BIOC40H3 Chapter Notes - Chapter 5: Chlorosis, Nitrogen Deficiency, Aeroponics

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Published on 2 Aug 2012
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Chapter 5 Mineral Nutrition
Essential Nutrients, Deficiencies and Plant Disorders
-Plant nutrients are divided into macronutrients and micronutrients based on their concentrations,
however scientists have said this is difficult to justify. Another classification system splits nutrients into
four groups:
1) N and S are first group of essential elements. Plants obtain these through redox reactions part of
carbon compounds.
2) Important in energy storage reactions or maintain structure. P, borate and silicate esters
covalently bound to OH.
3) Nutrients that remain in ionic form. Could be enzyme cofactors or regulate osmotic potential.
4) Nutrients involved in redox reactions, such as metals like iron involved in electron transfer.
-Some elements like Al, Se, Co are not essential but plants contain some and low levels may
stimulate plant growth. Co is part of vitamin B12, found in enzymes of N fixing organisms so Co
deficiency blocks development and function of N fixing nodules.
Special Techniques are used in nutritional studies
-Plants were grown in a nutrient solution with only inorganic salts and plants still grew, indicating
plants fulfill their needs from inorganic elements and sunlight. Technique of growing plants with
roots immersed in nutrient solution without soil is called hydroponics. Oxygen supply to root is
important and can be achieved by bubbling of air through medium.
-In a standard hydroponic system, plants are suspended by base of stem over a tank containing
nutrient solution. An air stone generates air stream of small bubbles. In nutrient film technique,
solution pump drives nutrient solution from reservoir along bottom of a tilted tank and down return
tube.
-In aeroponics high pressure pump sprays nutrient solution on roots enclosed in a tank exposed to
air. In ebb and flow system, pump periodically fills upper chamber containing plant roots with
nutrient solution. When pump is turned off, solution drains back through pump into reservoir.
Aeroponics and ebb flow system require higher nutrient levels.
Nutrient solutions can sustain rapid plant growth
-A Hoagland solution contains all known mineral elements needed for rapid plant growth like KNO3,
Ca(NO3)2, KH2PO4. Concentrations of these elements are set as high as possible without being toxic
or causing saline stress.
-Also, nitrogen is supplied as ammonium NH4+ and NO3- to reduce rise of pH that occurs when
nitrogen is supplied solely as nitrate ion. Allows for cation-anion balance. A problem in nutrient
solutions is maintaining availability of iron.
-When supplied as FeSO4 or Fe(NO3)2 iron can precipitate out of solution as iron hydroxide. If
phosphate is present, insoluble phosphate salts can form also. Citric acid is a chelator because it
forms soluble complexes with cations like iron and calcium and is held by ionic bonds. Modern
solutions use EDTA and DTPA as chelating agents.
-Iron may be released from chelator when it is reduced from Fe3+ to Fe2+ at root surface and chelator
may diffuse back into solution. After uptake in root, iron is kept soluble by chelation with organic
compounds present in plant cells. Citric acid is an important organic iron chelator.
Mineral deficiencies disrupt plant metabolism and function
-Elements like N, P, K can move from leaf to leaf, others such as B, Fe and Ca are relatively immobile.
If an essential element is mobile, deficiency symptoms appear first in older leaves. Deficiency of an
immobile essential element becomes evident in younger leaves.
Group 1: Deficiencies in mineral nutrients that are part of carbon compounds
-Nitrogen availability in soils limits plant productivity in most natural and agricultural ecosystems.
Nitrogen deficiency rapidly inhibits plant growth. Most plants show chlorosis which is yellowing of
the leaves esp. in older leaves near base of plant. Younger leaves might not show symptoms
because N can be mobilized from older leaves. If N deficiency develops slowly, plants may have
slender and woody stems due to buildup of carbohydrates that can’t be used in synthesis of amino
acids or N compounds. These carbs can be used in anthocyanin, leading to purple colour in leaves,
petioles and stems.
-Sulfur is found in some amino acids and coenzymes and vitamins. Sulfur deficiency has similar
symptoms of N deficiency including chlorosis, stunted growth and anthocyanin accumulation.
Chlorosis caused by sulfur deficiency arises first in young leaves because S is not remobilized to
younger leaves.
Group 2: Deficiencies in mineral nutrients that are important in energy storage or structural
integrity
-P and Si are macronutrients whereas B is a micronutrient. These elements usually present in plants
as ester linkage. Phosphorous is imp for intermediates of respiration and photosynthesis and
phospholipids. Deficiency symptoms include stunted growth in young plants and dark green
colouration of leaves, which may contain dead spots called necrotic spots. Excess anthocyanin
may give leaves dark greenish purple colour and slender stems.
-Silicon is essential for scouring rush plants. Deficiency symptoms include susceptibility to lodging
(falling over) and fungal infection. Silicon is deposited on ER, cell walls and intercellular spaces as
silica. Serves as alternative to lignin in reinforcement of cell walls. Can reduce toxicity of metals.
-Boron helps with cell elongation, nucleic acid synthesis and hormone responses. Symptoms include
black necrosis of young leaves and terminal buds. Necrosis of young leaves occurs at base of of leaf
blade. Stems may be stiff and brittle. Plant can become highly branched, but terminal apices
become necrotic due to inhibition of cell division.
Group 3: Deficiencies in mineral nutrients that remain in ionic form
-Potassium regulates osmotic potential of plant, activates enzymes in respiration and
photosynthesis. Symptoms include marginal chlorosis and develops into necrosis at leaf tips and
between veins, extending toward leaf base. Initially appear in old leaves since potassium is mobile.
Leaves may curl and crinkle. Stems are slender and weak with short intermodal regions. Roots have
susceptibility to root rotting fungi and higher tendency for lodging.
