assimilation of nitrogen in plants through nitrate and ammonia nutrition and the enzyemes envolved in the assimilation.

1. MINERAL NUTRITION
MACRONUTRIENTS
NITROGEN

2. ASSIMILATION OF NITROGEN:
• • THE MAJOR SOURCES OF INORGANIC NITROGEN ARE
• -NITRATE
• -AMMONIUM
• • MOST OF AMMONIUM ARE INCORPORATED INTO ORGANIC COMPOUNDS IN THE
ROOT, WHILE NITRATE IS MOBILE IN XYLEM OR STORED IN VACUOLES

3. NITRATE ASSIMILATION AND REDUCTION:
• NO3- + 8H+  NH3 + 2H2O + OH-
• BACTERIA USES NITRATE AS AN ELECTRON ACCEPTOR IN ANAEROBIC CONDITIONS
• • REDUCTION OF AMMONIA IS MEDIATED BY TWO SEPARATE ENZYMES:
• -NITRATE REDUCTASE WHICH REDUCES NITRATE INTO NITRITE
• -NITRITE REDUCTASE WHICH REDUCES NITRITE TO AMMONIA.
• •NITRATE REDUCTASE
• HIGH M.WT HAVE SEVERAL PROSTHETIC GROUPS
• REQUIRES NADP OR NADPH AS E-DONOR
• ACTIVITY CAN BE INHIBITED BY AMMONIUM AND CERTAIN AMINO ACIDS OR AMIDES.
• IT IS VERY LOW IN MOLYBDENUM-DEFICIENT PLANTS.

4. • NITRITE REDUCTASE:
• LOW M.WT
• IN LEAVES IT IS ASSOCIATED WITH CHLOROPLASTS WHERE THE REDUCED FERREDOXIN IS THE
E-DONOR AND IN ROOTS WITH PROTOPLASTIDS WHERE AN UNKNOWN E-CARRIER BETWEEN
NADPH AND THE ENZYME IS PRESENT.
• RARELY ACCUMULATES IN INTACT PLANTS, PRESENT IN MUCH HIGHER CONCENTRATION IN
TISSUE THAN NITRATE REDUCTASE. ROOT NODULES ARE EXCEPTION.
• CERTAIN HERBICIDES CAN INHIBIT NITRATE REDUCTASE IN LEAVES AND CORRESPONDINGLY
INCREASE NITRITE LEVEL IN TISSUES.
• C4 PLANTS MESOPHYLL AND BUNDLE SHEATH CELLS DIFFER IN NITRATE ASSIMILATION

5. LOCALIZATION IN ROOTS AND SHOOTS:
• 1- LEVEL OF NITRATE SUPPLY
• 2-THE PLANT SPECIES
• 3-THE PLANT AGE.
• AND HAVE EFFECT ON:
• 1-MINERAL NUTRITION
• 2-CARBON ECONOMY OF PLANTS

6. • WHEN NITRATE SUPPLY IS LOW REDUCTION IS HIGH WHEN INCREASED SUPPLY REDUCTION
CAPACITY LIMITED AND INCREASING PRO-PORTION OF NITROGEN IS TRANSLOCATED
• LARGE CARBOHYDRATE REQUIREMENT FOR REDUCTION IS ONE OF ITS LIMITING FAC-TORS.
• LEAF AGE
• IN ROOTS
• LIGHT: CORRELATION BETWEEN LIGHT INTENSITY AND NITRATE REDUCTION
• CARBOHYDRATE LEVEL AND SUPPLY REDUCING EQUIVALENTS.
• THE ENZYME STABILITY.

