Chapter 5 Notes.doc

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University of Toronto Scarborough
Biological Sciences
Mary Olaveson

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 B ,12ound 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 KNO , 3 Ca(NO )3 2KH P2 .4Concentrations of these elements are set as high as possible without being toxic or causing saline stress. - Also, nitrogen is supplied as ammonium NH 4+ and NO 3 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 FeSO or Fe4NO ) iron c3 2precipitate 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 Fe 3+to Fe 2+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 cant 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 se
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