Plant physiology lecture notes



Mineral Nutrition

Objectives

• Nutritional needs of plants

• Symptoms of specific nutritional deficiencies

• Use of fertilizers to ensure proper plant nutrition

• Influence of soil structure and root morphology on the transfer of inorganic nutrients from the environment into plants

• Mycorrhizal symbiotic associations.

Mineral Nutrients

Mineral nutrients are elements acquired primarily in the form of inorganic ions from the soil. (example: N, P, and K). Mineral absorption by plants is a very efficient process.

Mineral Nutrition

Mineral Nutrition is the study of how plants obtain and use mineral nutrients.

Studying of mineral nutrition is central to research in modern agriculture and environmental protection. Less than half of the fertilizer applied is used by plants and the remaining minerals leach into surface waters or ground waters. Plants are also proving useful for removing deleterious minerals including heavy metals toxic-waste dumps.

Essential Nutrients

An essential element is an intrinsic component in the structure or metabolism of a plant.

An essential element is an element whose absence causes severe abnormalities in plant growth, development, or reproduction. Table 5.1 lists the essential elements for the vast majority of higher plants.

Classification of essential mineral elements:

- According to their relative concentration in plant tissue (Table 5.1):

- Macronutrients: required in large amount

- Micronutrients: required in small amount

- According to their biochemical role and physiological function (Table 5.2):

- G1: nutrients involved in oxidation and reduction biochemical reactions to form organic compounds (i.e. nutrients that are part of carbon compounds, e.g. N & S).

- G2: nutrients involved in ATP reactions, or contribute to cell wall structure or cell wall mechanical properties (i.e. nutrients that are important in energy storage or structural integrity, e.g. P, Si, & B).

- G3: Nutrients acting as enzyme cofactors and/or involved in regulation of osmotic potentials; free ions dissolved in the plant water (nutrients that remain in ionic form; e.g. K, Na, Ca, Cl, Mg, & Mn).

- G4: nutrients involved in electron transfer reactions, e.g. constituent of cytochrome systems involved in photosynthesis or respiration (i.e. nutrients that are involved in redox reactions, e.g. Fe, Zn, Cu, Ni, & Mo).

Hydroponics or Solution culture (Techniques of studying mineral nutrition)

Growing plants with their roots immersed in a nutrient solution containing only inorganic salts without soil. Successful hydroponic culture requires:

- a large volume of nutrient solution

- frequent adjustment of the nutrient concentrations and pH of the medium

- a sufficient supply of oxygen.

Types of Hydroponics:

- Hydroponic growth system: plants are grown in a supporting material, and nutrient solutions are then flushed through the supporting material. Old solutions are removed by leaching.

- Nutrient film growth system: plant roots lie on the surface of a narrow channel, and nutrient solutions flow in a thin layer along the channel over the roots.

- Aeroponic growth system: plants are grown with their roots suspended in air while being sprayed continuously with a nutrient solution.

- Ebb and flow system: the nutrient solution periodically rises to immerse plant roots and then recedes (withdraws, ebbs), exposing the roots to a moist atmosphere.

Nutrient Solutions

- Early formulations: KNO3, Ca(NO3)2, KH2PO4, MgSO4, & Iron salt.

- Modified Hoagland nutrient solution (Table 5.3): contains all of the known mineral elements needed for rapid plant growth.

Properties of the modified Hoagland formulation

- The concentrations of the elements are set at the highest possible levels that permit plants to grow in a medium for extended periods without replenishment of the nutrients.

- Nitrogen is supplied in two forms, ammonium (NH4+) and nitrate (NO3-), i.e. in a balanced mixture of cations and anions; reduce the rapid rise in the pH of the medium.

- Maintaining the availability of iron to plants; adding chelators forms soluble complexes with cations (e.g. Fe3+); chelated compounds remain physically available to a plant.

- EDTA and DTPA are chelating agents (see Fig. 5.2).

Mineral Deficiency

Nutrient deficiency is the inadequate supply of an essential element.