7. NITRATE ASSIMILATION AND OSMOREGULATION:
• ASSIMILATE IN SHOOTS AND SYNTHESIZE ORGANIC ACID ANIONS IN THE CYTOPLASM AND
STORED IN THE VACUOLE.
• MECHANISMS FOR EXCESS OSMOTIC SOLUTES REMOVAL
• 1-PRECIPITATION IN AN OSMOTICALLY INACTIVE FORM.
• 2-RETRANSLOCATION OF REDUCED NITROGEN WITH PHLOEM-MOBILE CATIONS TO GROWTH
AREAS
• 3-RETRANSLOCATION OF ORGANIC ACID ANIONS WITH POTASSIUM INTO ROOTS

8. ASSIMILATION OF AMMONIUM:
• AMMONIUM AND ITS EQUILIBRIUM PARTNER AMMONIA ARE TOXIC AT QUIT LOW
CONCENTRATIONS. NH3 ↔ NH4+ + OH-
• THE FORMATION OF AMINO ACIDS, AMIDES AND RELATED COMPOUNDS IS THE MAIN
PATHWAY FOR ITS DETOXIFICATION.
• STEPS IN ASSIMILATION AMMONIUM IONS
• -UPTAKE INTO ROOT CELLS WITH SIMULTANEOUS RELEASE OF PROTON FOR CHARGE
COMPENSATION
• -INCORPORATION INTO AMINO ACIDS AND AMIDES.

9. • ALL OF THE ASSIMILATED AMMONIA IS TRANSLOCATED TO THE SHOOTS AS AMINO ACIDS,
AMIDES AND RELATED COMPOUNDS.
• AMMONIUM ASSIMILATION IN ROOTS HAS A LARGE CARBOHYDRATE REQUIREMENT BECAUSE
OF THE NEED FOR CARBON IN AMINO ACID AND AMIDES.
• CARBON LOSS FROM NITROGEN TRANSPORT -WHICH OCCURS THROUGH XYLEM- ONE OR
RARELY TWO OR MORE OF THE NITROGEN COMPOUNDS (N/C > 0.4)
-THE AMIDES GLUTAMINE (2N/5C) AND ASPARAGINE (2N/4C)
• -THE AMINO ACID ARGININE (4N/6C)
• -THE UREIDE ALLANTION (4N/4C)
• THE LOW MOLECULAR WEIGHT ORGANIC COMPOUNDS USED FOR LONG DISTANCE
TRANSPORT OR FOR STORAGE IN INDIVIDUAL CELLS DIFFERS AMONG PLANT FAMILIES.
• TWO KEY ENZYMES ARE INVOLVED, GLUTAMINE SYNTHETASE AND GLUTAMATE SYNTHASE.

10. • IN THIS PATHWAY THE AMINO ACID GLUTAMATE ACTS AS AMMONIA ACCEPTOR AND GLUTAMINE
IS FORMED.
• GLUTAMINE SYNTHETASE: HIGH AFFINITY FOR AMMONIA (LOW KM VALUE)
• -IT IS ACTIVATED BY HIGH PH AND HIGH CONCENTRATION OF BOTH MAGNESIUM AND ATP. THESE
THREE FACTORS ARE INCREASED IN THE CHLOROPLAST STROMA.
• GLUTAMATE SYNTHASE (GOGAT): CATALYZE THE TRANSFER OF AMIDE GROUP (-NH2) FROM
GLUTAMINE TO 2-OXOGLUTARATE (PRODUCT OF TRICARBOXYLIC ACID CY-CLE)
• -EITHER REDUCED FERREDOXIN, NADH OR NADPH ARE REQUIRED.
• -CYCLE RESULTS IN TWO MOLECULES OF GLUTAMATE ONE FOR CYCLE MAINTENANCE AND THE
OTHER CAN BE UTILIZED FOR PROTEIN BIOSYNTHESIS.
• -WHEN AMMONIA SUPPLY IS LARGE BOTH MOLECULES ACT AS AMMONIA ACCEPTOR AND ONE
GLUTAMINE MOLECULE RELEASED.
• GLUTAMATE DEHYDROGENASE: LOCALIZED PRINCIPALLY IN MITOCHONDRIA OF ROOT AND LEAVES
AND HAVE LOW AFFINITY TO AMMONIA (HIGH KM¬) INCONSISTENT WITH THE NEED TO MAINTAIN
THE LOW INTRACELLULAR AMMONIA CONCENTRATION.

