Question

Long answer type questions.
Why were micronutrients discovered later than macronutrients? With one example each, mention the physiological role and deficiency disease caused by (i) nitrogen, (ii) phosphorus, (iii) copper, and (iv) iron.

Answer 1

Macronutrients are needed in large quantities in comparison to other nutrients which is why they are called “macro” nutrients and are commonly referred to as "macros". Micronutrients are needed only in very small amounts, these substances to not provide any calories but enable our bodies to produce enzymes, hormones, and other substances vital to development, disease prevention, and well-being.

(i) Nitrogen: Plants absorb nitrogen from the soil in three forms: nitrate (NO3- ), Nitrite (NO2-) or Ammonium (NH4+).

Functions:

(i) It is the major constituent of proteins, purines, pyrimidines, vitamins, chlorophyll and hormones, It is also present as a component of coenzymes like FAD, NAD, NADP etc., Older leaves when turning yellow, their nitrogen passes to younger parts in the form of amines and amides.

Deficiency Symptoms:

(i) Stunted growth due to reduced cell division and dormant lateral buds. Chlorosis (yellowing of leaves), Suppressed or late flowering, Increase in starch content but the decrease in protein content, Wrinkling of cereal grains, Purple coloration appears in shoot axis.

(ii) phosphorus: Soil contains phosphorus in organic and inorganic forms.

Functions:

It is a constituent of phospholipids (membrane lipids), nucleic acids, nucleotides, coenzymes, ATP, metabolic intermediates, sugar phosphates in photosynthesis etc.

Deficiency Symptoms:

Purple or red pigmentation on leaves, Premature fall of leaves and floral buds

Delay in seed germination, Older leaves affected first and become dark brown, Stunted and slender stem in young plants, Accumulation of carbohydrates in Glycine max (Soybean), Vascular tissues reduce in tomato plants.

(iii) Copper:

Copper is necessary for carbohydrate and nitrogen metabolism and, inadequate copper results in stunting of plants. Copper also is required for lignin synthesis which is needed for cell wall strength and prevention of wilting. Deficiency symptoms of copper are dieback of stems and twigs, yellowing of leaves, stunted growth and pale green leaves that wither easily.

Copper deficiencies are mainly reported on sandy soils which are low in organic matter. Copper uptake decreases as soil pH increases. Increased phosphorus and iron availability in soils decrease copper uptake by plants.

(iv) iron:

Iron is involved in the production of chlorophyll, and iron chlorosis is easily recognized on iron-sensitive crops growing on calcareous soils. Iron also is a component of many enzymes associated with energy transfer, nitrogen reduction and fixation, and lignin formation. Iron is associated with sulfur in plants to form compounds that catalyze other reactions. Iron deficiencies are mainly manifested by yellow leaves due to low levels of chlorophyll. Leaf yellowing first appears on the younger upper leaves in interveinal tissues. Severe iron deficiencies cause leaves to turn completely yellow or almost white, and then brown as leaves die.

Iron deficiencies are found mainly on high pH soils, although some acid, sandy soils low in organic matter also may be iron-deficient. Cool, wet weather enhances iron deficiencies, especially on soils with marginal levels of available iron. Poorly aerated or compacted soils also reduce iron uptake by plants. Uptake of iron decreases with increased soil pH, and is adversely affected by high levels of available phosphorus, manganese and zinc in soils.