Nutrient deficiency symptoms are the expression of metabolic disorders resulting from the insufficient supply of an essential element.

A particular deficiency may appear as a specific pattern of leaf discoloration.

Diagnosis of nutrient deficiency symptoms in soil-grown plants is more difficult and complex than those in solution culture-grown plants.

In general, the essential elements function in plant structure, metabolism, and cellular osmoregulation.

The extent to which an element can be recycled from older to younger leaves is an important clue relating acute deficiency symptoms to a particular essential element.

- N, P, & K are mobile elements (can readily move from leaf to leaf)

- B, Fe, & Ca are relatively immobile elements in most plant species.

Deficiency symptoms of a mobile element tend to appear first in older leaves.

Deficiency of an immobile essential element becomes evident first in younger leaves.

Nitrogen

- a constituent of many plant cell components, e.g. amino acids, proteins, & nucleic acids

- nitrogen deficiency rapidly inhibit plant growth

- Chlorosis (yellowing of the leaves), especially in the older leaves near the base of the plant.

- nitrogen –deficient plants:

- may have light green upper leaves and yellow lower leaves.

- may have slender and woody stems

- may reveal purple coloration in leaves, petioles, and stems (due to accumulation of anthocyanin pigment)

Sulfur

- many of the symptoms of sulfur deficiency are similar to those of nitrogen deficiency, including chlorosis, stunting of growth, and anthocyanin accumulation.

- sulfur Chlorosis may occur simultaneously in all leaves (mature and young leaves)

Phosphorus

- sugar-phosphate intermediates of respiration and photosynthesis

- phospholipids

- ATP, DNA, & RNA

- Phosphorus deficiency symptoms include:

- stunted growth in young plants

- dark green coloration of the leaves with necrotic spots (small spots of dead tissue)

- purple coloration of leaves (not associated with chlorosis)

- slender stems and death of older leaves

Silicon

- plants deficient in silicon are susceptible to lodging (falling over) and fungal infection.

Boron

- Evidence suggests that boron plays roles in cell elongation, nucleic acid synthesis, and membrane function

- Boron deficiency symptoms:

- A characteristic symptom is black necrosis of the young leaves and terminal buds.

- Stems may be unusually stiff and brittle.

- fruits, fleshy roots, and tubers may exhibit necrosis

Potassium

- present within plants as cation, K+.

- plays an important role in regulation of the osmotic potential of plant cells

- activates many enzymes involved in respiration and photosynthesis

- Symptoms of potassium deficiency:

- mottled (spotted) or marginal chlorosis, which then develops into necrosis primarily at the leaf tips, at the margins, and between veins.

- symptoms appear initially on the more mature leaves toward the base of the plant.

- leaves may curl and crinkle.

- stems may be weak, with abnormally short internodes.

- roots may be susceptible to root-rotting fungi.

- weak stems and rotted roots result in lodging of the plants.

Calcium (Ca2+)

- used in the synthesis of new cell walls, specially the middle lamella.

- used in the mitotic spindle during cell division

- required for normal functioning of plant membranes

- Symptoms of calcium deficiency:

- necrosis of young meristematic regions where cell division and cell wall formation are most rapid.

- necrosis may be preceded by general chlorosis and downward hooking of the young leaves.

- death of meristematic regions results in severe stunting growth.

Magnesium (Mg2+)

- plays a specific role in the activation of enzymes involved in respiration, photosynthesis, and the synthesis of DNA and RNA.

- Mg is part of the chlorophyll molecule.

- Symptoms of magnesium deficiency:

- chlorosis between the leaf veins, occurring first in the older leaves

- premature leaf abscission

Chlorine (Cl-)

- required for the water-splitting reaction of photosynthesis.

- required for cell division in both leaves and roots.

- Deficiency symptoms:

- wilting of the leaf tips followed by general leaf chlorosis and necrosis

- reduced growth and “bronzing” of leaves

- roots may appear stunted and thickened near the root tips

Manganese (Mn2+)

- activation of enzymes, particularly enzymes involved in the tricarboxylic acid cycle (Krebs cycle)

- functions in the water-splitting reaction of photosynthesis

- Deficiency symptoms:

- inter-venous chlorosis associated with small necrotic spots

Sodium (Na+)

- vital for regenerating phosphoenolpyruvate (PEP), the substrate for the first carboxylation in the C4 and CAM pathways of carbon fixation.