-Calcium is used in synthesis of new cell walls (middle lamella) and mitotic spindle during mitosis
and a second messenger. Calcium may bind to calmodulin which binds to several proteins such as
kinases, phosphatases, etc. and regulates cellular processes from transcription to cell survival.
Deficiency symptoms include necrosis of young meristematic regions, i.e. tips of roots or young
leaves, where mitosis and wall formation is rapid. Necrosis may be preceded by chlorosis and
downward hooking of young leaves. Young leaves may be deformed. Root system may be brownish,
short, highly branched.
-Magnesium involved in activation of enzymes in respiration and photosynthesis. Also part of
chlorophyll. Deficiency symptoms include chlorosis between leaf veins in older leaves because of
mobility of element. Leaves may become yellow or white and premature leaf abscission.
-Chlorine needed for water splitting reaction. Symptoms: wilitng of leaf tips followed by chlorosis and
necrosis. Leaves may have bronze colouration. Roots appear stunted and thickened near tips.
-Manganese activate several enzymes (decarboxylases and dehydrogenases in Kreb Cycle).
Symtoms: intervenous chlorosis with small necrotic spots. Occurs in younger or older leaves
depending on species and growth rate.
-Sodium used in C4 and CAM pathway for regenerating PEP. Symptoms: Chlorosis or necrosis and fail
to form flowers.
Group 4: Deficiencies in mineral nutrients that are involved in redox reactions
-Iron involved in transfer of electrons such as cytochromes. Can be reversibly oxidized. Symptom:
Intervenous chlorosis. Appear in younger leaves because not easily mobilized. After a while, veins
may become chlorotic, causing leaf to turn white. Leaves become chlorotic because iron is required
for synthesis of some chlorophyll protein complexes. Low mobility due to precipitation in older
leaves.
-Zinc required in enzymes and chlorophyll biosynthesis. Symptoms: reduction in internodal growth
and leaves form circular cluster radiating at or close to ground. Small or distorted leaves with leaf
margins having puckered appearance because of insufficient auxin indole acetic acid (IAA). Older
leaves become intervenously chlorotic and white necrotic spots.
-Copper associated with enzymes such as plastocyanin involved in electron transfer. Symptom: dark
green leaves, may have necrotic spots. Necrotic spots appear at tips of young leaves then extend
toward base. Twisted or malformed.
-Nickel required in N fixing organisms. Symptoms: urea accumulation in leaves and leaf tip necrosis.
-Molybdenum required in enzymes such as nitrate reductase and nitrogenase. Symptoms: chlorosis
between veins and necrosis of older leaves. Leaves may be twisted and die (whiptail disease).
Flower formation may be prevented or abscise. Molybdenum is involved with nitrate assimilation and
nitrogen fixation so may cause nitrogen deficiency.
Analysis of plant tissues reveals mineral deficiencies
-Soil analysis is chemical determination of nutrient content in soil sample from root zone. Soil
analysis does not tell us how much of a particular mineral nutrient the plant actually needs or can
absorb. Plant tissue analysis requires understanding of relationship between plant growth and
mineral concentration.
-When nutrient concentration is low, growth is reduced. This deficiency zone, an increase in
nutrient leads to increase in growth. The adequate zone is where further increases in concentration
don’t result in any more growth. Transition between deficiency and adequate zones is the critical
concentration which is the minimal tissue content of nutrient that is correlated with maximal
growth. As concentration increases, growth may decline because of toxicity, referred to as the toxic
zone.
- Plant analysis has been useful in establishing fertilizer schedules that sustain yields and ensure food
quality of crops.
Treating Nutritional Deficiencies
-In acid soils, leaching that carries ions away with drainage water can be decreased by adding lime
(CaO, CaCO3, and Ca(OH)2) to make soil more basic because many compounds form less soluble
compounds when pH is higher than 6. However, decreasing leaching may lead to decreased availability
of iron. Most nutrients are available in pH 5.5-6.5.
Crop yields can be improved by addition of fertilizers
-Fertilizers contain N, P, K. Straight fertilizers contain only one nutrient. Compound/mixed fertilizers
contain more than one nutrient. The numbers such as 10-14-10 are percentages of N, P as P2O5 and
K as K2O.
-Some micronutrients like B, Cu, Zn, Mn, Mo, Fe are needed in acidic and sandy soils in humid
regions. Sulfur can lower pH of soil and lime can increase pH. Organic fertilizers originate from
residues of plant or animal life from rock deposits. These compounds must be broken down by soil
microorganisms, a process called mineralization.
- Slow rate of mineralization hinders efficient fertilizer use, so farmers may need additional N or P,
although organic fertilizers improve structure of soil and enhance water retention during drought
and increasing drainage in wet weather.
Some mineral nutrients can be absorbed by leaves
-Plants can absorb mineral nutrients applied to their leaves as sprays in a process called foliar
application. Reduces lag time between application and uptake. Can prevent restricted uptake since
Fe, Mn, Cu are adsorbed on soil particles and less available to roots.
- Nutrient uptake on leaves most effective if nutrient solution remains on leaf as thin film, which
requires surfactant chemicals that reduce surface tension. Uptake involves diffusion through cuticle
and uptake by leaf cells. Stomatal pore prevents liquid penetration, so it can’t enter through
stomata.
- If foliar sprays are applied on hot day when evaporation is high, salts may form on leaf surface and
cause burning. Spraying on cool days and evenings is better. Adding lime to spray diminishes
solubility and limits toxicity. Foliar application can save an orchard or vineyard when fertilizers would
be to slow to correct a deficiency.
Soil, Roots and Microbes
Negatively charged soil particles affect the adsorption of mineral nutrients