Chapter 12 & 13 Transport, Soil and Mineral Nutrition Topics Methods of transport Xylem transport Phloem transport Soils properties and nutrient absorption Macro and micro essential nutrient elements Too much or too little nutrients Mobile or immobile nutrients within the plant Deficiency symptoms Special adaptations in N-poor soils Plant mineral storage Short distance - Diffusion, Osmosis, and Active Transport Diffusion = random movement of particles from areas of high concentration to low concentration Diffusion of water through a selectively permeable membrane = osmosis selectively permeable membranes allow only certain substances to pass through water molecules pass through all membranes, but pass more rapidly if the membrane has protein channels called aquaporins To move molecules against their gradient, energy (via ATP) is necessary - this is active transport 1
Water Potential - ψ Water has free energy, capacity to do work, chemical potential Chemical potential of water = water potential (symbolized as ψ) When water adheres to a substance, the water molecules form hydrogen bonds with the material and are not as free to diffuse as are other water molecules So, water s capacity to work has decreased when in solutions Water moves from higher to lower ψ Guard Cells For guard cells to open, K + are actively transported from surrounding cells into them Guard cell ψ becomes more negative and the adjacent cells become less negative; results in a net movement of water into the guard cell Guard cells become turgid and swollen, bending and opening the pore due to uneven thickening of guard cell wall Once open, pumping stops and water movement brings guard cells and adjacent cells into water potential equilibrium, and net water movement stops Guard Cells The process is reversed for the stomatal pore to close Guard cells of fully opened and fully closed stomata are both in equilibrium with surrounding cells, even though they all have different internal conditions 2
Control of Water Transport - Guard Cells Numerous mechanisms have evolved that control stomatal opening and closing If the leaf has an adequate moisture content, then light and carbon dioxide are the normal controlling factors Blue light triggers stomatal opening Decrease in internal carbon dioxide concentration may lead to stomatal opening Decreased air humidity - high wind - may close stomata partially High T leads to stomatal closure e.g. CAM plants Control of Water Transport - Guard Cells These mechanisms in healthy plants are completely overridden by a much more powerful mechanism triggered by water stress Roots under water stress synthesize hormone, abscisic acid (ABA) transported to leaves, which immediately causes guard cells to close the stomatal pore (ABA is synthesized by apical buds and senescing tissues too) In water stress - pores are closed even under blue light and low concentrations of CO 2 Long-Distance Transport: Xylem 3
Xylem Transport Tension cohesion adhesion model Coleus Plant Root xylem Leaf = 1.5 MPa Stem = 0.7 MPa Root = 0.4 MPa Atmosphere = 80 MPa Soil water = 0.1 MPa Transpiration loss of water vapor mainly thru stomata for nutrient uptake and cooling Casparian strip forces selective absorption of solutes (keep unwanted solutes out) and help hold water in xylem Transpiration generates tension on soil-plantatmosphere water path water flows along water potential gradient Phloem transport - Münch Pressure Flow hypothesis XYLEM PHLOEM Sieve tube running through length of plant Companion cell Sieve tube element Direction of water Direction movement of sucrose movement Active loading by STM/CC/P complex, and polymer trapping in STM at source/leaf greater sugar conc. In STM water absorption from xylem increased turgor pressure mass flow toward sink - active and passive unloading in sink along pressure gradient pressure flow hypothesis by Ernst Münch for phloem transport Soils and Plants Soil has both abiotic (chemical + physical) and biotic properties - minerals, water, air, T, flora and fauna Right soil is crucial for plants Supplies minerals Holds water Supplies air, T to roots Acts as a matrix that stabilizes plants Harbors nitrogen-fixing bacteria, mycorrhiza, other microbes Animals for plants Texture a crucial physical property Soil Particles Sand, Coarse Size range (mm) 2.0 0.2 Sand, Fine 0.2 0.02 Silt 0.02 0.002 Clay < 0.002 (micelles) 4
Soils and Mineral Availability: CE CO 2 from root respiration reacts with soil water to produce carbonic acid H+ from carbonic acid disrupt cations from soil micelle (negatively charged mineral/clay matrix or organic matter) Roots cannot absorb cations directly from micelle cation exchange is crucial Essential Elements Research in mineral nutrition involves growing the plant in hydroponic solution in which the chemical composition is carefully controlled e.g. except one element see picture Elements that are necessary for plant growth = essential elements/nutrients Essential Elements Macro - needed in large amounts Micro - needed in smaller amounts Criteria for essentiality Must be needed for normal plant development through a full life cycle No substitute can be effective Must be acting within the plant, not outside it 5
Too Much Salty regions - some excrete salt from salt glands on leaves Desert soils sometimes too much minerals too alkaline too negative water potential Toxicity caused by elevated levels of single minerals: Aluminum toxicity in acid soils High levels of heavy metals on mine tailings, polluted soils Too Little Some soils - low concentrations of certain essential elements - plants are unable to thrive on them Deficiency diseases are most commonly encountered in crop plants or ornamentals Harvesting crops leads to soil depletion Fruits, seeds, tubers, and storage roots often have the greatest concentration of minerals in a plant Symptoms of Deficiency One symptom common in many elements = chlorosis Leaves lack chlorophyll, tend to be yellowish, and are often brittle and papery Deficiencies of either nitrogen or phosphorus cause accumulation of anthocyanin - coloration Leaves become dark green or purple Lack of potassium or manganese causes necrosis Patches of tissue die 6
Mobile and Immobile Elements Chlorine, magnesium, nitrogen, phosphorus (picture below), potassium, and sulfur - mobile elements After been incorporated into a tissue, they can still be translocated to younger tissue If soil is exhausted - salvaged and moved to growing regions Mobile and Immobile Elements Boron, calcium, and iron (picture below) are immobile elements They remain in place after being incorporated into plant tissue. In deficient soils - newer tissues show symptoms 7
Nitrogen from Animals Soils in bogs and swamps have very little nitrogen available because of nitrifying and denitrifying bacteria Many bog-adapted, carnivorous plants get reduced nitrogen by catching animals Hydnophtum A mutualistic ant plant Storage of Minerals within Plants All plant parts (except seeds) store minerals in soluble form in central vacuoles Nitrogen is converted to compounds with multiple amino groups Phosphates, sulfates, and other mineral nutrients - simply sequestered in the same forms in which they are used Seeds store minerals as polymerized forms, usually in protein bodies 8