- Sodium-deficient plants exhibit chlorosis and necrosis, and may fail to form flowers.

Iron (Fe2+ or Fe3+)

- a component of enzymes involved in the transfer of electrons (redox reactions)

- required for the synthesis of chlorophyll-protein complexes in the chloroplast

- Symptoms of iron deficiency:

- inter-venous chlorosis, appears initially on the younger leaves.

- chlorotic veins and the whole leaf turns white under extreme and prolonged deficiency.

Zinc (Zn2+)

- required for enzymes activity

- required for chlorophyll biosynthesis

- Zinc deficiency is characterized by:

- small and distorted leaves with margins having wrinkled appearance.

- reduction in internodal growth; plants display a rosette habit of growth (a circular cluster of leaves radiating at or close to the ground)

Copper (Cu+ or Cu2+)

- required for enzymes involved in the transfer of electrons (redox reactions)

- Symptoms of cupper deficiency:

- production of dark green leaves, which may contain necrotic spots

- necrotic spots appear first at the tips of the young leaves and then extend toward the leaf base along the margins.

- leaves may be twisted or malformed.

- premature leaf abscission

Nickel

- Urease is the only enzyme known in higher plants that contains Ni2+.

- Nickel-deficient plants accumulate urea in their leaves and, consequently, show leaf tip necrosis.

Molybdenum

- Mo ions are components of several enzymes (e.g. nitrate reductase & nitrogenase)

- involved in nitrate assimilation and nitrogen fixation by microorganisms

- Molybdenum deficiency symptoms:

- general chlorosis between veins and necrosis of the older leaves.

- flowers formation may be prevented

- premature flowers abscission

- Molybdenum deficiency may bring about nitrogen deficiency.

Analysis of the Ninerals (nutrients) Levels in Plant Tissue and in Soil

- Requirements for mineral elements change during the growth and development of a plant.

- In crop plants, nutrient levels at certain stages of growth influence the yield of the economically important tissues.

Soil analysis

- Chemical determination of the nutrient content in a soil sample from the root zone.

- Soil analysis may reflect the levels of nutrients potentially available to the plant roots from the soil.

Plant tissue analysis

- How much does the plant actually need of a particular mineral nutrient? or

- How much does the plant actually able to absorb a particular mineral nutrient?

- Understanding of the relationship between plant growth (or yield) and the mineral concentration (nutrient content) of the plant tissue.

For a given nutrient, one can define three zones according to the response of growth or yield to the nutrient content (concentration) of tissue samples (Fig. 5.3).

1- Deficiency zone

- Low nutrient concentration in a tissue sample is correlated with reduced growth i.e. an increase in nutrient availability is directly related to an increase in growth or yield.

2- Adequacy zone

- A point at which further addition of nutrients is no longer related to increase in growth or yield

- further addition of nutrients is reflected in increased nutrient concentrations of the tissue

- The Critical concentration of a nutrient is the minimum nutrient content of the tissue that is correlated with maximum growth or yield.

- Critical concentration is located at the transition point between the deficiency and adequate zones

3- Toxicity zone

- Increasing of the nutrient concentration in the tissue, beyond the adequate zone, is correlated with declines of growth or yield.

If a nutrient deficiency is suspected (especially in agricultural soils that are often limited in the elements N, P, & K), steps are taken to correct the deficiency before it reduces growth or yield.

Treating Nutritional Deficiencies

Traditional farming practices

- Recycling of mineral elements

- (crop residues and manure from humans and animals return the nutrient to the soil)

- leaching of dissolved ions away with drainage water results in loss of nutrients from such agricultural systems

- leaching may be decreased (or treated) by the addition of lime (a mix of CaO, CaCO3, & Ca(OH)2), Fig. 5.4.