11. AMINO ACID AND PROTEIN BIOSYNTHESIS:
• THE ORGANICALLY BOUND NITROGEN OF GLUTAMATE AND GLUTAMINE CAN BE UTILIZED FOR
THE SYNTHESIS OF OTHER
• AMIDES,
• UREIDES,
• AMINO ACIDS AND
• PROTEINS.

12. ROLE OF LOW MOLECULAR WEIGHT ORGANIC
NITROGEN COMPOUNDS:
• INTERMEDIATES BETWEEN THE ASSIMILATION
• EXCRETE ORGANIC NITROGEN
• THE TRIPEPTIDE GLUTATHIONE FUNCTION IN CHLOROPLAST REDOX SYSTEM AND IN LONG
DISTANCE TRANSPORT OF REDUCED SULFUR IN THE PHLOEM.
• SEVERAL ANTIBIOTICS SUCH AS VALINOMYCIN ARE LOW MOLECULAR WEIGHT POLY-PEPTIDES.
• THE LONG DISTANCE TRANSPORT OF CERTAIN HEAVY METALS IN XYLEM.
• THEY ARE PRECURSORS FOR AMINE SYNTHESIS.
• THEY ARE ALSO INVOLVED IN OSMOREGULATION IN HIGHER PLANTS.

13. AMMONIUM VERSUS NITRATE NUTRITION:
• CALCIFUGUS PLANTS ACID SOILS AND LOW SOIL REDOX POTENTIAL AND HAVE PREFERENCE FOR AMMONIA.
• CACICOLE PLANTS HIGH PH AND CALCAREOUS SOILS AND HAVE PREFERENCE FOR NITRATE.
• THE UPTAKE AND UTILIZATION OF AMMONIA ARE HIGHER AT LOW TEMPERATURE FOR ALL PLANTS.
• HIGHEST GROWTH RATES ARE GAINED WHEN COMBINED AMMONIUM AND NITRATE NUTRITION OR
AMMONIUM ONLY.
• CATION-ANION BALANCE, ON ROOT INDUCED RHIZOSPHERE CHANGES AND ON ENERGY METABOLISM.
AMMONIUM GENERALLY INHIBIT CATION UPTAKE AND CAN DEPRESS GROWTH BY INDUCING MAGNESIUM
DEFICIENCY. AMMONIUM ALSO UNLIKE NITRATE INCREASES ROOT RESPIRATION WHICH RESULTS IN ENHANCED
ROOT EXUDATION AND HENCE INCREASED BACTERIAL GROWTH.
• GROWTH INHIBITION BY AMMONIUM IS CLOSELY RELATED TO THE FALL IN SUBSTRATE PH. AT LOW PH THE
AMMONIUM UPTAKE IS NOT DEPRESSED AS OTHER CATIONS WHICH INCREASES THE CATION ANION
IMBALANCE.

14. NITROGEN SUPPLY, PLANT GROWTH AND PLANT
COMPOSITION:
• NITROGEN CONTENT REQUIRED FOR OPTIMAL GROWTH 2-5% DEPENDING ON THE PLANT SPECIES,
DEVELOPMENTAL STAGE AND ORGAN.
• SUB-OPTIMAL SUPPLY:
• 1.PLANT GROWTH RETARDED
• 2.ENHANCED SENESCENCE OF OLDER LEAVES.
• SUPRA-OPTIMAL SUPPLY:
• 1.INHIBIT ROOT ELONGATION WHICH IS UNFAVORABLE FOR NUTRIENT GAINING AND WATER UPTAKE.
• 2.ENHANCE SHOOT ELONGATION WHICH INCREASES SUSCEPTIBILITY TO LODGING AND YIELD LIMITING
FACTOR
• 3.INCREASE LENGTH, WIDTH AND AREA OF THE LEAVES THUS INTERFERES WITH LIGHT INTERCEPTION
• 4.INDUCE CHANGING IN PHYTOHORMONES BALANCE.