plants to accept greater nutrient inputs without excessive upwards growth – Semi-dwarf plants
- Pioneered by Dr. Norman Borlaug of the University of Minnesota
- His innovations are credited with saving over a billion people from starvation and malnutrition
- Used pure breeding techniques and developed semi-dwarf, high-yielding, and disease-resistant rice and wheat varieties
- Relied on this innovation for the last ~80 years, but demand is increasing
Researchers bred cereals (wheat, rice, maize etc.) crossing with other cultivated and wild varieties to
select for natural resistance traits against pests and disease as well as smaller, more compact plants
Additional traits were often selected for to improve
the nutritional content of the crop to combat malnutrition in developing countries
Led to development of semi-dwarf breeds of both rice and wheat which
effectively reduced a hormone involved in stem elongation, allowed the dwarf crops to accept the same or greater fertilizer input to produce more yield but experienced no lodging problems
Eutrophication
Enrichment of an ecosystem with chemical nutrients such as compounds containing nitrogen and phosphorous
Only ~10% of fertilizer added to croplands ends up in plant biomass, the rest is
lost as surface or ground water runoffs and end up in bodies of water
Increased available nutrients allow for massive algal blooms which eventually
sink to the bottom where bacteria feed on them leading to depletion of oxygen -> anoxic conditions, Leads to catastrophic consequences to aquatic animal and plant life
Trade-off: tremendous agricultural benefit by
applying Nitrogen fertilizers but hurts the ecosystem due to eutrophication
Soil
- Contains soil-mineral particles, compounds, ions, decomposing organics, water, air, and microorganisms
- Soil particles vary in size:sand (2-0.02mm), silt (0.02-0.002mm), and clay particles (<0.002mm)
Humus
Decomposing organics, holds water and nutrients
The relative thickness of each layer and their
composition varies greatly
The relative amount of soil particles determine
soil properties: water availability and
mineral availability
Soil solution
- a combination of water and dissolved substances that coat soil particles and partially fill pore spaces
- available for plant uptake after-gravity drainage
Soil Properties – Water Availability
- Water molecules are attracted to clay and organic particles
- Clay is alkaline and holds a negative charge at typical soil pH (~7-8)
- Humus greatly increases water availability due to the heterogeneous mixture of charged or hydrophilic organic compounds
- Sandy soil is looser and holds less water than clay soils
- Increased drainage results in lower amounts of available soil solution
Soil Properties: Mineral Availability
- All available minerals have to be dissolved in water
- Some passively enter plant roots (facilitated diffusion) while others are selectively absorbed by roots (Active ion-specific transport proteins)
- Both anions and cations are present in soil solution but are not equally available to plants
Anions: (NO3-, SO42-, PO43-)
- Move freely into root hairs
- Weakly bound to soil and remain dissolved in the soil solution-> leach easily by excess water application
Cations (Mg2+, Ca2+, K+, Fe2/3+)
- Mineral cations are adsorbed to negatively charged soil particles (i.e. clay)
- Cation exchange replaces mineral cations with H+ produced by roots as excreted H+ or carbonic acid from CO2 release
In addition to root branching and the production of root hairs to increase the root surface area, many plants can
can form symbiotic relationships with fungi
Mycorrhizae
- Fungal hyphae (branching filamentous network) greatly extends the accessible soil area for increased water absorption and finding of mineral nutrients
- Fungi are highly capable of mobilizing soil nutrients (organic and inorganic) through external digestion and acid release
Both partners benefit from the two-way exchange of nutrients
- The plant provides the fungus with photosynthates
- Fungus increases the supply of soil nutrients (mobilization of P is key)
What happens when the soil holds no nutrients?
- Highly acidic soils cause leaching of cationic mineral nutrients and limit water retention by clay particles
- Water that is applied quickly drains most mobile nutrients away
- Inability or inefficiency to complete anion exchange is common
- Plants that survive in these conditions often require nutrient supplements by other means than soil absorption = heterotrophic/predatory plants
Heterotrophic/predatory plants
Trap and digest insects and microorganisms for sources of mineral nutrients and organic compounds
Primary producers
Plants incorporate and concentrate CO2, H2O and mineral nutrients to provide the basis of the food chain
There are 17 essential nutrients for plants:
Macro (10) and Micro (7)
Macronutrients are critical building blocks or are
are important for metabolic processes and cell maintenance
Micronutrients are
are typically enzyme cofactors or catalysts that are not incorporated or used up
Nitrogen limitation is commonly the largest barrier to plant growth and reproduction, followed by
Phosphorus and Potassium
Bacterial-mediated Nitrogen cycling incorporates atmospheric N2 into plant-available compounds such as NH4+ or NO3-
Symbiotic relationships with aquatic blue-green algae or soil diazotrophs greatly improve Nitrogen availability and uptake
Dr. Borlaug: Semi-dwarf plants that accept
greater nutrient inputs (i.e. N,P,K fertilizers) saved millions of people from malnutrition and starvation
Excessive fertilizer usage leads to
eutrophication due to the soil’s inability to retain the applied nutrients = <90% lost to runoff
Topsoil with humus is the most
important layer for water availability and nutrient uptake
Soil pH plays a large role in determining
the available nutrients for plants
Alkaline soil:
clay is negatively charged and holds on to cations, Cation Exchange releases cations for plant uptake; anions often leached
Acidic soil:
clay is positively charged and holds anions, plants have limited abilities to cope (Anion exchange is rare); cations often leached
Root hairs and symbiotic mycorrhizae greatly improve
root surface area and improve nutrient mobilization (greatly improves P-uptake by plants)
Heterotrophy is an adaptation to dealing with
highly acidic soils that lack key nutrients