High-production agricultural systems of industrial countries

- significant unidirectional removal of nutrients from the soil to the crop

- addition of fertilizers (containing one or more of the nutrients) to restore the lost nutrients.

Application of nutrients to the soil

- Addition of chemical (inorganic) fertilizers

- contain inorganic salts of the macronutrients N, P, & K.

- Straight fertilizers (e.g. superphosphate, ammonium nitrate, & muriate of potash)

- compound or mixed fertilizers.

Addition of organic fertilizers

- plant and animal residues contain many of the nutrient elements in the form of organic compounds

- Mineralization (breaking down of organic compounds by microorganisms) is required before plants can acquire the nutrient elements from the organic residues.

- The slow rate of mineralization hinders the efficient use of fertilizer.

Modification of the soil pH

- addition of lime can raise the pH of acidic soils

- addition of elemental sulfur can lower the pH of alkaline soils

Foliar application of mineral nutrients

- most plants can absorb mineral nutrients applied to their leaves as sprays

- foliar application can reduce the lag time between application and uptake by the plant

- foliar application can overcome the problem of restricted uptake of a nutrient from the soil (eg. Iron, Mn, & Cu are adsorbed on soil particles and hence are less available to the root system)

- Precautions should be considered for foliar nutrient application.

Soils, Roots, and Microbes

- the soil contains three phases: solid, liquid, and gaseous phases

- the solid phase contains inorganic and organic particles that provide a reservoir of the mineral nutrients

- the liquid phase includes the soil solution that contains dissolved mineral ions.

- the gaseous phase contains gases such as O2, CO2, & N2 that are dissolved in the soil solution and are found in the air spaces (gaps) between soil particles.

Soil particles

- Soil particles, both inorganic and organic, have mainly negative charges on their surfaces

- Many inorganic soil particles are crystals of aluminates and silicates (Al3+ and Si4+ bound to oxygen atoms)

- Replacement of Al3+ and Si4+ by cations of lesser charges within the crystals results in negatively charged inorganic soil particles.

- The negative surface charges of organic particles result from the dissociation of H+ from the carboxylic and phenolic groups present in components of the soil.

- Inorganic soils are categorized by particle size into gravel, coarse sand, fine sand, silt, and clay

Soil mineral ions

- Mineral cations (e.g. NH4+ and K+) adsorb to the negative charges on the surfaces of inorganic and organic soil particles; thus, they provide a nutrient reserve available to plant roots; causing soil fertility.

- Cation exchange is the process through which adsorbed mineral nutrients can be replaced by other cations.

- Cation exchange capacity (CEC) of a soil is the degree to which a soil can adsorb and exchange ions

- Mineral anions (such as NO3- and Cl-) tend to be repelled by the negative charge on the surface of soil particles and remain dissolved in the soil solution.

Soil pH

- Soil pH affects the growth of plant roots and soil microorganisms.

- Root growth favors pH values between 5.5 and 6.5

- Fungi (pH below 7)

- Bacteria (pH above 7)

- Acidity promotes releasing of K+, Mg2+, Ca2+, and Mn2+ form rocks.

- Acidity increases the solubility of carbonates, sulfates, and phosphates.

- Microbial decomposition of organic material and the amount of rainfall lower the pH by releasing of H+.

- H+ displace K+, Mg2+, Ca2+, and Mn2+ form the cation exchange complex in a soil.

Excess minerals in the soil

• Saline soil: excess minerals.

• Plant growth can be restricted if it affects water availability or exceed the adequate zone.

• Major problem in arid and semi-arid regions.

• Salt-tolerant plants: can survive high levels of salts.

• Halophytes: grow vigorously under such conditions.

• Accumulation of heavy metals

Extensive root systems

• Growth of plant roots depends on availability of water and minerals in the rhizosphere.

• Crop plants (under fertilization and irrigation) allocate more resources to the shoot and reproductive structures than to roots.

• They differ in form but are based on common structures.

• Monocots; dicots.

................
................

In order to avoid copyright disputes, this page is only a partial summary.

Google Online Preview   Download